Download Phakic Intraocular Lenses

December, 2003
244
DOS Times - Vol.9, No.6
EDITORIAL
Dear friends,
There have
been extraordinary developments in the
field of refractive surgery in
the past few
years. State of
the art technology is being developed and used for the treatment of
refractive errors. It all started with man’s
desire to get rid of his dependence on
spectacles which drove ophthalmologist
in the early seventies to experiment with
procedures that changed the corneal curvature to achieve emetropia, the ultimate
goal of any eye surgeon. Radial Keratotomy was considered a revolution of sort
when it was first introduced. But as the
side affects began to show up newer techniques surfaced. First came PRK and
then Lasik. Emergence of excimer laser
and its dominance in refractive corneal
surgery is one of the most significant
advances in recent times. Harnessing the
laser to safely perform corneal surgery
has been a major technical achievement.
Though initially Lasik was recommended for all degrees of myopia it is
now clear that this procedure is not recommended in very high myopias (>10D). This is because of limitation in
night vision, loss of best spectacle corrected usual acuity, visual aberrations
and diminished quality of vision. The
surgeon’s goal is not only to provide a
satisfactory postoperative visual acuity
but also a good quality of vision.
We are entering a new age with the
introduction of phakic IOLs for correction of high refractive errors where we
implant IOL leaving crystalline lens intact. Apart from providing excellent refractive accuracy, it preserves corneal
sphericity and allows rapid visual recovery. Compared with techniques that
change corneal curvature like Lasik,
phakic IOL styles & designs are capable
of providing the patient with best visual
acuity, better quality of vision and quality of image. Its main advantage over
corneal surgery is that it is reversible.
However there is some concern about its
safety in the long run and also that it
demands more surgical skill than the
routine cataract surgery.
Though not practiced in our country
till recently, there are few centers that
have started adopting this new technique. R.P.Centre has initiated a project
to study the role of Phakic IOL in our
Indian setup with special emphasis on
its long-term safety.
It is a well-observed fact that refractive surgery is often patient driven and
sometimes they push the ophthalmologist to provide surgical relief from their
dependence on optical devices. Therefore the ophthalmologists has a special
responsibility towards the patients, to be
sure that the patient has a realistic expectation about surgery and that they are
not solely guided by enthusiastic media
claims. Patients must understand both
the advantages and disadvantages of the
surgery and accept that there may be
complications from any surgical procedure. These patients must be counseled
and must be told of the risk involved
even though the percentage of success
may be high.
Thanks,
Dr. Jeewan S. Titiyal
Secretary
Programme for DOS Monthly Clinical Meeting for December 2003
Venue: Library, Venu Eye Institute & Research Centre, New Delhi
Date & Time : 27th December, 2003 (Saturday) at 2.30 P.M.
Case Presentation
1. Combined Penetrating Keratoplasty and R.D. Surgery................... Dr. Reny
using a Temporary Keratoprosthesis
2. Macular Ischaemia following .............................................................. Dr. Arundhati
Intravitreal injection of an aminoglycoside
10 Mins.
10 Mins.
Clinical Talk
l Dry Eye .................................................................................................... Dr. Ashu Agarwal
Mini Symposium: Low Vision
20 Mins.
Chairman : Dr. S.C. Gupta,
Convenor : Dr. Anil Tara
1. Assessment of a Low Vision Patient .................................................. Dr. Sunita
2. Optical & Non-optical Low Vision Devices ...................................... Dr. Gaurav
3. Indications & Therapeutic Options .................................................... Dr. Ajay
Panel Discussions : 15 min.
Followed by Tea
December, 2003
245
10 Mins.
10 Mins.
10 Mins.
DOS Times - Vol.9, No.6
LETTERS TO EDITOR
Letters to Editor
Low Vision in India
Dear Editor,
Low Vision corresponds to visual
acuity of less than 6/18, but equal to
or better than 3/60 in the better eye
with best possible correction, according to the International Classification
of Diseases (10th edition).
In the National Survey on Blindness (2001-02) conducted under the
National Programme Control of
Blindness, 63337 persons aged 50
years and above were examined in
15 states of the country1. The prevalence of Low Vision was estimated
to be 10.3%. Extrapolating this figure
to the entire population, the prevalence of Low Vision in the general
population approximates 1.3%.
In a population-based study in
urban Delhi, children aged 5-15 years
were examined2. The prevalence of
Low Vision was 0.22% in this agegroup. In a similar study in rural
Andhra Pradesh, the prevalence of
Low Vision was found to be 0.15%
in the age-group 7-15 years3.
In another population-based
study in Andhra Pradesh, persons of
all ages were examined 4. In this
study, Low Vision was defined as
permanent visual impairment that
was not correctable with refractive
error or surgical intervention. The
participants with best-corrected distance visual acuity of <6/18 to perception of light or central visual field
<10 degrees because of an
untreatable cause in both eyes were
considered as having Low Vision.
The prevalence of Low Vision was
estimated to be 1.05%.
The available data thus suggests
an estimated prevalence of Low Vision of 1-1.5% in India, in the general population, though there is wide
geographic variation, and there is a
paucity of data on Low Vision in the
young adult population. This is approximately 2.1/2-3 times the prevalence of blindness, when blindness is
defined by the W.H.O. criteria. Adequate attention needs to be paid for
provision of available and accessible
Low Vision Services in our country.
References
1. Murthy, GVS, Gupta, S, Tewari, HK,
Jose, R, Bachani, D. Eds. National Survey on Blindness & Visual Outcomes
after Cataract Surgery 2001-02. Report.
National Programme for Control of
Blindness, Directorate General of Health
Services, Ministry of Health and Family Welfare, Government of India, New
Delhi.
2. Murthy, GVS, Gupta, SK et al. Refractive error study in children in an urban
population in New Delhi. IOVS, March
2002, Vol. 43, No.3: 623-631.
3. Dandona, R, Dandona, L et al. Refractive error study in children in a rural
population in India. IOVS, March 2002,
Vol. 43, No.3: 615-622.
4. Dandon, R, Dandona, L et al. Planning
Low Vision Services in India: a population based perspective. Ophthalmology
2002 Oct; 109(10): 1871-8.
Dr. Sanjeev K. Gupta,
Dr. G.V.S. Murthy,
Dr. Praveen Vashist
Community Ophthalmology Deptt.
Dr. R.P. Centre for Ophthalmic Sciences,
AIIMS, New Delhi
Pre-Retinal Blood Aspiration-hydraulic Help – A New Surgical Technique
Dear Editor,
After pars plana vitrectomy a thin
layer of blood may often be present
on the retinal surface. It is generally
easily aspirated with back flush
needle or an end opening extrusion
needle connected to active aspiration
module of the vitrectory machine
(Vacuum cleaning). 1,2 If the blood is
tenacious because of clotting, forceps
removal may be necessitated.
We experienced difficulty sometimes in removal of pre-retinal blood
with Vacuum cleaning technique.
The our surprise when back flush
mechanism was actuated, blood of-
December, 2003
ten dispersed and could be easily aspirated. This step is repeated in the
area where pre-retinal blood could
not be aspirated otherwise. We have
been using this technique routinely
in removing pre-retinal blood whenever vacuum-cleaning technique
does not work before resorting to
forceps removal. Often times the
technique does work obviating forceps blood removal, which can be
traumatic to the retina or even cause
retinal tears. We believe hydraulic
back pressure helps in breaking up
tenuous blood clot and hence simplifies its aspiration.
246
We do no recommended that hydraulic technique be used over the
optic disc and fovea for fear of inducting traumatic hydraulic damage.
References
1. Charles S. Vitreous microsurgery
Williams & Wilkins, 1987, 83-84.
2. O’Malley, C. Extrusion Method.
Octome fragmatome newsletter,
1987, 3.
Dr. Yog Raj Sharma
Dr. R.V. Azad,
Dr. Deependra V. Singh
Dr. R.P. Centre, AIIMS, New Delhi
DOS Times - Vol.9, No.6
CURRENT PRACTICE
Phakic Intraocular Lenses
Namrata Sharma MD, Jeewan S. Titiyal MD, Nishant Taneja MBBS,
Rasik B. Vajpayee MS, FRCS (Ed)
Phakic intraocular lens
(IOL) is any lens located between the cornea and the
crystalline lens, which is left
undisturbed in the eye.
Anterior chamber phakic
intraocular lenses have been
used since 1950s but the surgical procedure failed due to
lack of microsurgery devices
and poor understanding of
the endothelial function. In
1987, there was a comeback
of these lenses and today
phakic intraocular lenses are
an important area of refractive surgery.
Types of Phakic IOLS
There are three types of
phakic IOLs. These include:
1. Anterior chamberangle fixated IOL e g. ZB M5,
NuVita MA20, Phakic 6.
2. Anterior chamber- Iris
fixated IOL e g. VerisyseTM
Phakic IOL (Artisan lens)
3. Posterior chamber sulcus fixated IOL e g. STAAR
Implantable contact lens and
phakic refractive lens (PRL)
Selection of patients for
phakic IOLs
1. Age above 18 years
2. Moderate to high
myopes (>-9.00D) & hyperopes (> 4.5 D)
3. Also indicated in
lesser degrees of ametropias
if LASIK is contraindicated
such as
Cornea and Refractive Surgery
Services, Dr. R.P.Centre for
Ophthalmic Sciences, AIIMS,
New Delhi.
December, 2003
Ø Corneas thinner than 500
microns
Ø Steep or flat corneas
Ø Topographic change suggestive of keratoconus
4. Endothelial cell density: at least 2250-2500mm3.
5. Pupil smaller than 6
mm in scotopic luminance.
6. Stable refraction for at
least 1 year
7. Anterior chamber
depth (excluding corneal
thickness) at least 2.8mm
8. Angle width at least 30
degrees
9. No eye pathology except refractive
10. No systemic pathology such as diabetes, collagen diseases etc
Angle Supported Phakic
IOLS
First generation angle
supported IOLs were developed by Baikoff & Joly in
1997. The first model (ZB DOMILENS) was a modified
Kelman type lens with a
4.0mm optic and 2 haptics
with a 4-point fixationin the
angle. However, this lens
had a high vault and was so
close to the endothelium that
it had to be explanted in 50
% of the eyes due to endothelial damage.
In the line of Kelman type
angle supported anterior
chamber phakic IOL, the
only available at present is
PHAKIC 6which is made of
PMMA, has a 6.0mm optic
and 2 haptics with four point
fixationin the angle.
Two types of foldable intraocular lenses in this group
include:
1. Vivarte (Ciba Switzerland) and the Duet (Tekia,
USA) in one group and
2. ICARE
(Corneal,
France) and Acrysof (Alcon,
USA)
Vivarte and Duet have a
foldable optic (acrylic) of
5.5mm and 2 haptics made
of PMMA with 3 points for
the angle fixation. The difference between these two IOLs
is that in the Duet the haptic
and optic are implanted
separately in the bag and the
lens is assembled in the eye
whereas in the ICARE and
Acrysof the folding occurs
outside the eye.
Selection of angle supported IOL
To select an IOL we need
to calculate the power and
the size of the implant. As
these IOLs are supported by
the angle and the size of the
anterior chamber varies
from patient to patient, the
correct size of the anterior
chamber must be defined.
Most surgeons measure horizontal or vertical white to
white with the help of the
calipers, Holladay discs or
Orbscan.
To this white-to-white, a
correction factor is added to
determine the correct length.
For example 1 mm in Phakic
6, 0.5-1.0mm in Vivarte and
1.5mm in Acrysof.
To calculate the lens
247
Fig.1: Phakic 6 IOL
Fig. 2: Vivarte
power we use Van der
Heijde nomogram, which
takes into account the spherical equivalent, the corneal
power and the anterior
chamber depth.
Surgical Technique
To implant an angle supported anterior chamber
phakic IOL , the following
steps are performed:
1. Pre-operative miosis
2. Incision size of 6.2 mm
for Phakic 6 (corneal/
corneoscleral)
3. Incision of 3.2mm
(clear corneal self sealing )
4. Fill anterior chamber
with standard viscoelastic
5. Introduce the IOL
6. No iridotomy / iridectomy required
7. Wash out the viscoelastic
8. Suture in case of
DOS Times - Vol.9, No.6
CURRENT PRACTICE
Fig.4: Nuvita lens
Problems of anterior chamber phakic IOLs
Fig. 3: Duet IOL
1.
Phakic 6 IOL
9. Postoperatively, topical steroids and antibiotics
are given for 1 week
The problems, which
have been encountered with
these lenses, are as follows:
Iris Supported Anterior
Chamber Phakic IOLS
Iris fixated IOLs have
haptics in the form of lobster
claw that fixate the IOL to the
mid peripheral iris. The classical type of this phakic IOL
is the Artisan (Ophtec, Netherlands & Verisyse AMO).
The Artisan lens is a onepiece UV wavelength absorbing PMMA compression
molded lens with a diameter
of 8.5 mm. The optic is
vaulted suitably (0.5mm) to
stay clear of the iris cone. It is
available in 5.0mm optic (for
myopia -3.00 to -23.00 and
hyperopia + 3.0 to +12.00) or
6.0mm (available for myopia
-3.0 to - 14.50 D). There is also
a toric Artisan available in
5.0mm model, which corrects upto astigmatism of 7.00 D. Two models are available: Model A: axis of cylinder in the axis of IOL and
Model B: Axis of cylinder 90
degrees of the axis of IOL.
December, 2003
Endothelial cell loss ® Intermittent endothelial
touch
2. Pupillary ovalisation (4-42%)
a) Immediate post operatively ® Iris tuck/ oversized IOL
b) Late onset ® Iris root ischemia
3 Iris depigmentation(2.3-4.5%) ® Iris protrusion
during surgery
4. Halos & glare® Small optic zones
5. Surgically induced astigmatism® Long incisions
Selection of iris fixated IOLs
Concerning the power,
the rules are same as for anterior chamber fixated
phakic IOLs. Concerning the
size: ' one size fits all' as these
implants are not dependent
on the eye dimensions.
Surgical Technique
The steps of the surgery
are as follows:
1. Pre-operative miosis
2. Two side ports are
made (2 mm away from each
extremity of main incision)
3. Main incision 5.2 or 6.2
mm) corneal or corneoscleral
4. Fill the anterior chamber with standard viscoelastic
5. Introduce the IOL
6. Grasp the iris tissue
into the claws
7. Wash out the viscoelastic
Problems of Iris fixated phakic IOLs
1. Anterior chamber inflammation: early post-op- 6.4
to 16% of eyes (Fechner et al 1992)
2. Glaucoma
3. Iris atrophy: on fixation sites - 81% cases( Santonja
et al)
4. Implant dislocation: lens instability & haptic
disincarceration in 9.3% ( Santonja et al.)
5. Decentration : 23.4-56% (Manejo et al.)
6. Endothelial cell loss: mean endothelial cell loss
5.3%, 7.63% &17.9% at 1, 2 &5 years respectively.
7. Cystic wounds and subconjunctival fistulas- rare
248
8. Iridectomy/ Iridotomy recommended
9. Postoperatively, topical steroids and antibiotics
are given for 1 week
The advantages of these
lenses are as follows:
1. Enclavation of the loop
tips produce a pillow of iris
tissue over the most peripheral part of the haptics
2. The angle of the anterior chamber, the crystalline
lens, and the corneal endothelium are not at risk because they are far away
3. The Pupil can be dilated
for fundus examination. Iris
claw lens does not affect the
movements of the iris and the
pupil, except at the point
where the iris passes through
the claw. The crystalline lens
is not affected since the implanted lens remains far
away from it.
These lenses are very easy
to explanted of needed new
lenses can be important any
increased risk.
Posterior Chamber Phakic
IOLS
In the years between 19901998 a new material came
into vogue known as the
Collamer. This was a hybrid
of silicone and collagen and
the lens was called as the
Implantable contact lens or
the ICL. The current version
available is the V4, which
was introduced in 1998.
Fig.5: Artisan lens Phakic
IOL (VerisyseTM)
DOS Times - Vol.9, No.6
CURRENT PRACTICE
Fig.6: Artisan lens Phakic
IOL (VerisyseTM)
The PRL or the phakic refractive lens is another posterior chamber phakic IOL
available with Ciba vision. It
is made of new generation
ultra thin hydrophobic silicone. It has no anatomical
fixation sites and floats on the
layer of aqueous humor inside the posterior chamber
exerting no traction on the
ciliary structures and without coming in contact with
the anterior capsule of the
crystalline lens.
The Sticklens (IOLTECH,
France) is available which is
made up of hydrophilic
acrylic material, which sticks
firmly to the anterior surface
of the crystalline lens.
Surgical Technique
1. To prevent angle closure 2 fully patent sufficiently wide Nd: YAG laser
iridotomies must be done 1
to 2 weeks before surgery.
2. Peripheral
iridotomies should be placed at
11 and 1 0'clock under the
upper lid to avoid the risk of
monocular diplopia, ghost
images and for aesthetic reasons.
3. Intraoperative surgical iridectomy is a less recommended option.
4. Both ICL and PRL are
injected through a less than
2.5 mm incision to provide
December, 2003
the patient with the benefits
of 'minimally invasive implant surgery'. The current
PRL has 2 tiny dots on the
haptics in order to avoid inverted implantation.
5. Smooth and gentle
retro positioning of the
footplates involves maneuvering the haptics through
the 1 mm sideports with sand
blasted visco cannula or specifically designed 'tuckers'.
Bimanual irrigation aspiration is then done to remove
the viscoelastic agent.
Phakic Iols: Whre Are We
Today?
Roughly 60,000 phakic
IOLs have been implanted
(equally divided among
angle fixated, iris supported
and posterior chamber
phakic intraocular lenses. If
properly sized and implanted, all these phakic
IOLs are likely to produce
more or less the acceptable
similar results in terms of
precision, predictability and
stability of the refractive outcome. The anecdotal incidence of catastrophic events
like endophthalmitis and
retinal detachment is acceptably low if considered in
terms of risk/benefit ratio in
a population who is also
Fig. 7: Staar ICL Collamer
Fig. 8: PRL
Problems of phakic posterior chamber IOLs
Ø
Ø
Ø
Ø
Ø
Ø
Ø
Inverted implantation- iatrogenic
Endothelial cell damage (2.3-3.0% at 2 yrs)
Inflammation
Pigment dispersal
Elevated IOP
Cataractogenesis -0.82 to 4.38% at 5 years
Decentration
naturally prone to posterior
segment disorders. Nighttime symptoms are frequent
but disabling only in a few.
With latest and future generations wide optic lenses
and adequately sized lenses
these problems including the
problem of cataractogenesis
should decrease. Although
the short-term results are reassuring, the long-term results are awaited.
Sugested Reading
1: Guell JL, Vazquez M,
Malecaze F, Manero F, Gris O,
Velasco F, Hulin H, Pujol. J. Artisan toric phakic intraocular
lens for the correction of high
astigmatism. Am J Ophthalmol.
2003 Sep;136(3):442-7.
4: Sanchez-Galeana CA, Smith
RJ, Sanders DR, Rodriguez FX,
Litwak S, Montes M, Chayet AS.
Lens opacities after posterior
chamber phakic intraocular
lens implantation. Ophthalmology. 2003 Apr; 110(4):781-5.
5: El-Sheikh HF, Tabbara KF.
Cataract following posterior
chamber phakic intraocular
lens. J Refract Surg. 2003 JanFeb;19(1):72-3.
7: Sanders DR, Vukich JA; ICL
in Treatment of Myopia (ITM)
Study Group. Incidence of lens
opacities and clinically significant cataracts with the implantable contact lens: comparison
of two lens designs. J Refract
Surg. 2002 Nov-Dec; 18(6): 67382.
Summary of posterior chamber Phakic IOLs
Model
ICL ( STAAR)
PRL( CIBA Vision)
Sticklens(IOLTECH)
Optic
Single piece
Single piece
Single piece
Geometry
Planospherical
Planospherical
Meniscus
Diametrs
5.5/5.25/5.0/4.6
5/4.5
6.5
Material
Collamer
Hydrophillic Silicone
Hydrophillic Acrlic
Special features
Toric custom
No
No
Power
-3 to –21
+3to+17
-3to-20
+3to+15
-7to-25
+4to+7
Power calc. Formula
Olson-Feingold
Holladay Refractive
Van der Hejide
Incision size
2.5mm
1.8mm
3.0mm
249
DOS Times - Vol.9, No.6
CURRENT PRACTICE
Contrast Sensitivity Measurement
and its Practical Implications
Parul Sony MD, Vandana Kori BSc
Contrast is created by the
difference in luminance, and
the amount of reflected light,
reflected from two adjacent
surfaces. It provides critical
information about the edges,
borders and variation in the
brightness of two objects. It
can be defined with the following formula:
Contrast = Lmax - Lmin
Lmax + Lmin
Lmax = Luminance on the
lighter surface
Lmin = Luminance on the
darker surface
Contrast sensitivity is
the reciprocal of the contrast
at threshold, i.e., one divided by the lowest contrast
at which forms or lines can
be recognized. If a person
can see details at very low
contrast, his or her contrast
sensitivity is high and vice
versa. Contrast sensitivity
measures the ability to see
details at low contrast levels.
Depending on the structure
of the stimulus used in the
measurement - either gratings of different size or symbols - contrast sensitivity of
a person gets different values. When the darker surface is black and reflects no
light, the ratio is 1. Contrast
is usually expressed as percent, then the ratio is multiplied by 100. The maximum
contrast is thus 100% conDr. R.P.Centre for Ophthalmic
Sciences, AIIMS,
New Delhi.
December, 2003
trast. The symbols of the visual acuity charts are close
to the maximum contrast. If
the lowest contrast perceived is 5%, contrast sensitivity is 100/5=20. If the lowest contrast perceived by a
person is 0.6%, contrast sensitivity is 100/0.6=170.
Contrast sensitivity function (CSF) is defined as the
visual function that is measured using a range of sinusoidal grating pattern as the
visual stimulus. CS assesses
the patient’s sensitivity to
large, intermediate and
small objects (spatial frequency) under the conditions of varying contrast. It
is a subjective measurement
of a person’s ability to detect
a low contrast pattern
stimuli and it gives a more
accurate representation of
the eyes’ visual performance. The CSF usually decreases with age especially
for high and intermediate
spatial frequencies. A low
CSF reduces the amount of
detail that can be seen especially in lower levels of lighting or at a distance like, difficulty in reading poor contrasting print materials, difficulty in moving around
safely in dim light. The
pathological loss of CS is of
four types
1. generalized loss at all
frequencies
2. high frequency loss
3. mid frequency loss
4. low-mid frequency
loss
Snellen visual acuity Vs
Contrast sensitivity: Snellen’s
charts test VA at a very high
contrast thus many patients
have a good snellens VA but
they may be visually handicapped in real life situations.
The VA drops in the situations with low contrast and
the quality of vision is not
good. CS is a useful index of
pattern vision. It provides
the information that cannot
be obtained from visual acuity, and it is often a better
predictor of visual performance then VA. There are
various causes that may decrease the CS
1. uncorrected refractive
errors,
2. keratoconous
3. cataract
4. posterior segment dis-
orders: AMD, Diabetic retinopathy, Glaucoma
5. Optic neuritis
Methods
Testing contrast sensitivity is important as it subjectively assesses the patient’s
visual function in day to day
life. Various charts and electronic devices are available
to measure the contrast sensitivity. The key difference is
target type. For example, the
Pelli-Robson chart determines the contrast required
to read large letters of a fixed
size. With the Pelli-Robson
chart, the contrast varies
while the letter size remains
constant. The Regan chart, a
low-contrast letter chart
having different size letters,
reduces the contrast levels of
a standard Snellen-type letter acuity chart resulting in
several charts. The Functional Acuity Contrast Test
uses sine-wave gratings.
Most common stimulus
used for the clinical evaluation of spatial CS is repetitive patterns of alternating
light and dark bars where
Contrast sensitivity is the reciprocal of
the contrast at threshold, i.e., one
divided by the lowest contrast at which
forms or lines can be recognized.
Fig. 1: Cambridge low contrast charts
250
DOS Times - Vol.9, No.6
CURRENT PRACTICE
Plate no
Contrast %
Demo
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
13
5
2.7
1.6
1
0.72
0.52
0.37
0.27
0.19
0.14
l
Bailey Lovie chart
Vision contrast test
system (VCTS) by
Vistech
l Cambridge low contrast grating
l Regan charts
l FACT charts
l
Cambridge low contrast gratings
It is a rapid and
simple screening test for
contrast sensitivity. Performed at a distance of
6m. It comprises of 12
pair of plates consisting
of stripes of varying contrast. First one is for
demonstration and rest
are for the proper testing
and are numbered from
1-10. The plates are
changed sequentially
starting from plate 1 till
the patient fails to respond. Then a new series is begun starting 4
plates prior to where the
patient failed to respond. Four such series
Fig. 2: Pelli robson contrast sensiare completed and the
tivity charts and scoring pad
score of each series is
noted (numbered as per
the luminance of the bars
varies sinusoidally along the the number of plate read)
single axis (known as sine and added. The final total
wave grating). The patient value is converted into consignals when the pattern is trast sensitivity from the
first detected. Typically the provided table.
spatial frequency varies
from 0.5 to 23 cpd. The PELLI ROBSON contrast
graphic representation of sensitivity charts
This chart utilizes letters
the mean and standard deof
the
same size but with reviation of the threshold of
different spatial frequencies ducing contrast to provide a
is called the CS curve or CSF. quick means of assessing
The CS peaks around 4 cpd patient contrast sensitivity.
of spatial frequency. Con- There are two charts and
trast sensitivity measures two scoring pads. The two
the ability to see details at charts have different letter
low contrast levels. The vari- sequence but other wise
ous methods available to they are similar. Each chart
has 6 letters in each row ormeasure CS include
ganized into two triplets of
l Pelli Robson test
December, 2003
varying contrast. Thus there
are two contrast levels in
each row. The illumination
of the chart is 85 cd/mm2
and glare should be avoided. The test is carried at a
distance of 1 meter with patient wearing the best correction and before dilating
the pupils. The patient is
asked to read the alphabets
starting from left hand corner, when he fails to respond
several seconds are given to
him to retry and guess the
alphabet. The score of the
test is recorded by the faintest triplet out of which at
least 2 letters are correctly
identified. The log CS value
for this triplet is given by the
Contrast
sensitivity
measures the ability
to see details at
low contrast levels
number on the scoring pad
nearest to the triplet, either
on the left or the right side.
Each eye is tested separately
and then the both eyes together. The three measurements should not take
longer then 8 minutes. Usually the binocular log CS is
higher by 0.15 units then
monocular CS. However development of cataract may
result in poorer binocular CS
when compared to the CS of
each eye being tested separately.
Functional Acuity Contrast
Testing (FACT)
FACT charts were developed by Dr. Arthur Ginsburg. FACT comprises of a
chart with sine-wave gratings of varying contrast as
well as varying spatial fre-
251
quencies. It is an accurate
and comprehensive grating
chart that tests functional
visual acuity. The chart tests
five spatial frequencies
(sizes) and nine levels of
contrast. The Contrast varies in a row, decrease from
left to right. And the spatial
frequencies increase as one
move down the various columns from top to bottom.
This test is performed at a
distance of 10 feet. The patient determines the last
grating seen for each row (A,
B, C, D and E) and reports
the orientation of the grating: right, up or left. The last
correct grating seen for each
spatial frequency is plotted
on a contrast sensitivity
curve. FACT contrast sensitivity scores can also be used
to generate images with
VSRC’s EyeView™ Functional Analysis Software
which demonstrates how
the world looks to your patient based on their contrast
sensitivity scores. FACT
Contrast Sensitivity Charts
are available in two sizes:
Studies have shown that
the contrast sensitivity curve
provided by sine-wave grating tests is more sensitive
and informative than the results obtained from lowcontrast letter acuity systems. Pelli Robson is more
sensitive and most reproducible methods for studying and detecting the mid
range loss of CS.
Practical application
Importance of CSF lies in
various practical situations
like:
l Driving in winter in fog
or rain
l Radiographic diagnosis
l In near vision tasks like
DOS Times - Vol.9, No.6
CURRENT PRACTICE
Monthly Meetings Calendar
For The Year 2003-2004
27th July, 2003 (Sunday)
Army Hospital
30th August, 2003 (Saturday)
Sir Ganga Ram Hospital
27th September, 2003 (Saturday)
Hindu Rao Hospital
19 October, 2003 (Sunday)
DOS Midterm Conference
Fig. 3: FACT chart
1st November, 2003 (Saturday)
R.P. Centre for Ophthalmic Sciences
Distance FACT Chart and Near FACT Chart
Stimulus
type
Cambridge
low contrast
gratings
Pelli
robson
FACT
Contrast
gratings
Alphabets
Contrast
gratings
1m
85-150cd/
mm
10 feet
—
1 chart
1 near,
1 distance
Test distance 6m
Luminance
100cd/mm2
Number of
Plate
l
l
l
12 chart
reading and writing, if the
information is at low contrast
In every day tasks, where
there are numerous visual
tasks at low contrast, like
cutting an onion on a light
colored surface, pouring
coffee into a dark mug,
checking the quality of
ironing, etc
Development of cataract
causes a decrease in the
contrast sensitivity. The
CS shows improvement
after the cataract surgery.
Various IOLs have different effect on the postoperative CS. The CS recovery
is best with all PMMA
IOL>Acrylic IOL> silicone
IOL
Diffractive multifocal IOL
December, 2003
l
l
29th November, 2003 (Saturday)
Dr. Shroff’s Charity Eye Hospital
27th December, 2003 (Saturday)
Venu Eye Hospital & Research Centre
31st January, 2004 (Saturday)
Safdarjung Hospital
have poorer CS when
compared to the monofocal IOL especially for
near vision
The aspheric IOL (Technis) provides a significant improvement in retinal image contrast. This
improvement is significantly better when compared to the performance
of conventional spherical
silicone and acrylic IOLs.
CS decreases after refractive surgery like RK, PRK
and LASIK. Lasik has the
least effect on CS, though
it has been seen that CS improves slightly still it does
not return to baseline even
by 6 months postoperatively.
252
28th February, 2004 (Saturday)
M.A.M.C. (GNEC)
28th March, 2004 (Saturday)
Mohan Eye Institute
3-4th April, 2004 (Saturday & Sunday)
Annual DOS Conference
Attention DOS Members
The Hi-tech DOS Library has started functioning on
Ground Floor, Dr. R.P. Centre, Delhi Ophthalmic Sciences, AIIMS, New Delhi-110029 from 12.00 Noon to
9.00 P.M. on week days and 10.00 A.M. - 1.00 P.M. on
Saturday, Sunday. The Library will remain closed on
Gazetted Holidays. Members are requested to utilise
the facilities available i.e. Computer, Video Viewing,
Latest Books and Journals. We are planning to subscribe two journals. Member can give suggestion in this
regard.
Dr. Lalit Verma
Library Officer, DOS
DOS Times - Vol.9, No.6
MANAGEMENT PEARLS
Analysis of Problem Situations:
Management of IOL Power Calculation Problems
After Refractive Surgery
Sudipto Pakrasi MD, DNB
These days, cataract patients who have previously
undergone refractive procedures are taking center stage
in many ophthalmic practices. As the popularity of refractive surgeries such as
LASIK and PRK grows, surgeons will increasingly be
confronted with the problem of calculating the proper
IOL power after previous
corneal refractive surgery.
Surgeons are dancing as fast
as they can to try to avoid
all-too-common refractive
surprises in these patients.
Postrefractive surgery patients often bring with them
the same high expectations
they had for their refractive
procedures. They’re going to
want the same sort of visual
outcomes after cataract surgery that they achieved after refractive surgery. This
requires a different approach from that used with
the typical cataract patient.
The difficulty of IOL
power calculation after refractive surgery lies in the
ability to accurately calculate corneal powers. The altered corneal shape can result in multiple ranges of
corneal power throughout
the central corneal surface.
The average keratometric
Eye Microsurgery Associates
Aashlok Hospital, Safdarjung
Enclave, New Delhi
December, 2003
readings from the standard
intraocular lens formulas
are inaccurate for these patients.
Conventional manual
keratometry measures a limited number of points at a 3
to 4 mm annulus, which can
vary with steeper or flatter
corneal shapes. Another factor introduced by corneal
tissue removing surgeries,
such as LASIK, has been the
change in the standardized
refractive index of the cornea due to the change in the
corneal power of the front
corneal surface.
Why a problem?
The true corneal power
following RK, ALK, PRK
and LASIK is difficult to
measure by keratometry, or
corneal topography. The
reasons for this are as follows:
Keratometry and topography assume a normal relationship between the anterior and posterior corneal
curvatures and measure the
anterior corneal radius.
Keratorefractive surgery for
myopia flattens the anterior
corneal radius, but leaves
the posterior corneal radius
mostly unchanged. This disparity is greater with LASIK
and somewhat less with RK.
Because the central cornea
has been flattened, keratometry may read a falsely
large area. Rather than measuring several points within
a central 2.5 to 3.5 mm area,
the measurement may instead be at 6.0 mm. With the
non-incisional forms of
keratorefractive surgery,
such as LASIK and PRK, the
index of refraction of the cornea is probably changed.
Automated keratometry
and corneal topography
analysis therefore use incorrect assumptions in measuring corneal power. The cornea can no longer be compared to a sphere centrally.
The back surface is no longer
1.2mm steeper than the front
surface. The reading measure the curvature at 3mm.
This may not reflect the
power in the centre of the
cornea. In normal corneas
the posterior curvature of
the cornea is 1.2 mm less
than the anterior surface.
Anterior corneal radius of
7.5 mm, using the Standardized Keratometric Index of
Refraction of 1.3375, the corneal power would be 45 D.
Overestimates total power
by 0.56 D. Most IOL calculations today used a net index of refraction of 1.3333
(4/3) and the anterior radius
of the cornea to calculate the
net power of the cornea. Using this total power of a cornea = 44.44 D. The major
source of IOL error in such
patients therefore is accu-
253
rately determining the
keratometric power. Manual and automated keratometers evaluate the radius
of curvature designated by
four points in orthogonal
meridians separated about
3.2 mm apart with manual
keratometers, or about 2.6
mm apart in automated
keratometers. Corneal optics
in keratometers is assumed
to be spherocylindrical. Normal corneas are nearly
spherical, or prolate. A patient who, prior to the refractive surgery, is myopic is
going to have a relatively
flatter cornea in the center
and it’s going to get steeper
in the periphery. Postrefractive corneas, however,
are oblate.
Setting The Stage For
Trouble
For these reasons,
keratometry, and corneal
topography will typically
over-estimate central corneal power following keratorefractive surgery for
myopia. For low power myopic corrections by RK (less
than -2.00 D), this effect is
minimal. But for higher
power myopic corrections,
and especially those by
LASIK, this over-estimation
can be quite significant. Failure to keep this important
fact in mind will often result
in an unexpected and unDOS Times - Vol.9, No.6
MANAGEMENT PEARLS
K Value Problems Figures:
ractive keratectomy.
Direct Change of Central
Anterior Corneal Curvature
During excimer laser
photorefractive keratectomy
(PRK), selective removal of
tissue across the anterior corneal surface results in a
change of the anterior corneal curvature. Although the
central anterior surface of the
cornea may become flatter
(to treat myopia) or steeper
(to treat hyperopia), the posterior surface is presumed to
remain stable. The same may
be true for uncomplicated
laser in situ keratomileusis
(LASIK) in which direct flattening or steepening of the
central anterior corneal surface is achieved by focal keratectomies
under
a
planohinged flap.
(A) Normal Cornea (Fig A) Corneal thickness - reduced
B) AFTER PRK/LASIK (Fig B)
w Anterior curvature changed
w Posterior curvature - No change
C) AFTER RK (Fig C)
w Anterior & posterior - similar changes
w No change of corneal thickness
pleasant post-operative hyperopic surprise.
With the postmyopic refractive laser in-situ
keratomileusis patient, this
means the practitioner will
be
measuring
the
keratometry values from the
steeper part of the cornea
rather than the centrally flattened portion. The K is going to estimate high, so
you’ll estimate your IOL
power to be lower than it
should be, and you’re going
to get a hyperopic surprise.
In post-hyperopic LASIK
patients, the corneas are
December, 2003
steeper in the center and flatter in the periphery. Practitioners are going to be measuring keratometry values
from the flatter part of the
cornea and estimating low.
Therefore, IOL power will
be overestimated and practitioners will get a myopic
surprise. Potentially adding
to the error is the fact that
manual and automated
keratometers don’t take into
account irregular astigmatism, which may occur after
radial keratotomy, or central
islands and decentrations
with LASIK or photoref-
Indirect Change of Central
Anterior and Posterior Corneal Curvature
To correct myopia with
radial keratotomy (RK),
deep radially oriented incisions are applied in the
midperiphery of the cornea
to induce midperipheral
bulging of the cornea. Indirectly, the central cornea becomes flatter. Since no tissue
is removed it is assumed
that the anterior and posterior surfaces of the cornea
react in an analogous way.
Principles In Intraocular
Lens Power Calculation
Calculation of intraocular
lens (IOL) power in cataract
surgery is based on measurements of corneal
power/radius of curvature,
axial length, and estimation
of postoperative anterior
chamber depth—termed effective lens position (ELP).
254
The term keratometric diopters are used to avoid confusion with optical diopters,
since keratometric diopters
and optical diopters are not
equivalent. There are a number of formulas available for
the calculation of IOL
power. They can be classified as either empirical regression formulas (most
popular are the SandersRetzlaff-Kraff formulas)
with (SRK/T) or without
(SRK I and II) the inclusion
of additional terms accounting for non-linearity or theoretical optical formulas
(Holladay, Hoffer, or
Haigis).
There are five variables in
such a formula:
1) Intraocular
lens
power, which is generally
chosen as the dependent
variable;
2) Keratometric diopters (average keratometry
reading)—the most crucial
variable after refractive corneal surgery;
3) Axial length, measured by A-scan ultrasonography or by optical means;
4) Effective lens position; and
5) Target refraction, typically ranges between -2.00
and 0 diopters (D).
Calculation of the resulting spectacle correction for
a given IOL is typically
based on a vertex distance of
12 or 14 mm. The only variable that cannot be measured preoperatively is the
effective lens position. Improvements in IOL power
calculations over the last 30
years are mainly a result of
improving the predictability
of effective lens position by
additional measurement of
horizontal white-to-white
DOS Times - Vol.9, No.6
MANAGEMENT PEARLS
corneal diameter, preoperative anterior chamber depth,
and lens thickness, since the
anterior segment dimensions often are not proportional to the axial length
Types of Formulae
1. Regression Formulas
These are based upon
mathematical analysis of a
large sampling of post-operative results. Most familiar is SRK formula: works
well 22.5 to 25.0 mm in axial
length. The formula does not
work well for “long” (>25
mm) or “short” (<22.5 mm)
eyes.
2. Theoretical Formulas
Theoretical formulas are
optical formulas based on the
optical properties of the eye.
They do a better job of predicting post-op outcomes for
long and short eyes.3 widely
used theoretical formulas,
each with their own
strengths and weaknesses.
These formulas are included
in
various
combinations with many Ascan instruments. They are
also available in software
products
· Holladay between 22 and
26mm
· Hoffer Q shorter than
22.5mm
· SRK/T for long eyes
(>26mm).
3. Recent Developments
· Holladay II Tackles severe myopia
· Holladay Consultant
IOL program – (HIC)
uses Holladay 2 formula
(uses7 variables)
· New Haigis formula
optimised for axial length
dependency of the type of
IOL to be used
December, 2003
K = Preoperative Average K - Change in Manifest Reaction Prerefractive and Postrefractive Surgery
Ktrue
=
Kpre + Rpre – Rpost
Generally holds true, if the following are known:
n Ktrue = the true corneal power after refractive surgery
n Kpre = the average corneal power in diopters before refractive surgery, and
n Rpre = the spherical equivalent in diopters before refractive surgery, and
n Rpost = stable spherical equivalent in diopters after refractive surgery, then
To give accurate information, the refractive numbers (Rpre
and Rpost) must retain their corresponding plus (hyperopic)
and minus (myopic) signs, and be corrected for vertex distance.
K = Base Curve + (Difference in Refractive Error without Contact Lens and with Contact Lens)
Ktrue = Cbase + Cpower + Rcl - Rbare
Generally holds true, if the following are known:
l Ktrue
= the true corneal power after refractive surgery
l Cbase = base curve of the contact lens in diopters, and
l Cpower = spherical refractive power of the contact lens
in diopters,
l Rcl
= spherical equivalent refractive error with the
contact lens, &
l Rbare = spherical equivalent refractive error without the
contact lens, then
To give accurate information, the refractive numbers ( Rcl
and Rbare) must retain their corresponding plus (hyperopic)
and minus (myopic) signs, and be corrected for vertex distance.
· Gaussian Optics Formula
No matter what type of
[refractive] procedure the
patient has had, the stability
of the refraction must be determined. This determination includes daily fluctuations from morning to night,
as well as long-term changes.
… Each of these factors must
be used in determining the
desired postop target refraction and to prepare the patient for the visual changes
and realistic expectations
after the procedure.
In these patients, use
biomicroscopy, retinoscopy,
corneal topography, and endothelial cell counts. The
first three help evaluate the
amount of irregular astigmatism, which may be contributing to the reduced vision, as well as the cataract.
The irregular astigmatism
may also limit the patient’s
vision after cataract surgery.
The endothelial cell count
can identify patients with
low cell counts from the previous surgery who may be
at a higher risk for corneal
decompensation or prolonged visual recovery. Secondary tests include the potential acuity meter, super
pinhole, and hard contact
lens trial. In addition, patients should be informed
that only glare from the cata-
255
ract will be eliminated. Any
glare from the keratorefractive procedure will remain unchanged.
How to Deal with the Problem?
1. Measuring K
w A) Manual Keratometry
w B) Automated Keratometry
w C) Corneal Topography
2. Calculating K
w A) Clinical history
method
w B) Contact Lens method
w C) Double K method
3. Other methods
w Intraoperative Retinoscopy
w Hand held Autorefractometers
w Secondary IOL Implant
w Piggyback IOL
w IOL Exchange
1. Measuring K:
Methods to Assess
Keratometric Diopters as A
Measure of Central Corneal
Power
Keratometry
1. A) Manual-Manual
keratometry is the method
that IOL power calculation
formulas were originally
based upon. Keratometry
evaluates only four points
separated 3 to 4 mm in two
orthogonal meridians on the
paracentral cornea, and the
corneal optics are assumed
to be spherocylindrical.
Thus, asphericity or asymmetry of corneal shape cannot be measured with standard keratometry.
Manual keratometry
probably represents the least
accurate method since current instruments, such as the
Zeiss, Javal- Schiötz, or
Bausch & Lomb keratoDOS Times - Vol.9, No.6
MANAGEMENT PEARLS
meters make too many assumptions that do not take
into account irregular corneal astigmatism. Nevertheless, the examiner is actually
able to see the reflected
mires and the amount of irregularity. Seeing the mires
does not help get better measurements, but does allow
the observer to discount the
measurement as unreliable.
1. B) Automated-Automated keratometers may be
more accurate than manual
keratometers in corneas after RK that have small clear
zones (i.e., less than 3 mm)
because they sample a
smaller central area of the
cornea (nominally 2.6 mm).
The smaller the clear zone
and the greater the number
of RK incisions, the greater
the probability and magnitude of error. This error occurs because the samples at
2.6 mm are close to the paracentral transition zone
(knee) after RK. After PRK
or LASIK, automated
keratometers accurately
measure the front radius of
the cornea, because the transition areas are far outside
the 2.6- mm zone that is
measured. However, derivation of keratometric diopters from the radius of
curvature is still wrong.
1. C) Corneal Topography Analysis
w Videokeratography
derived Simulated Keratometry Readings (Sim-K) using the topography system, the simulated keratometry value (Sim-K) is determined from the power of
Placido mires 7, 8, and 9 of
the videokeratoscope for 128
equally spaced meridians.
Measuring more than 5000
points over the entire cornea
December, 2003
256
DOS Times - Vol.9, No.6
MANAGEMENT PEARLS
and more than 1000 points
within the central 3 mm,
videokeratography provides greater accuracy in
determining the power of
corneas with irregular astigmatism compared to keratometers. However, Sim-K
value derivation may vary
among different videokeratoscopes, and these values should be compared to
the results of conventional
keratometry before using
them interchangeably.
w Average
Central
Power (ACP) - A new parameter, average central
power, from the average of
corneal powers inside the
region demarcated by the
entrance pupil of the TMS-1
videokeratoscope. Because
the density of measured
points with the videokeratoscope is highest in the central cornea and decreases
toward the periphery, areacorrected power was used
for compensation. This
modified method is supposed to have major advantages over the classic Sim-K
after RK with small clear
zones, but not after PRK and
LASIK.
Other similar values are
provided by other videokeratoscopes. For example, the
EyeSys videokeratoscope
has a display called the
Holladay Diagnostic Summary in which there is an
analogous value calculated
called effective refractive
power.
w Scanning Pancorneal
Slit Topography -Using the
Orbscan scanning slit-beam
videokeratoscope , a threedimensional location of several thousand points of the
corneal and anterior chamber surfaces can be deterDecember, 2003
mined. Two scanning slit
lamps project 40 calibrated
beams onto the eye, angled
at 45° to the left and to the
right of the video camera
axis, covering the whole cornea from limbus to limbus
and overlapping in the central 5-mm zone. This system
has the potential to provide
topographical height and
power maps of the anterior
and posterior corneal surface as well as a corneal
thickness profile, allowing a
direct correlation of
pachymetry and height
data.
2. Calculating K:
Combination of the clinical history & contact lens
methods are considered the
most accurate ways of determining the IOL power after
refractive surgery. It should
be remembered that these
methods will give an estimation of the true central corneal power and may not be
exact. At present, these techniques represent the best
clinical methods available.
Hopefully, in the future we
may have a more accurate,
and less time-consuming,
method for measuring these
challenging eyes.
2. A) Clinical history
method
Requires the following information:
Ø Pre-Refractive refraction
Ø Pre-Refractive K readings
Ø Post-Refractive refraction
(preferably current, but
before significant cataract
formation).
Using K reading before
and after refractive surgery,
refraction derived K is calculated at the corneal plane,
and used in the IOL formula.
2B) Contact Lens Method
The current refraction = +0.25 D.
With a plano hard contact lens BC of 35 D placed on the
cornea, the spherical refraction changes to -2 D. Because the
patient had a myopic shift with the contact lens, the cornea
must be weaker than the base curve of the contact by 2.25 D.
Therefore, the cornea must be 32.75 D (35 - 2.25), which is
slightly different than the value obtained by the calculation
method.
SEQ refraction without hard contact lens = +0.25 D
Base curve of plano hard contact lens = 35.00 D
SEQ refraction with hard contact lens = -2.00 D
Change in refraction = -2.00 - (+0.25) = -2.25 D (myopic
shift)
Mean corneal power = Base curve of plano HCL + change
in refraction
Mean corneal power = 35.00 + (-2.25)
Mean corneal power = 32.75 D
2 B) Contact lens method
(Overrefraction) :
After refracting the patient, a plano hard CTL with
known base curve is placed
on the eye and an over-refraction is performed. If the
refraction does not change
with the lens, then the power
of the cornea must be the
same as the base curve of the
CTL. If there is a myopic
shift, the power of the base
curve is greater than that of
the cornea by the amount of
the shift. If there is a hyperopic shift, the power of the
base curve of the CTL is less
than that of the cornea by the
amount of the shift. This
works only if visual acuity is
better than 6/24.
2 C) Double K method
A new method for calculating postrefractive IOL
power, developed by Jaime
Aramberri, MD, (Spain),
uses a modified SRK-T formula. With these standard
calculations, in postrefractive surgery patients, a
hyperopic refractive error is
the rule.
257
Postrefractive K readings
are at the center of the problem. However, no matter
how flat the cornea gets after refractive surgery, the anterior chamber depth does
not change. After corneal refractive surgery, the distance
from the cornea to the intraocular lens must remain
similar. Therefore, instead of
using postrefractive surgery
K values alone, which will
lead to inaccurate results,
they should be paired with
the original preoperative Kvalue for the first step of the
calculation. Practitioners are
advised to use the
postrefractive K-value for
the remaining calculation —
in effect splitting the K values.
Each of these methods
will assist the cataract surgeons to more accurately
select the proper IOL power
for patients who have had
previous corneal surgery.
The most difficult challenge
for refractive surgeons is that
much of the data necessary
are often not present because
of the long time interval between a patient’s cataract
DOS Times - Vol.9, No.6
MANAGEMENT PEARLS
surgery and refractive surgical procedure. A working
knowledge of these three basic methods combined with
trying to gather as much preoperative patient information as possible, will help
surgeons overcome this
problem, which will certainly grow in significance as
the number of patients receiving refractive surgery
increases worldwide.
The first two methods,
the clinical history method
and hard contact lens
method, can be used for
incisional surgery as well as
excimer laser surgery. Care
must be taken with the topographic method when
analyzing patients who
have had previous PRK or
LASIK due to a change in
effective refractive corneal
index of refraction.
In the future, our ability
to more accurately measure
the corneal surfaces including the anterior and posterior curvature, as well as
new IOL technologies that
may allow for more surgeon
customization will help us
improve the accuracy of this
difficult postrefractive surgical situation.
1. Other Methods
Ø 3A) Intraoperative Retinoscopy:
Ø 3B) Hand held Autorefractometers: These intraoperative measures may be utilized to reconfirm the IOL
power calculated in the preoperative examination.
Large refractive surprises
can be avoided by intraoperative retinoscopy or
handheld autorefractors. Because there are many factors
at surgery that may change
in the postoperative period,
December, 2003
these refractions should not
be relied upon, however, for
fine-tuning the IOL power.
Factors such as the pressure
from the lid speculum, axial
position of the IOL, intraocular pressure may cause the
intraoperative refraction to
be different than the final stabilized postoperative refraction. If the intraoperative refraction is within 2 D of the
target refraction, no lens exchanges should be considered unless intraoperative
keratometry can also be performed.
Ø 3C) Secondary IOL Implant: After completing the
initial surgery, IOL can be
implanted at a secondary
stage.
Ø 3D) Piggyback IOL:
Another IOL can be placed
in the sulcus after the primary IOL implantation after
3 – 6 months, when the refraction stabilizes.
Ø 3E) IOL Exchange: The
most drastic way out can be
undertaken, in case the patient is miserable with the
initial results.
Results after PRK/ LASIK
Very few results of cataract surgery following PRK
and LASIK are available.
Hyperopic shift on the
first day and daily fluctuations appear to be much less,
similar to the early postoperative period following
these procedures. In most
cases, the stability of the cornea makes these cases no
different than patients who
have not had keratorefractive surgery.
Postoperative after RK
First Postoperative Day –
Leads to hyperopic shift,
due to the transient corneal
edema. Daily fluctuations
cause a myopic shift during
the day (due to the regression of corneal edema after
awakening in the morning).
Long-term Results after RK
Long-term results of cataract surgery are very good.
The long-term hyperopic
shifts and against-the-rule
astigmatism over time following cataract surgery
should be the same as following RK. Glare and
starburst patterns are usually minimal - adjusted to
these unwanted optical images following the initial
RK. If the patient’s primary
complaint before cataract
surgery is glare and
starbursts, it should be made
clear to the patient that only
the glare due to the cataract
will be eliminated by surgery, and the symptoms that
are due to the RK will remain unchanged.
Post Myopic LASIK
Center flatter & periphery steeper, hence, the various methods measure K
from steeper part of cornea
causing higher K values,
giving rise to lower IOL
power estimates making the
person HYPERMETROPIC.
Post Hypermetropic LASIK
Center steeper & periphery flatter hence, the various
methods measure K from
flatter part of cornea causing
lower K values, giving rise
to higher IOL power estimates making the person
MYOPIC.
Summary & Practical Recommendations
1. Just like you do before
every refractive surgery procedure, make sure that the
258
patient has realistic expectations.
2. Make sure that the refraction is stable (after refractive surgery and before
cataract surgery), before you
take up the patient for surgery.
3. Discuss the desired
target refraction. Aiming for
-1.00 D seems to be a good
compromise. Do remember
that trying to produce
monovison in a patient who
has never experienced this
condition may cause intolerable anisometropia and may
require further surgery.
4. Perform corneal topography analysis in all patients, in case you have access to it, to assess the
amount of irregular astigmatism and asphericity.
5. If keratometric diopters and refraction before refractive surgery are known,
use the clinical history
method, considering the
change of spherical equivalent refraction at the corneal
plane after RK, PRK, or
LASIK. If those values are
not known, you may want to
use the respective calculations at the spectacle plane
to be on the safe (ie, myopic)
side. Some experts in this
field even recommend the
use of spherical equivalent
refraction change at the
spectacle plane routinely
and add another 1.00 to 2.00
D to the resulting IOL
power, being sure to avoid
any potential undercorrection. The clinical history
method seems to be the most
reliable method after RK,
PRK or LASIK.
6. If keratometric diopters but not refraction before
refractive surgery is known,
use the change in anterior
DOS Times - Vol.9, No.6
MANAGEMENT PEARLS
surface keratometry readings after PRK or LASIK.
7. If preoperative keratometric diopters and refraction are not known and
the visual acuity is 6/24 or
better, try the hard contact
lens method after RK.
8. If preoperative keratometric diopters and refraction are not known and
visual acuity is less than 6/
24 or plano hard contact
lenses are not available, use
average central power or the
average keratometric diopters at multiple paracentral
cursor points of videokeratography after RK, but
use refined calculation of
keratometric diopters from
radius of anterior and posterior corneal surface after
PRK or LASIK.
9. Use more than one
modern third-generation
formula (Hoffer Q, Holladay
2, SRK/T, Haigis). Do not use
a regression formula (SRK I
or SRK II) to calculate the IOL
power and choose the highest value for your implant.
10. During the first days
after cataract surgery following RK, patients may experience a significant hyperopic
shift similar to the first post-
operative days following
their RK. This is due to corneal edema. These patients
may also exhibit diurnal fluctuations of refraction during
the early time period after
cataract surgery. No lens exchange should be contemplated until the refraction
has stabilized (1 week to 3
months).
11. Following PRK and
LASIK, early hyperopic shift
or diurnal fluctuations appear to be much less after
cataract surgery. In most
cases, stability of the cornea
makes these cases no different than patients who have
had no previous refractive
corneal surgery.
Future Challenges
In many situations today,
patients may ask for cataract
surgery in centers other than
those where the refractive
procedure has been performed. Thus, neither the
preoperative keratometry
reading nor the exact amount
of refractive correction may
be available.
In these eyes, it might be a
good option to consider the
keratometric diopters of the
anterior and posterior cor-
neal curvature separately after PRK or LASIK. This can
be achieved by means of
scanning slit topography,
although the accuracy of the
Orbscan unit for posterior
corneal curvature measurements has not yet been fully
validated. However, measuring the accuracy of an instrument on posterior curvature is not possible since the
true shape of these surfaces
cannot be referenced.
However, to be on the safe
side, refractive surgeons
might consider giving their
patients a wallet card indicating their preoperative
keratometric reading, preoperative refraction, and postoperative refraction at some
stable time point to allow for
the application of the clinical history method.
An additional approach
to adjust keratometry readings after PRK and LASIK
may be to apply multiple regression analysis comparing
the corrected keratometric
diopters to the measured values and to the spherical
equivalent refraction change
in a large number of cases.
Thus, correction of keratometric diopters after PRK
or LASIK may be realized by
entering the measured value
into an empirical quadratic
regression formula. Certainly, such a theoretical approach must be refined after
clinical data become available from more cataract operations.
At this time, no definite
statement can be made concerning IOL power calculation in patients with intracorneal rings. However, reversing the refractive corneal
effect by timely removal of
the ring before assessment of
keratometric diopters might
be a valid option. This seems
even more reasonable in the
face of the multiple reports
about complete reversibility
of the refractive effect after
ring removal.
The foolproof methodology has not yet been discovered as yet. Since the problem is made more complex
by the fact that there are different types of refractive surgeries, the non-availability of
most of the pre refractive
data, we have to wait for
accurate solutions to be developed – till then; the above
mentioned suggestions may
be followed.
Attention DOS Members
Applications are invited for DOS Fellowship for partial financial
assistance to attend international conference(s). The last date for
receiving application is 31st January 2004.
For details please see page no. 263.
December, 2003
259
DOS Times - Vol.9, No.6
MANAGEMENT PEARLS
Management of Common Intra-Operative
Complications of Phacoemulsification
Sarmi Malik MS, AK Grover MD, FRCS
Phaeoemulsification is a
technically demanding surgery with often a long learning curve. Complications,
especially in the early stages
of the learning curve are not
uncommon, which gradually reduce to make it an extremely safe procedure.
However, one must be aware
of all the possible complications, so as to minimize their
occurrence and to manage
them competently, should
they arise. The complications
of phacoemulsification may
occur at any stage of the procedure.
Problems related to the incision
1) Wound burn
A wound burn is caused
by excess heat energy at the
phaco tip. The vibrations of
the titanium tip, while causing emulsification of the
nucleus, also produce a tremendous amount of heat in
the anterior chamber even
exceeding 100 degrees centigrade. This heat leads to
swelling of the superficial
corneal stroma and thus its
opacification. Some of the
risk factors include hard
nucleus, more phaco power,
a tight wound and insufficient irrigation (too low
flow). Viscoelastic agents
Vision the Eye Clinic,
12/27, West Patel Nagar,
New Delhi-8
December, 2003
may block the tip and lead to
its clogging. Therefore, some
amount of viscoelastic must
be aspirated before starting
the irrigation. Wound burns
lead to high astigmatism and
inadequate wound closure.
Sometimes, they may even
necessitate suturing.
adjacent conjunctiva balloons up, making the procedure more difficult. If the
side port incision is too big,
there will be continuous
leakage. In such cases, it is
advisable to place a suture
and continue after making a
new side port incision.
2) Wound leak
An inadequate corneal
valve is responsible for
wound leak. As mentioned
Descemet’s detachment
Descemet’s detachment
may occur during phacoemulsification by the follow-
Descemets detachment occurs due to
the use of blunt instruments in the
anterior chamber, accidental injection of
viscoelastic under the Descemet’s membrane or attempted insertion of an IOL
through an incision that is too small.
above, a wound burn may
also cause a wound leak. Hydration of the anterior lip of
the incision as advocated by
Fine, may help to close the
incision. In case the wound
still leaks despite hydration,
it is prudent to place sutures.
3) Induced astigmatism
Postoperative astigmatism may be avoided by using either a temporal incision
or an incision place in the
steeper axis of preexisting
astigmatism.
Problems with the side port
If the side port incision is
made too close to the limbus,
it impinges on the iris. The
ing ways. It may be due to
the use of blunt instruments
in the anterior chamber, accidental injection of viscoelastic under the Descemet’s membrane or attempted insertion of an IOL
through an incision that is too
small. The management in
such a case would depend on
the extent of detachment. If
the detachment is small, one
can proceed with the
phacoemulsification, taking
care not to cause further
damage. In case of larger detachments, it can be reattached using air or viscoelastic and depending on the
situation, the surgery may be
postponed or continued as
260
planned. In case of very large
detachments transcorneal
sutures may be required.
Problems with the rhexis
In order to achieve the
perfect rhexis, there has to be
good visibility (good illumination and magnification)
and a chamber filled with
viscoelastic. The patience of
the operating surgeon is of
utmost importance during
this part of the procedure. A
good viscoelastic material
such as Sodium hyduranate
(Healon) is very helpful. Methyl cellulose, if cooled has a
greater viscosity, which
helps in capsulorhexis. The
most common problem during rhexis is extension into
the periphery. This can be
managed by using a fine forceps. The advancing edge is
grasped and pulled sharply
inwards. (Fig. 1 & 2) It can
also be managed by making
a cut in the intended direction of the rhexis, using a fine
scissors. Following this, the
rhexis is completed using
forceps.
Management of a small
pupil
Performing a phacoemulsification procedure in a patient with a small pupil poses
special difficulties during
rhexis and greater chances of
iris damage. Therefore, the
pupil size must be increased
using non-surgical or surgical means. Non-surgical
DOS Times - Vol.9, No.6
MANAGEMENT PEARLS
methods include viscomydriasis (injection of viscoelastics into the anterior chamber), pharmacological
treatment and the use of iris
hooks. Viscoelastics produce
a mechanical mydriasis.
Pharmacological mydriasis is produced by instilling
intracameral preservativefree adrenaline through a
side port incision. It is used
in a concentration of I ml of
1: 1000 that is diluted in 10ml
of BSS. The most popular iris
hooks are De Juan and
Mackool hooks. De Juan
hooks consist of segments in
monofilament nylon threads
that are curved at one end. A
silicone device is fitted at one
end to block the hook at the
desired length. Hooks are
inserted through a paracentesis site and after retracting
the iris, are blocked by the
silicone device. Usually four
hooks are used to create a
square shaped pupil. (Figure
3,4,5) Mackool hooks consist
of titanium and are already
blocked at a preset distance.
Surgical methods of increasing the pupil size include
sphincterotomy, pupilloplasty, midperipheral iridectomy, radial iridectomy and
keyhole iridectomy. The disadvantages of these surgical
maneuvers include bleeding, postoperative inflammation, iris traction and
sometimes the need to leave
permanent ins sutures in
situ.
Iris injury
Iris injury may be due to
performance of the procedure in a patient with a small
pupil, shallowing of the anterior chamber or facing the
bevel of the phacoprobe
December, 2003
downwards. It can be prevented by the following
methods. The phaco procedure must only be carried out
in the central 5-6mm zone,
the rhexis should never be
crossed, the phacoprobe
should always be parallel to
the iris and the bevel of the
probe should be pointed
upwards.
Rupture of the posterior
capsule
Some principles are to be
followed as soon as posterior
capsule rupture is detected.
a) Stop working as soon
as the problem is detected.
b) Maintain a closed
chamber
c) Preserve as much of
the posterior capsule as possible.
Three possible situations
include
1) Rupture without vitreous loss
2) Rupture with vitreous
prolapse
3) Rupture with dislocation of nuclear material into
the vitreous
1) Rupture without vitreous
loss
This occurs most commonly during the stage of irrigation and aspiration.
Once it is detected, the illumination and magnification
of the microscope should be
increased. The anterior chamber is filled with an air
bubble or viscoelastic and
the probe is removed at the
same time in order to maintain the anterior chamber. A
dry aspiration using a oneway cannula is done. The
one-way cannula is attached
to a syringe containing BSS.
In case the rupture occurs at
the stage of early nuclear
phaco, viscoelastic is injected
beneath the nuclear fragment until the fragment
comes into the anterior
chamber. Under cover of viscoelastic, phaco may be done
in the anterior chamber
while using a Lens glide.
Alternatively, the incision
may be enlarged and the
nuclear fragment brought
out.
2) Rupture with vitreous loss
The remnants of the
nucleus are extracted manually with the help of a forceps or gentle phaco after
injecting viscoelastic under
the fragments and coaxing
them into the anterior chamber. Alternatively, the incision may be extended and
the fragments removed using a wire vectis and a
spatula. A large fragment
may be removed using the
sandwich method, in which
a wire vectis is used below
and an iris repository is used
above to take out the entire
fragment.
Depending on the size of
the opening and the amount
of vitreous loss, a vitrectomy
is performed using an anterior or a posterior approach.
At the end of irrigation and
aspiration, an anterior
vitrectomy is done.
3) Rupture with dislocation
of nuclear material into the
vitreous
In such a situation, it is always wise to let an experienced vitreoretinal surgeon
take over. The vitrectomy
performed may be anteriorly
limited if the nuclear material is limited to the anterior
portion.
A
limited
261
vitrectomy may be done depending on the location of
the nuclear material. However, if the nuclear material
is posteriorly located, a total
vitrectomy is done followed
by one of the following options:
a) Perflorocarbon liquids are used to float up the
nucleus and subsequently
remove it. It is very important that the PFCL should be
completely removed at the
end of the procedure. The
disadvantages of using
PFCL besides the high cost
include requirement of a
third port and possible toxic
effects on the retina.
b) A phacofragmenter is
used to fragment the nucleus
within the vitreous and subsequently remove it.
c) The nucleus may be
speared and brought up into
the anterior chamber with
the help of light from an
endoilluminator.
Suggested reading
1) Textbook of Ophthalmology
Volume 3. Eds Sunita
Agarwal, Athiya Agarwal,
David J Apple, Lucio
Buratto, Jorge L Alio, Suresh
K Pandey and Amar
Agarwal
2) Phacoemulsification- Principles and Technique. Lucio
Buratto
3) Phacoemulsification, Laser
Cataract Surgery and Foldable IOLS. Eds Sunita
Agarwal, Athiya Agarwal,
Mahipal Singh Sachdev,
Keiki R Mehta, Howard
Fine, Amar Agarwal
4) Advances in Ophthalmology 1. Ashok Garg, Suresh
K. Pandey, Vidushi Sharma,
David J. Apple.
DOS Times - Vol.9, No.6
“International Symposium on Uveitis and Intraocular Inflammations”
and “3rd Annual Conference of Uveitis Society of India”
February 6 – 8, 2004, Venue : PGIMER, Chandigarh
FACULTY
1. Dr Narsing A Rao, USA
15. Dr D.N. Shah, Nepal
2. Dr Anita Agarwal, USA
16. Dr J Biswas, Chennai
3. Dr Janet Davis, USA
17. Dr S.R. Rathinam, Madurai
4. Dr Carl P Herbort, Switzerland
18. Dr S.P.Garg, New Delhi
5. Dr Marc de Smet, The Netherlands
19. Dr V.S.Sangwan, Hyderabad
6. Dr Tran Van Tao, Switzerland
20. Dr Alay Banker, Ahmedabad
7. Dr Manfred Zierhut, Germany
21. Dr Vinita Rao, Jalna
8. Dr Ilknur Tugal Tutkun, Turkey
22. Dr M.R.Dogra, Chandigarh
9. Dr Manabu Mochizuki, Japan
10. Dr Khalid F Tabbara, Saudi Arabia
11. Dr James E Puklin, USA
12. Dr Ahmed M Abu EI-Asrar,S Arabia
23. Dr Jagat Ram, Chandigarh
24. Dr Sunil Arora, Chandigarh
25. Dr P.Bambery, Chandigarh
26. Dr Vishali Gupta, Chandigarh
13. Dr Luca Cimino Oculista, Italy
27. Dr Amod Gupta,Chandigarh
14. Dr Peizeng.Yang, China
HIGHLIGHTS
Plenary Sessions: VKH Syndrome, Serpiginous Choroditis and Ocular Tuberculosis
Updates: Comprehensive Coverage of Uveitis and Intraocular Inflammation
Current techniques: Diagnosis, Investigations, Pharmacotherapy and surgery
·
Interactive case presentations by International Faculty
·
Free paper Narsing A Rao Award
Free paper submissions are invited from all 100 words abstract may be
e-mailed to the Organising Secretary before 30.12.2003.
Registration Fee:
Delegates
Students
Upto 30.11.2003
Rs 800/Rs 600/-
After 1.12.2003
Rs 1000/Rs 800/-
Spot
Rs 1200/Rs 1000/-
For more details contact:
Dr Vishali Gupta, Organising Secretary, Department of Ophthalmology,
Postgraduate Institute of Medical Education and Research, Chandigarh 160 012.
Telefax: 0172-2747837, E-mail: [email protected]
December, 2003
262
DOS Times - Vol.9, No.6
Delhi Ophthalmological Society Fellowship for Partial
Financial Assistance to Attend Conferences
Conferences
International: Two fellowships per year (two fellowships can
be awarded at a time if committee feels that papers are very
good)
· Maximum of Rs. 25,000/- per fellowship will be sanctioned
National: Three fellowships per year (only for AIOS)
· Maximum of Rs. 5,000/- per fellowship will be sanctioned
Eligibility:
· DOS Life Members (Delhi Members only)
· 75 or More DCRS Points
· Accepted paper for oral presentation, poster, video or instruction course.
Time since last DOS Fellowship:
Preference will be given to member who has not attended
conference in last three years. However if no applicant is found
suitable the fellowship money will be passed on to next year.
Members who has availed DOS fellowship once will not be
eligible for next fellowship for a minimum period of three
years.
Authorship
The fellowship will be given only to presenting author. Presenting author has to obtain certificate from all other co-authors that they are not attending the said conference or not
applying for grant for the same conference. (Preference will
be given to author where other authors are not attending the
same conference). If there is repeatability of same author group
in that case preference will be given to new author or new
group of authors. Preference will also be given to presenter
who is attending the conference for the first time.
Quality of Paper
The applicant has to submit abstract along with full text to
the DOS Fellowship Committee. The committee will review
the paper for its scientific and academic standard. The paper
should be certified by the head of the department / institution, that the work has been carried out in the institution. In
case of individual practitioner he or she should mention the
place of study and give undertaking that work is genuine. The
fellowship committee while scrutinizing the paper may seek
further clarification from the applicant before satisfying itself
about the quality and authenticity of the paper. Only Single
best paper has to be submitted by the applicant for review (6
copies). Quality of the paper will carry 50% weightage while
deciding the final points.
Poster and Video
The applicant will need to submit poster and video for review.
Credit to DOS
The presenter will acknowledge DOS partial financial assistance in the abstract book / proceedings.
The author will present his or her paper in the immediate
next DOS conference and it will be published in DJO/DOS
Times.
December, 2003
Points Awarded
1) Age of the Applicant
Points
a) £ 35 years
10
b) 36 to 45 years
07
c) 45 years plus
05
2) Type of Presentation
a) Instructor/ Co-instructor of Course
12
b) Free Paper (Oral) / Video
07
c) Poster
05
3) Institutional Affiliation
a) Academic Institution
15
b) Private Practitioner
20
4) DCRS Rating in the immediate previous year
a) 75-150
05
b) > 150
08
c) < 75
not eligible for fellowship
Documents
· Proof for age. Date of Birth Certificate
· Original / attested copy of letter of acceptance of paper for
oral presentation / video / poster or instruction course.
· Details of announcement of the conference
· Details of both International & National Conferences attended in previous three years.
· Copy of letter from other national or international agency
/ agencies committing to bear partial cost of conference if
any.
· At least one original document should be provided, that is
ticket, boarding pass or registration certificate along with
attendance certificate of the conference.
· Fellowship Money will be reimbursed only after submission of all the required documents and verified by the committee.
· Undertaking from the applicant stating that above given
information’s are true.
· If found guilty the candidate is liable to be barred for future fellowships.
Dr. J C Das (President DOS), Dr. Gurbax Singh (Vice President DOS), Dr. Kamlesh (Editor) Dr. Lalit Verma (Library Officer),
Dr. Sudipto Pakrasi (Member) and Dr. Jeewan S. Titiyal (Secretary
DOS) will be the members of DOS Fellowship for Partial Financial Assistance to Attend Conferences Committee.
Application should reach Secretary’s office addresses to
President DOS before 31st July and 31st January for international conference and before 30th September for national conference. The committee will meet thrice in a year in the month
of August, October and February with in 2 weeks of last date
of receipt of applications. The committee will reply within four
week of last date of submission in yes/no to the applicant. No
fellowship will be given retrospectively, that means prior sanction of executive will be necessary.
Dr. Jeewan S. Titiyal, Secretary
Delhi Ophthalmological Society
R.No. 476, 4th Floor,
Dr. R.P. Centre for Ophthalmic Sciences
AIIMS, Ansari Nagar, New Delhi – 110029
263
DOS Times - Vol.9, No.6
REVIEW
Ophthalmic Viscosurgical Devices and Anterior Segment Surgery:
Surgical Applications and Complications
Suresh K. Pandey M.D., Jaya Thakur M.D., Liliana Werner MD Ph.D., David J. Apple M.D.
Background
Viscoelastic substances
are solutions with dual
properties; they act as viscous liquids as well as elastic solids or gels. The ideal
viscoelastic substance in
ophthalmology should be
viscous enough to prevent
collapse of the anterior
chamber at rest, yet liquid
enough to be injected precisely through a small cannula. It should be elastic or
shock absorbing and should
enhance coating yet has
minimal surface activity. It
should be cohesive enough
to be removed easily from
the anterior chamber but not
so cohesive that it is aspirated during irrigation and
aspiration, which would
provide no protection to endothelial cells during surgical manipulations. It should
be eliminated from the eye
in the postoperative period
without any effect on intraocular pressure.
Viscosurgery was a term
coined by Balazs to describe
the use of these solutions
that had viscous, elastic and
pseudo plastic properties
during and after surgical
procedures. During viscosurgery, viscoelastic substances are used as a fluid or
a soft surgical instrument.
Moran Eye Center, Department of
Ophthalmology and Visual
Sciences, Fifth Floor, University of
Utah, 50 North Medical Drive Salt
Lake City, Utah-84132, USA.
December, 2003
The viscoelastic sodium hyaluronate was first used in
ophthalmic surgery in 1972,
when it was introduced as a
replacement for vitreous
and aqueous humor. Since
then ophthalmic surgical
procedures had undergone
considerable advancement.
The use of viscoelastic materials has become commonplace in anterior and posterior segment surgeries.
These agents facilitate delicate and often difficult intraocular manipulations
during various ophthalmic
gery using phacoemulsification with IOL implantation. Some of these details
are shown in the schematic
photograph (Figures 1, 2).
Capsulorhexis
In order to perform an intact and successful capsulorhexis, the contents of the
anterior chamber have an
important role. Till date balanced salt solution (BSS®),
air and OVDs have been
used. Out of these three the
best is viscoelastic as it is
considered the easiest, saf-
OVD should be viscous enough to
prevent collapse of the anterior chamber
at rest, yet liquid enough to be injected
precisely through a small cannula. It
should be elastic or shock absorbing
and should enhance coating yet has
minimal surface activity
surgical procedures.
Surgical Application of the
OVDs
In recent years the field of
viscosurgery has broadened
rapidly. It has been used
both intraocularly as well as
extraocularly, which includes cataract, cornea,
glaucoma, viteroretinal,
strabismus and oculoplastic
surgeries.
Use of OVDs in Cataract
Surgery
OVDs are helpful in each
step of modern cataract sur-
est, and the most reproducible method in both routine
and difficult cases (Figures
2A, 2B). To perform a good
capsulorhexis, the viscoelastic to be used should have
the four basic features1- High molecular weight
and high viscosity at zero
shear rate, which maintains the anterior chamber.
2- Excellent visibility provided by high transparency.
3- Make surgical maneuvers
easy, due to high elasticity and pseudoplasticity.
264
4- It should give a good capsular flap control, providing the soft and permanent spatula effect.
Cleavage of lens structure
It is best performed with
the use of OVDs. The ideal
viscoelastic material keeps
the anterior chamber shape
unchanged during BSS® injection and also avoids increase in pressure, which
can be produced with excessive amount of BSS® known
as capsular blockade.
Nuclear emulsification
During phacoemulsification, the viscoelastic is
likely to remain in the anterior chamber instead of leaking out of the eye (Figure
2C). OVDs help in preserving the space and also because of their low cohesiveness, they remain in the anterior chamber despite high
irrigation flow. Moreover
OVDs adhere to the corneal
endothelium, thus protecting the corneal endothelial
cells. Healon® and HealonGV® does not trap the air
bubble and provide excellent endothelial protection
(Figure 2D). This is because
of1- Scavenger effect- This effect captures the free radicals released during
phaco with consequent
inactivation.
2- Binding sites- There are
chemical receptors for
viscoelastic materials on
DOS Times - Vol.9, No.6
REVIEW
the corneal endothelium.
A molecular bond seems
to occur between the viscoelastic solution and the
corneal endothelium.
3- High Elasticity- This also
smoothes the possible impacts of the lens material
against the endothelium.
The phaco tip being in a
closed system, its vibrations
are transmitted to the internal structures of the eye but
viscoelastic provides a
smothering shield against
them.
Fig.1A: Injection of the viscoadaptive OVD in the anterior chamber through a 25 G cannula.
Irrigation and aspiration
The role of viscoelastic
during this procedure is the
protection of the endothelium. This is possible due to
high adhesiveness. It remains where it is placed,
without mixing with the cortex because of its low cohesiveness thus helping in easy
removal of cortex.
Fig.1B: Capsulorhexis is in progress.
Capsular bag filling and
IOL implantation
During IOL implantation,
it is necessary to expand the
capsular bag with a viscoelastic. It allows the surgeon to keep the bag well
opened and formed thus allowing the easy IOL implantation. OVD is also helpful
in correct positioning, centering and allowing for possible IOL rotation maneuvers (Figs. 1E, 1F). Beside
posterior chamber IOL implantation, OVD has also
been used for implantation
of other IOL designs (e.g.
anterior chamber, iris fixated, artisan lenses, etc.) (Fig.
2).
Cataract Surgery in Pediatric Cases
Pediatric cataract surgery
December, 2003
Fig.1C: Phacoemulsification in progress.
like the adult surgery has
undergone major changes in
recent years with the evolution of techniques including
small incision and the development of modern IOLs
(Wilson ME, Pandey SK,
Werner L, Apple DJ. Pediatric cataract surgery: Past,
present and future, Third
Prize for “Special Interest”,
Annual Video Festival, XX th
Congress of the European
Society of Cataract and Refractive Surgeons, Nice,
France, September 2002).
The main principle lies in
the control of the very elas-
265
tic nature of ocular tissues.
It is difficult to perform a
good capsulorhexis in the
presence of high capsular
elasticity. Moreover there is
low scleral rigidity, greater
intravitreal pressure that
makes the capsulorhexis
even more difficult, as the
pressure tends to curve the
capsulorhexis. But with the
use of viscoelastic, e.g.
Healon-GV ® the effective
push is in the opposite direction and hence completion
of capsulorhexis can be
done.
In pediatric cases, the
capsulorhexis must be
started in the central portion
and not towards the equator,
in order to prevent radial
extension. The high density
viscoelastic agents stabilize
the posterior chamber and
push back the vitreous face
during the posterior capsulorhexis. During IOL implantation, the capsular bag
is kept open and the anterior
chamber is well formed thus
ensuring easy and safe implantation of the IOL in the
bag. These agents also help
to dilate the pupil thus
maintaining a good intraoperative mydriasis.
OVDs like Healon-GV®
can easily be removed at the
end of the surgery including
the position which is behind
the IOL due to its high cohesiveness thus preventing
capsular blockade.
Use of the OVDs in Glaucoma Surgery
Viscocanalostomy
Viscocanalostomy is a
new surgical procedure for
glaucoma therapy. Viscoelastics play an important
role in this procedure. Figure 3 illustrates the surgical
DOS Times - Vol.9, No.6
REVIEW
Fig.1 D: Viscoadaptive OVD is transparent and easy to see during removal (left). Note the presence of the air bubbles within the anterior chamber after use of dispersive viscoelastic solution
(right).
Fig.1 E: Implantation of a posterior chamber intraocular lens in
the capsular bag.
vented and also the related
discomfort with it. It minimizes the risk of late infections and is independent
from conjuntival and episcleral scarring.
Viscocanalostomy allows
the aqueous to leave the eye,
through Schlemn’s canal
and episcleral veins thus restoring the natural outflow
pathway. This procedure
creates a bypass by which
aqueous humor reaches
Schlemn’s canal, skipping
the trabecular meshwork. A
chamber is produced inside
the sclera, which is in direct
communication with the
Schlemn’s canal. There is
also a communication through the Descemet’s membrane with the anterior
chamber.
The OVDs should have
high pseudoplasticity to allow injection into Schlemn’s
canal through a small needle
and should have high viscosity at shear rate of zero
to maintain the spaces as
long as possible. HealonGV ® and Healon-5 ® are
viscoelastics of choice for
this procedure.
OVDs for Intraocular Delivery of Dyes or Anesthetic
Agents
Researchers and vision
scientists have been using
OVDs as a vehicle to deliver
capsular dyes for use during
cataract surgery. Mixing
these substances with the
viscoelastic agent was attempted to prolong their effect and to limit the adverse
effect on ocular tissues. Ciba
Vision Corp. (Duluth, GA,
USA), has recently proposed
mixing an OVD with
lidocaine.
This was termed “viscoa-
Fig.1 F: Removal of the viscoadaptive OVD using irrigation-aspiration tip.
steps of viscocanalosotomy.
Viscocanalostomy literally
means “opening of the canal
by means of viscoelastic substance”. This procedure is a
non-penetrating and independent from external filtration. The advantages are
December, 2003
decreased risk of infection,
and decreased incidence of
cataract, hypotony and flat
anterior chamber as the anterior chamber is not opened, and moreover, with the
absence of external filtration
the bleb formation is pre-
Fig. 2: Beside posterior chamber IOL fixation in the capsular bag,
OVDs can also be used for implantation of the various phakic and
aphakic IOL designs in the anterior chamber, ciliary sulcus etc.
Use of the OVD facilitated the implantation of the Artisan® IOL as
shown in this photograph. (Courtesy: Camil Budo, M.D.).
266
DOS Times - Vol.9, No.6
REVIEW
Fig.3. Surgical steps of viscocanalosotomy. (Courtesy: Dr. Med. Tobias Neuhann, M.D., Munich, Germany).
Fig.3.A: Deep block construction incision.
Fig.3.B: Cutting the deep block in a single
plane with a spoon blade.
Fig.3.C: Proximal to Schlemm’s canal
there is a subtle change in the scleral fibers, from a crossing pattern to a tangential pattern, with an increased opacity.
Fig.3.D: Descemet’s window.
Fig.3.E: Cannulating Schlemm’s canal
with three puffs of viscoelastic directed at
the osteum.
Fig.3.F: Tight closure suture of the flap.
nesthesia” and was intended
to prolong the anesthetic effect
of
intracameral
lidocaine, as a complement
to topical anesthesia. Also,
the steps of intracameral injection of OVDs and of
intracameral injection of
lidocaine, as a complement
to topical anesthesia, would
be combined in only one step.
Removal of the OVDs
Several techniques have
been reported in the literature for removal of the
OVDs. These include: Rock
and roll technique, twocompartment technique and
bimanual irrigation/aspiration technique.
We would like to emphasize that a careful and thorDecember, 2003
ough removal of the OVDs
from the capsular bag and
the anterior chamber of the
eye is must after the end of
the surgery. This is important to avoid complications
such as rise in intraocular
pressure, crystallization of
the IOL surface. Studies
have shown that complete
removal of viscoelastic material from the capsular bag
can be more difficult when
some hydrophobic acrylic
lenses are used because of
the IOL’s tacky surfaces
(Apple DJ, Auffarth GU,
Pandey SK. Miyake posterior view video analysis of
dispersive and cohesive
viscoelastics, video presented at the Symposium on
Cataract, IOL, and Refrac-
tive Surgery, Seattle, WA,
April 1999).
Complications of OVDs:
OVDs have many positive attributes but their
drawbacks and complications must be given careful
considerations. Some of the
important complications are
as follows1. Increase in intraocular
pressure
Increase in intraocular
pressure is the most important postoperative complication of OVDs. It was first
267
noted with Healonâ . The increase in pressure can be severe and prolonged, if the
material is not thoroughly
removed at the end of the
surgery. The rise in pressure
occurs in the first 6 to 24
hours and resolves spontaneously within 72 hours
postoperatively. The rise in
pressure is due to the mechanical resistance of the trabecular meshwork to the
large molecules of the viscoelastic material, which decreases the outflow facility.
Hence to decrease the incidence of this complication,
Increase in intraocular pressure is the
most important postoperative
complication of OVDs.
DOS Times - Vol.9, No.6
REVIEW
many have advocated removing and aspirating the
viscoelastic material from
the eyes at the end of the surgery.
2. Capsular block syndrome or capsular bag
distension syndrome
Capsular block syndrome
(CBS), is a newly described
complication of cataract-IOL
surgery. It is characterized
by accumulation of a liquefied substance within a
closed chamber inside the
capsular bag, formed because the lens nucleus or the
posterior chamber IOL optic
occluded the anterior capsular opening created by the
capsulorhexis. Depending
on the time of onset, CBS is
classified as intraoperative
(CBS seen at the time of lens
luxation following hydrodissection), early postoperative (originally described
CBS), and late postoperative
(CBS with liquefied aftercataract or lacteocrumenasia).
Recently use of high-density viscoelastic agents, such
as Healon-GV ®, has been
found to be associated with
complication of late CBS.
Main ingredient of the transparent liquid in capsular
bags is sodium hyaluronate
and that the distention is
caused by aqueous humor
being drawn into the capsular bag by an osmotic gradient across the capsule when
the capsulorhexis diameter
is smaller than that of the PC
IOL and by viscoelastic material retained and trapped
in the bag intraoperatively.
Suggested Reading:
1. Arshinoff SA, Opalinski YAV,
Ma J. The pharmacology of
lens surgery: ophthalmic viscoelastic agents. In: Yanoff M,
Ducker JS, eds, Ophthalmology.
St Louis, Mosby-Yearbook,
1998; 4:20.1-21.6
2. Pandey SK, Thakur J, Werner
L, Sharma V, Izak AM, Apple
DJ. Ophthalmic viscosurgical
devices: An update. In: Garg
A, Pandey SK, Sharma V,
Apple DJ, eds., Advances in
Ophthalmology. Jaypee Brothers, New Delhi, India 2003, (in
press).
3. Liesegang TJ. Viscoelastics.
Surv Ophthalmol 1990; 34:268293
4. Pandey SK, Werner L, Apple
DJ, et al. Dye-enhanced pediatric cataract surgery. J Pediatr
Ophthalmol Strabismus 2003 (in
press).
5. Ram J, Pandey SK. Anesthesia for cataract surgery. In:
Dutta LC. Modern Ophthalmol-
ogy. Jaypee Brothers, New
Delhi, India, 2000, PP 325-330.
437-440, 378-384.
6. Saini JS, Pandey SK. Advances
in techniques of penetrating
keratoplasty. In: Nema HV,
Nema N, eds., Recent Advances
in Ophthalmology, Volume IV,
Jaypee Brothers, New Delhi,
India, 1998, pp 37-51
7. Pandey SK, Thakur J, Werner
L, Izak AM, Apple DJ. Classification, clinical applications
and complications of ophthalmic viscosurgical devices:
An update. In: Garg A, Pandey
SK, eds., Textbook of Ocular
Therapeutics. Jaypee Brothers,
New Delhi, India 2002, PP:
392-407
8. Pandey SK, Wilson ME, Apple
DJ, Werner L, Ram J. Childhood cataract surgical technique, complications and
management. In: Garg A,
Pandey SK, eds. Textbook of
Ocular Therapeutics. Jaypee
Brothers, New Delhi, India
2002, PP 457-486.
REQUIRED
In Glaucoma
Full time
Ophthalmologist
for a upcoming
Eye Hospital
in
East Delhi
Perimetry: Basics, single field and followup.
Interpretation, analysis and application in
day to day clinical situations .
Newer imaging techniques : basics and
interpretation : Utility in diagnosis and
followup
Groups of Ten Each
On Sunday :
December 14, 2003
February 8, 2004
Contact:
Dr. Devindra Sood
Glaucoma Imaging Centre
P-13, South Extension Part II
New Delhi 110049
Tel: (011 ) 26257803 : 26252000
Contact : Dr. Sanjay Bajaj
A-1/1, Nathu Colony Chowk,
Shahdara, Delhi – 110093
Ph : 9810623636, 9810015012
December, 2003
268
DOS Times - Vol.9, No.6
REVIEW
Macular Function Tests
Neena Kumar MD, Raj Vardhan Azad FRCS(Ed)
Macular function tests are
required for diagnosing as
well as for following up of
macular diseases and for
evaluating the potential
macular function in eyes
with opaque media such as
cataract and dense vitreous
hemorrhage. The retinal
function testing can be divided into psychophysical
and physiologic methods. A
psychophysical test is subjective. A physical stimulus is
presented to the patient and
the patient indicates verbally
or by other subjective means
his detection of the stimulus.
Physiologic methods are objective. A stimulus is presented and a response parameter is measured by electrophysiological or other
means.
Psychophysical tests
1. Visual acuity
2. Pupillary reactions
3. Photostress test
4. Amsler grid
5. Two point discrimination
test
6. Entoptic phenomenon
7. Tests dependent on macular pigment
8. Maddox rod test
9. Color vision
10. Foveal flicker sensitivity
11. Grating psychophysics
12. Dark adaptation
13. Perimetry
Electrophysiologic tests
a. Electroretinography
(ERG)
Dr. R.P.Centre for Ophthalmic
Sciences, AIIMS,
New Delhi.
December, 2003
b. Electrooculography
(EOG)
c. Visually evoked response (VER)
Psychophysical Tests
Visual Acuity
Visual acuity measurement is the most common
test of foveal function. The
best-corrected visual acuity
is a measure of actual foveolar function. Visual acuity is
measured by the visual resolution of a letter, symbol or a
pattern. The smaller the visual angle subtended by the
parts of the test letter correctly seen by the observer,
the better the resolution of
his visual apparatus. Another factor, which affects the
letter subtends 5 minutes of
visual angle on the retina.
Each component of this letter subtends 1 minute at this
distance.
Other methods of visual
acuity testing in cases of clear
ocular media are Landolt C
chart, grating acuity and
Optokinetic Nystagmus (in
children). Laser Interferometry and Potential Visual
acuity Meter (PAM) are used
to measure visual acuity, if
media is opaque due to cataract, vitreous hemorrhage
etc.
Laser Interferometry
Laser Interferometry can
be used in eyes with immature cataracts.1 The resolving
power of the macula is tested
by using two coherent beams
of light, which create a three
dimensional fringe pattern
on the retina. The beams produce two point sources be-
Laser interferometry over-predicts the
visual potential in amblyopic eyes
because laser fringe vision is better
than the letter acuity.
visual acuity, is contrast.
Contrast is defined as the
difference in the luminance
between dark and light parts
of the test area divided by the
mean luminance.
The Snellen chart
The Snellen score is expressed as a fraction. The
numerator is the test distance
and the denominator is determined by the size of the
smallest letter correctly identified. The numerical value
of the denominator is the
distance at which the height
of this smallest identified test
hind the lens opacity; the
light waves emitted from
these two points overlap.
Where the crest of one wave
overlaps the trough of the
other, the effect is cancelled
and a black band is produced. Where crests or
troughs coincide with one
another, the enhancement
produces bright bands of
light. The test is performed
as follows:
1. The pupils are widely
dilated.
2. The light beam is directed into the center of the
pupil in the plane of the iris.
269
3. The pupil is scanned
until the fringe pattern is
seen.
4. The patient is asked to
indicate the orientation of the
bands of light.
5. Initially, large gratings are used and then they
are gradually diminished
until the patient is unable to
detect their correct orientation.
6. The potential visual
acuity is estimated from the
width of the gratings.
Laser generated fringes
are not dependent on the optical components of the eye
for focusing. Therefore,
ametropia has little influence
on the patterns produced by
retina. It also over predicts
the visual potential in
amblyopic eyes because laser fringe vision is better than
the letter acuity.
The Potential Visual Acuity Meter (PAM)
PAM projects standard
Snellen chart through a small
clear area of an immature
cataract; it is most accurate
in eyes with visual acuities
of 6/60 or better.2,3 The main
components of the PAM are
an incandescent light source,
a miniature transilluminated
Snellen chart and a +12 D
lens. In performing the test
the pupils should be widely
dilated and the patient is
asked to read the letters on
the chart.
Pupillary reactions
The pupillary reactions
should be normal even in the
presence of a mature cataract. The defect usually indicates either a lesion of the
optic nerve or extensive retinal disease.
DOS Times - Vol.9, No.6
REVIEW
Photostress Test
Photostress test can differentiate visual loss caused by
macular disease from that
caused by an optic nerve lesion.4 The test is a gross version of the dark adaptation
test in which the visual pigments are bleached by light.
This causes a temporary state
of retinal insensitivity, which
is perceived by the patient as
a scotoma. The recovery of
vision is dependent on the
ability of the photoreceptors
to re-synthesize visual pigments. The test is performed
as follows:
1. The best-corrected
distance visual acuity is determined.
2. The patient fixates the
light of a pen torch or an indirect ophthalmoscope held
about 3 cm away for about
10 seconds.
3. The photostress recovery time (PSRT) is measured
by the time taken to read any
three letters of the pre-test
acuity line.
4. The test is performed
on the other, presumably
normal eye and the results
are compared.
In a patient with macular
lesion the PSRT will be
longer (50 seconds or longer)
as compared with the normal
eye whereas in a patient with
an optic nerve lesion there
will be no difference.
The Amsler grid
The Amsler grid test
evaluates the 10 0 of visual
field surrounding fixation.
The chart is composed of a
grid of lines containing a central black fixation spot. The
squares on the grid are 5 mm
in size and subtend a visual
angle of 10 at 30 cm viewing
distance.
December, 2003
The chart is to be viewed
in modest light monocularly
at a distance of 28-30 cm utilizing the correct refraction
for this distance. Viewing
should be accomplished
without previous ophthalmoscopy and without instillation of any drugs affecting
papillary size or accommodation. The test is performed
as follows:
1. The patient should
wear reading spectacles and
cover one eye.
2. The patient is asked to
look directly at the center dot
with the uncovered eye and
report any distortion, wavy
lines, blurred areas or blank
spots anywhere on the grid.
Two Point Discrimination
Test
The ability to distinguish
two illuminated points of
light suggests good retinal
function. Two illuminated
points of 2 mm diameter size
and 2 inches apart are placed
2 feet away from the patient’s
eye. The patient is then asked
to indicate whether he can
perceive the two points separately.
Entoptic Phenomenon
Entoptic phenomenon is
referred to visual perceptions that are produced or influenced by the native structures of one’s own eye.
Illumination of the fundus by
parallel light rays allows visualization of small opacities
located close to the retina.
Since the columns of blood
contained within retinal
blood vessels are linear
opacities situated in front of
the retinal photoreceptors,
this makes retinal blood vessels visible. If a focal source
of light (such as small pen-
light) is pressed firmly
against the exterior of the eye
through closed lids, the arborizing pattern of retinal
blood vessels can be made
briefly visible. This test is
used as test of retinal function.
The blue field Entoptic Phenomenon (Flying spots)
If one looks at a bright and
diffusely illuminated surface
with no contrasting features,
a series of fast moving, luminous points or spots can be
readily seen. The spots tend
to move in a generally
curved pattern, while trailing short, tapering segments
behind them. The spots are
best seen if the background
is illuminated by blue light
in the spectral region of 350
to 450nm. Since this region
contains the spectral absorption peak of hemoglobin, it
has been suggested that the
moving particles represent
red blood cells passing
through the retinal capillaries.5 In a normal subject 15 or
more of moving corpuscles
are seen. Abnormal Blue
Field Entoptic phenomenon
is constituted by the findings
of
(a) failure to see any corpuscles
(b) partial loss of corpuscles in one part of the
field
(c) visibility of less number of corpuscles
(d) slow corpuscular
movement
Tests dependent on the
Macular Pigment
Two tests are believed to
depend on the arrangement
of the yellow pigment in the
inner retinal layers of the
macula lutea. These are the
270
Haidinger’s brushes and the
Maxwell spot.
Haidinger’s Brushes
This test is commonly
used as a screening test for
retinal pathology in strabismus patients with amblyopia. If one views a diffusedly
illuminated source of planepolarized white or blue light,
brushes or sheaves radiating
from the point of fixation in
the form of Maltese cross can
be seen. The brushes have
contrasting yellow and blue
hues. The darker portions of
the Maltese pattern are yellow, whereas the brighter
portions are blue. This phenomenon is caused by variations in absorption of planepolarized light by oriented
molecules of xanthophyll
pigment in the foveal retina.
If the yellow pigment arrangement in the fovea is disrupted by pathology in the
inner retinal layers, the
brushes will not be seen.
Maxwell’s Spot
A subject with a normal
macula will see a 3- degree
dark ring surrounding fixation upon looking through a
blue filter at a brightly illuminated white surfaced.
Usually a smaller dark spot
is seen at the fixation point
itself. The dark ring and the
spot will fade as the eye
adapts to the blue light.
These can be restored by readapting to a yellow light; i.e.
by having the subject look
through a yellow filter for a
moment. Following this, the
blue filter will again elicit the
dark ring and spot.
Some visual physiologists
believe that perception of the
Maxwell’s Spot depends on
the screening of the photoreDOS Times - Vol.9, No.6
REVIEW
ceptors by the macular pigment while others believe
that variations in the relative
numbers of different types of
color receptors in the foveal
region explain this phenomenon.
The Maddox Rod Test
The Maddox rod consists
of a series of fused cylindrical red glass rods, which convert the appearance of a
white spot of light into a red
streak. The optical properties
of rods cause the streak of
light to be at an angle of 900
with the long axis of the rod;
when the glass rods are held
horizontally, the streak will
be vertical and vice-versa.
The rods are placed in front
of the right eye. This dissociates the two eyes because the
red streak seen by the right
eye cannot be fused with the
unaltered white spot seen by
the left eye. The amount of
dissociation is measured by
the superimposition of the
two images using prisms.
The base of the prism is
placed in the position opposite to the direction of the deviation. Both vertical and
horizontal deviations can be
measured in this way.
Color Vision
The central 30-60 degree
of the visual field processes
the trichromatic color vision.
Hereditary dystrophies of
the posterior pole, non-hereditary maculopathies and
certain optic nerve conditions often result in acquired
color defects.
As a rule, congenital
trichromatic color deficiencies are not particularly rare,
affect males, are symmetric,
involve red-green color system, and occur as isolated
December, 2003
visual defect. By contrast acquired dyschromatopsias
are asymmetric, are accompanied by other visual dysfunctions and most commonly show irregularity in
color testing not usually seen
in the congenital variety e.g.
acquired color defects are the
blue-yellow dyschromatopsia of central serous
chorioretinopathy and retinitis pigmentosa, or the
red-green defect of acquired
cone degeneration or optic
neuritis. Tests for color vision
include color confusion test
and in the clinical sphere include the Ishihara test plates,
the Farnsworth Penal D-15,
the American Optical
Hardy-Rand-Rittler pseudoisochromatic
plates
defective into protan ,
deutan, tritan (blue blind)
axes of confusion. Mild color
defectives score normally.
3. AOHRR Plates: The
AOHRR plates have colored
symbols and a gray background. The hues of the color
symbols occur at the neutral
points of color defective patients. The saturation of the
test symbol colors is progressively increased in successive plates. The colorblind
patient has difficulty discerning the test symbols
from the gray background.
This test is useful for screening and categorization of
red-green and blue-yellow
defectives in terms of severity of the defect and the axis
of confusion. It cannot differ-
Nagel Anamoloscope is considered to be
the best method for accurate
classification of red-green defects.
(AOHRR), and the Sloan
Achromatopsia Test.
Color Confusion Tests
1. Ishihara Plates: This
test is based on color confusion in the red-green colorblind patient. The color of the
test symbol and the background is of such hue, saturation and the intensity so as
to be confused by protans
(red blind) and deutans
(green blind).
2. Farnsworth Panel D-15:
This test is also based on
zones of color confusion. The
subject is asked to arrange a
series of 15 colored caps such
that each succeeding cap
appears most similar in appearance to the previous cap.
The D-15 Panel classifies the
moderate and severe color
entiate a dichromat (a patient
with only two cone pigment)
from an anomalous trichromat (one with three cone pigments).
4. Sloan Achromatopsia
Test: This test presents to the
observer a series of highly
saturated colors which he/
she is asked to match to a series of gray shades. The achromat, or totally colorblind
patient, makes characteristic
matches while a dichromat
or trichromat cannot make
any match.
Hue discrimination test
Hue (wavelength) discrimination for the color defective and normal is best at
spectral zones of intrinsic
desideration and poorest at
zones of best saturation
271
(Pitt’s law). The results are
diagnostic for the type of
dyschromatopsia. The color
blind subject exhibits characteristic spectral regions of
poor hue discrimination
which differ from the normal
subject since the intrinsic
saturation for the color defective differs from the normal.
Farnsworth-Munsell 100
Hue test
The subject arranges 85
colored caps (4 separate
boxes each containing about
20 caps) such that each succeeding cap is closest in hue
to the preceding cap. The FM 100 Hue test is based entirely on hue discrimination
and not on color confusion
pairs as the caps are balanced
for saturation and intensity.
The test rates the degree of
color discrimination in normal subjects in addition to
measuring zones of poor hue
discrimination in color
defectives. Specific zones of
poor hue discrimination
identify the nature of the dyschromatopsia.
Nagel Anomaloscope
This test is based on color
matching. It is considered to
be the best method for accurate classification of redgreen defects. The subject
must match a standard yellow light in color and brightness with a mixture of red
and green. Normal subjects
give a narrow matching
range, the so-called Rayleigh
match. Characteristic matching patterns are elicited for
the different types of redgreen dyschromatopsias and
achromats.
DOS Times - Vol.9, No.6
REVIEW
Foveal Flicker Sensitivity
There is a variation of
foveal sensitivity to flicker as
a function of temporal frequency. A small flickering
test light is superimposed on
a constant background luminance (fig.1). The luminance
of the flickering test light is
sinusoidally modulated; i.e.
it increases and decreases in
a sinusoidal fashion above
and below the mean luminance resulting in maximum
and minimum peak luminance. The mean luminance
of the test light is equal to that
of the surround. The test spot
is usually 0.5 – 2.0 degrees in
size for foveal flicker sensitivity.
For each temporal frequency value (cycles per second or Hz), the threshold is
the minimum modulation
depth at which detection of
flicker occurs. The reciprocal of the threshold is the sensitivity. The falloffs in sensitivity were found at high
temporal flicker frequencies
in certain retinal disorders,
even when visual acuity was
normal.6
Grating Psychophysics
Traditionally gratings are
composed of alternating discrete dark and light bars.
These bars have a changing
luminance described by a
square wave function. In order to explore spatial processing by the visual system,
gratings have been introduced with bar luminance
constantly changing to follow a sine wave function.
Discrete edges are not seen
in such gratings.
A grating detection test is
performed by having the patient fix on the center of a sinusoidal grating, preferably
small enough to fall within
the macular region. The
mean luminance of the grating is held constant at all
times. The pattern is alternated at a regular rate with a
homogenous screen of the
same mean luminance. The
patient indicates when he
can or cannot see the on – off
bar pattern. This test has
been applied to visual problems resulting from cerebral
and retinal pathology.7
The bar width at the cutoff frequency correlates
closely with the component
width of the smallest Snellen
letter correctly identified.
However, in patients with
cerebral lesions involving
the visual pathways and
with the presumed maculopathy of retinitis pigmentosa, the curve is no longer
predictable from the visual
acuity measurement i.e. different types of contrast sen-
sitivity loss can be seen in
different patients with the
same Snellen acuity score.
Dark Adaptation
Dark adaptation refers to
the ability of visual system
both rods and cones to recover sensitivity following
exposure to light. Dark adaptation is tested by
Goldmann – Weekers adaptometer (fig.2).
The eye is exposed to a
standard light bleach (preadaptation for 15 minutes). This
precedes the plotting of the
dark adaptation curve. Fol-
lowing the bleach, the patient fixates on a test light
located 12-15degrees eccentric to the stimulus light
which itself can be varied in
diameter. This retinal location is in an area of good
cone-rod mix. The intensity
of the test light is slowly increased from below threshold until the patient detects
it. The minimal intensity for
detection is the threshold at
this point in time. By taking
a threshold reading every
minute a curve of the changing threshold versus time of
dark adaptation is obtained.
Highlights for January Issue of DOS Times
Ø Problems in RD Surgery
: Dr. C.M. Shroff
Ø Ultrasound Biomicroscopy in Glaucoma
: Dr. Tanuj Dada
Ø Phaconit: Current Prespective
: Dr. Amar Agarwal
Ø Complication & Management of Contact Lens
: Dr. J.S. Saini
Ø Focus on AMRD
December, 2003
272
DOS Times - Vol.9, No.6
REVIEW
a generalized pathologic
process and in such cases peripheral field may also show
abnormalities.
The curve is normally
biphasic with a “rod-cone
break” at about 10 minutes.
The early part of the curve
represents cone adaptation
and the later part of the curve
represents the rod adaptation. With regard to the
macular area this test can be
modified so that foveal function can be studied.
December, 2003
Perimetry
Perimetry can also test the
retinal function. Central or
paracentral scotomas are
elicited in posterior pole disease and the plotting of several isopters by variation of
test object; color, size and intensity help to better characterize field defects. Macular
disease is sometimes part of
Electrophysiological Tests
1. E l e c t r o r e t i n o g r a p h y
(ERG)
The clinical electroretinogram is the recording of the
electrical potential waveform generated by the total
(preganglionic) retina in response to a diffuse light
stimulus (fig.3). The test is
performed using a corneal
contact lens electrode which
records changes in the
corneo-retinal potential with
each light stimulus. 8 The
clinical important components are the initial corneal
negative a-wave and the second positive b-wave. The awave is believed to emanate
from the inner segments of
the photoreceptors and the
b-wave from Muller cells in
the bipolar region. The peak
amplitudes and latencies as
well as waveform shape are
considered in the interpretation of the ERG. Since the
ERG monitors preganglionic
retinal activity, a patient due
to optic atrophy may have a
normal ERG.
ERG is a mass retinal response; an isolated lesion of
273
the macula would not be expected to affect this test.
2. E l e c t r o o c u l o g r a p h y
(EOG)
The EOG measures the
changes in the corneoretinal
potential of the eye under
varying conditions of illumination. To record this , two
electrodes are placed on the
skin, one at the lateral canthus and the other at the medial canthus. The subject is
asked to shift fixation back
and forth between two red
lights that turn on and off
sequentially in an alternating fashion. The angular distance between the lights and
consequently the eye movement excursion is generally
30 degrees. Since the
corneoretinal potential is
positive with regard to the
cornea, the EOG recording is
positive when the patient’s
cornea (gaze) is turned toward the canthus with the
recording electrode. The
record is negative when the
eye turns to the opposite direction (toward the reference
electrode). Several measurements are taken every
minute for a total of 15 minutes in darkness and then 15
minutes in the light. A plot
of the average amplitude
DOS Times - Vol.9, No.6
REVIEW
value for each minute
against time normally shows
a minimum trough value
during the dark period and
a maximum peak value in the
light (fig.4).
The dark trough of the
standing potential is believed to depend on the integrity of the pigment epithelial outer segment region.
The light rise depends on the
function of the mid-retinal
layers in addition to the outer
retina-pigment epithelial
complex. The absolute value
of the corneoretinal potential
is variable, the light peak/
dark trough ratio is found to
be a reliable parameter of
retinal function and the normal lower range is 170% (calculated by taking the ratio of
the maximum light adapted
to the minimum dark
adapted response and multiplying it by 100). This ratio
is known as Arden ratio.
EOG is a reflection of the
generalized retinal responsiveness. Therefore, it is abnormal in most of those conditions in which ERG is ab-
normal.9 There are, however
several clinical conditions in
which this does not hold true
which include Best’s
Vitelliform macular dystrophy, Butterfly dystrophy,
fundus flavimaculatus and
generalized drusen. In these
conditions ERG is normal
but EOG is abnormal.
3. Visually Evoked Response (VER)
The visually evoked response is a measure of the
electrical potential generated in response to a visual
stimulus. It is recorded with
scalp electrodes placed over
the occipital lobe region, a
cortical area with primarily
a macular representation.10
There are many different
techniques of foveal stimulation in VER testing. In one
method diffuse light stimulus flashes intermittently.
This method is most useful
in the assessment of suspected monocular pathology. In second method retina
is stimulated by a patterned
stimulus. One such pattern
represents alternating dark
and light bars in the form of
a sinusoidal grating. The
light bars become dark and
the dark bars become light
with the pattern reversing at
a constant rate. A third technique employs a checkerboard pattern stimulator
with alternation of dark and
light checks.
The usefulness of the VER
in detecting lesions of the
macula must be inferred by
the fact that the ultimate
record reflects neural transmission along the entire visual pathway from retina to
occipital lobe. Therefore, abnormality anywhere along
the system may result in abnormal VER.
References
1. Cohen M.M.: Laser interferometry: evaluation of potential visual acuity in the presence of cataracts. Ann. Ophthalmol 1976; 8: 845-849.
2. Minkowski J.S. and Guyton
D.L.: Potential acuity meter using a minute aerial pinhole aperture (poster abstract). Ophthalmology 1981; 88(9S): 95.
3. Guyton D.L.: Instuments for
measuring retinal visual acuity behind cataracts. Ophthalmology 1982; 89(8S): 98-103.
4. Severin S., Harper J., Culver
J.: Photostress test for the
evaluation of macular function. Arch. Ophthalmol. Otolaryngol. 1975; 79: 701.
5. Loebl M. and Riva C.E.: Macular circulation and the flying
corpuscles phenomenon. Ophthalmology 1978; 85: 911-917.
6. Tyler C.W.: Flicker frequency
response functions in the diagnosis of retinal disorders.
Presented at ARVO meeting.
Sarasorta Fla. 1976.
7. Bodis-Wollner I., Diamond S.:
The measurement of spatial
contrast sensitivity in cases of
blurred vision associated with
cerebral lesion. Brain 1976;
99:695.
8. Lawwill T.: The bar pattern
electroretinogram for clinical
evaluation of the central
retina. Am. J. Ophthalmol. 1974;
78: 121.
9. Francois J., Verriest G.,
deRouck A.: Electrooculography as a functional test in
pathological conditions of the
fundus. I. First results. Br. J.
Ophthalmol. 1956; 40: 108.
10. Sokol S.: Visually evoked potentials: theory, techniques
and clinical applications. Surv.
Ophthalmol. 1976; 21; 18.
Congratulations!
Ø Dr. V.K. Malik, Eye Surgeon, Distt. Hospital, Meerut has got the appreciation award
from Govt. of U.P. for doing highest number of I.O.L. Surgeries, year 2002-03.
Ø Dr. Pradeep Sharma, Additional Professor, R.P. Centre has been recently awarded the
following:
w
w
w
Awarded International Membership of American Association of Pediatric Ophthalmology and
Strabismus.
Awarded Retina Foundation Medal Oration by the Gujarat Ophthalmological Society, 11th October, 2003.
Awarded Rustumji Ranji Medal Oration by the Andhra Pradesh State Ophthalmological Society, 18th October, 2003.
Ø Dr. Rajiv Mohan, Joint Director & Consultant Mohan Eye Institute, on Being Awarded
FRCS (Ophthalmology) and Invited as Examiner at Glasgow by the Royal College of
Physicians & Surgeons of Glasgow
December, 2003
274
DOS Times - Vol.9, No.6
APPLIANCES
Ocular Ultrasonography
Atul Kumar MD, Nikhil Pal MD,
Sanjeev Nainiwal MD,
Raj Vardhan Azad MD, FRCS (Ed)
History
Developed in the late
1950s and early 1960s, diagnostic ophthalmic ultrasonography remains a critical ancillary test for the clinical evaluation of the opaque
media globe and abnormal
orbit.Ultrasound was first
used in ocular diagnosis in
1956 by Mundt and Hughes,
who employed the A-scan
technique. Their work was
refined in the late 1950s and
early 1960s by Oksala and coworkers from Finland, who
did extensive work on the
detection and differentiation
of various ocular disorders,
also using the A-scan
method. At about the same
time, Baum and Greenwood
developed the first two-dimensional B-scan for use in
ophthalmology, using the
immersion technique. Further pioneering work using
the immersion B-scan method was performed by Edward Purnel and Jackson
Coleman and co-workers.
The first commercially available contact B-scanner, in
which the handheld probe
was applied directly to the
closed lid, was introduced by
Nathaniel Bronson in 1972.
Indications for Echography
Indications for ophthalmic ultrasound (i.e.,
echography) have steadily
increased as the capabilities
Dr. R.P.Centre for Ophthalmic
Sciences, AIIMS,
New Delhi.
December, 2003
and accuracy of the equipment and methods have improved (see box). Echography is indicated whenever
complete or partial opacification of the media prevents
an adequate clinical examination of either the anterior
or posterior segment. It has
also proved to be very useful in clear media for the differentiation and measurement of intraocular tumors
and inflammatory lesions.
Before beginning any examination, the echographer
should be informed of the
of diseases, including primary and secondary tumors
of the orbit, inflammatory
diseases, and changes secondary to thyroid disease.
Physics And Instrumentation
Ultrasound is an acoustic
wave that consists of an oscillation of particles. By definition, ultrasound waves
have a frequency greater
than 20 kHz (i.e., 20,000 oscillations per second), which
renders them inaudible. For
diagnostic ophthalmic ultra-
Echography is indicated whenever
complete or partial opacification of the
media prevents an adequate clinical
examination of either the anterior or
posterior segment.
pertinent history and clinical findings.
Biometry is another important contribution of ultrasound in ophthalmology.
Reliable intraocular lens
(IOL) calculations depend on
accurate and precise axial
length measurements. Measurements of the globe have
also become important in
evaluating conditions such
as congenital glaucoma, microphthalmia and nanophthalmos, persistent hyperplasia of primary vitreous
(PHPV), phthisis bulbi, and
myopia. Orbital ultrasonography is most useful in evaluation of patients with exophthalmos related to a number
sound, frequencies used are
generally in the range 8 to 10
MHz (1 MHz = 10G cps). The
very high frequencies used
in ophthalmology produce
short wavelengths, in the
range of 0.2 mm. These very
short wavelengths allow sufficient resolution of the
minute structures in the eye
and orbit. In contrast, abdominal and obstetric ultrasound require frequencies in
the range 1 to 5 MHz, which
provide the longer wavelengths that are necessary for
deeper penetration of soft
tissue.The speed at which
ultrasound travels depends
on the medium through
which it passes. For example,
275
aqueous and vitreous conduct sound at a velocity of
1532 m/sec, intraocular and
orbital soft tissue at a velocity of 1550 m/sec, and the
normal lens at 1641 m/sec.
As the ultrasound passes
through tissues, part of the
wave may be reflected back
toward the probe; this reflected wave is referred to as
an echo. Echoes are produced by acoustic interfaces
that are created at the junction of media with different
sound velocities.The greater
the difference in sound velocity of the media that create the acoustic interface, the
stronger the echo. For example, the lens (velocity =
1641 m/sec) produces a
stronger echo when adjacent
to aqueous (velocity = 1532
m/sec) as opposed to blood
(velocity = 1550 m/sec), as
in a hyphema.
The returning echoes are
affected by many factors, including the size and shape
of acoustic interfaces, the
angle of sound beam incidence, absorption, scattering, and refraction. An understanding of these principles is important for the
performance of accurate ultrasound examinations.
Clinical echography depends on technology that
emits an ultrasound wave
and then detects and processes the returning echoes.
The basis of the system is the
piezo-electric material (e.g.,
quartz crystal) that is located
near the tip of the transducer
(i.e., probe). (Fig 1) This
transducer emits pulses of
ultrasound waves (i.e., the
sound beam) and then receives the returning echoes.
The detected echoes are processed in the instrument
(e.g., filtered and amplified)
DOS Times - Vol.9, No.6
APPLIANCES
and are then displayed on the
screen as echograms. This
signal processing differs
from one instrument to another and is critical in determining the character of the
echogram. The ultrasound
instruments used most commonly in ophthalmology are
the A-scan and the B-scan.
A-scan
A-scan is a one-dimensional acoustic display in
which echoes are represented as vertical spikes from
a baseline. Spacing of the
spikes depends on the time
it takes for the sound beam
to reach a given interface and
for its echo to return to the
probe. The time between any
two echo spikes can then be
converted into distance by
knowing the sound velocity
of the media from which the
echoes are received. The
height of the displayed
spikes indicates the strength
(i.e., amplitude) of the echoes (Fig. 2).
B-scan
B-scan differs from Ascan in that it produces a twodimensional acoustic section
(such as a photograph) by
using both the vertical and
horizontal dimensions of the
screen to indicate configuration and location. A section
of tissue is examined by an
oscillating transducer that
emits a sound beam that
“slices” through a tissue,
much like a slice with a knife.
Ophthalmic B-scan instruments require a focused
beam that functions at a frequency in the range of 10
MHz. An echo is represented
as a dot on the screen rather
than as a spike. The strength
of the echo is depicted by the
brightness of the dot. The
December, 2003
OPAQUE OCULAR MEDIA
Anterior segment
Ø Corneal opacification
Ø Anterior chamber hyphema or hypopyon
Ø Miosis or pupillary membrane
Ø Cataract
Posterior segment
Ø Vitreous hemorrhage or inflammation
CLEAR OCULAR MEDIA
Anterior segment
Ø Iris lesions
Ø Ciliary body lesions
Posterior segment
Ø Tumors and masses: detection and differentiation
Ø Retinal detachment: rhegmatogenous vs. exudative
Ø Intraocular foreign bodies: detection and localization
Ø Optic disc abnormalities
FOLLOW-UP STUDIES
BIOMETRY
Ø Axial eye length
Ø Anterior chamber depth
Ø Lens thickness
Ø Tumor measurement
Indication of orbital USG
Ø Orbital pseudo tumour
Ø Orbital lymphoma
Ø Cavernous hemangioma
Ø Orbital lymphangioma
Ø Orbital abscess
Ø EOM evaluation
Ø Thyroid ophthalmopathy Vs Myositis
Ø Optic nerve evaluation-Optic disc coloboma
coalescence of multiple dots
on the screen forms a twodimensional representation
of the examined tissue section (Fig. 3).
How to get best image
l Beam must fall perpendicular to the area of interest
l Lesion must be centered
to the beam path
l Select lowest gain compatible with clear image
l Place the probe through
open eye lid (preferable)
l Avoid scanning through
lens
Examination technique
l Brief clinical history
l Set the instrument(Globe1,Orbit-5)
l Supine/recline position
l Close the eye lid
l Apply methyl cellulose
l Give fixation target
l Probe in desired direction
& position
l Begin the examination
l Start with med – high gain
276
-(70-80db)
l Select lowest gain compatible with clear image
Types of B-Scan (Globe)
q Fundamental (Fig 4a)
l Axial(Fig 4b)
l Longitudinal(Fig 4c)
l Transverse(Fig 4d)
q Special
l Topographic
l Kinetic
l Quantitative
Special examination technique
q Topographic echography
q Quantitative echography
q Kinetic echography
Examination Techniques
Specific examination
techniques have been carefully designed to allow thorough evaluation of the anterior and posterior segments
of the globe. The type of examination performed is determined by the indication
for examination. The contact
method (i.e., the probe is
applied directly to the globe)
is used to evaluate the posterior segment. In those cases
in which the anterior segment also needs evaluation,
an easy immersion technique
has been developed that utilizes the same contact instrument
Basic screening and special examination techniques
for lesion differentiation (topographic, quantitative, and
kinetic echography) have
been developed for the
evaluation of posterior segment disorders.
Positioning the patient
If separate A-scan and
contact B-scan instruments
are used, they may be placed
on one large or two small
carts. The patient is seated in
a reclining examining chair
of adjustable height. While
DOS Times - Vol.9, No.6
APPLIANCES
most examinations are performed with the patient reclined, it is occasionally helpful to have the patient in a
sitting position.
The basic screening examination is performed to
detect lesions of the posterior
segment. It is best to use both
A-scan and B-scan for lesion
detection, but it is suggested
that the novice begin with Ascan; this is to allow the new
echographer to master the Ascan examination techniques
in the normal eye, a prerequisite to using the standardized A-scan for the more difficult differentiation of lesions. Furthermore, the
small, pencil-like A-scan
probe allows thorough
screening of the peripheral
fundus, including the ciliary
body.
Special examination techniques
Topographic echography—
shape, location, and extension
As soon as a lesion is detected, topographic echography is performed to determine location, general classification, and specific configuration. B-scan is ideally
suited for the initial topographic evaluation since the
moving, focused sound
beam provides a two-dimensional display. It is also important, however, to appreciate a lesion’s topography
with the A-scan in order to
carry out subsequent special
examination techniques (i.e.,
quantitative and kinetic). It
is essential that scanning
techniques be systematically
performed with both B-scan
and A-scan if reliable results
are to be achieved
The probe face is always
represented by the initial line
December, 2003
In the axial scan, the
on the left side of the
patient fixates in priechogram. The fundus,
mary gaze and the
located on the side of
probe is placed on the
the globe opposite the
center of the cornea,
probe, is represented
thus displaying lens
on the right side of the
and optic nerve in the
echogram. The upper
center of the echogram.
part of the echogram
The axial scan does not
corresponds to the poryield as much information of the globe where
tion and thus is perthe probe marker is di- Fig.1: Parts of ultrasonography machine
formed at the end of the
rected. The center of
topographic examinathe screen corresponds
tion.
to the central portion of
the probe face. Since
Transverse scans
this provides the best
With transverse scans,
resolution, a lesion
the probe is placed on
should always be centhe globe with the longtered within the
est diameter of the oval
echogram.
probe face positioned
In practice, the
parallel (i.e., tangenechographer applies
tial) to the limbus. In
methylcellulose to the
Fig.2. Normal standardized A-scan
this way, the back-andprobe face as a couechograms. I, Initial spike corresponding
forth movement of the
pling medium. The B- to probe tip (corneal spike hidden by initransducer also occurs
scan probe is then tial spike); A, anterior lens; P, posterior
parallel to the limbus.
placed directly on the lens; M, multiple signal; V, echo-free vitreThe sound beam then
globe (i.e., conjunctiva ous cavity; R, retina; S, sclera; O, orbital
oscillates back and
or cornea). Examina- soft tissue.
forth across the oppotion through the lids is
site fundus, producing
generally avoided bea circumferential slice.
cause of sound attenuThis orientation is apation by the lids. In adpropriate for showing
dition, with the lids
the lateral extent of a leclosed,
the
sion (e.g., extending
echographer can never
from 2- to 4-o’clock mebe certain which porridians, from 7- to 9:30tion of the globe is bemeridians, etc.)
ing evaluated. The parThe designation of the
ticular section of ocutransverse scan is delar tissue displayed on
Fig.3: Normal standardized B-scan
termined by the meridthe screen depends on echograms
ian that lies in the
how the probe is posimiddle of the scanning
tioned and how the
marker is directed. The three cornea. Thus the sound beam section. For example, if the
basic probe orientations that bypasses the lens, allowing probe is held horizontally
are used to evaluate in- better sound penetration. with its face centered on the
traocular lesions are trans- These scans are performed 6-o’clock meridian, the
verse, longitudinal, and with the patient’s gaze di- middle of the echogram (on
rected away from the probe, the right) will display the 12axial.
The transverse and longi- toward the meridian being o’clock meridian of the funtudinal scans are used most examined. This allows a dus; this probe position is
commonly because the wide surface of globe on called a transverse scan of the
probe is placed on the con- which to place and shift the 12-o’clock meridian. If the
probe is placed in a vertical
junctiva peripheral to the probe.
277
DOS Times - Vol.9, No.6
APPLIANCES
Fig.4a: Schematic diagram of
fundamental USG scan (H:
Horizontal scan V: Vertical
scan O: Oblique scan)
Fig.4b: Axial scan
Fig.4c: Longitudanal scan
Fig.4d: Transverse scan
orientation at the 3-o’clock
limbus, the sound beam
sweeps across the 9-o’clock
meridian; this is called a
transverse scan of the 9o’clock meridian.
By convention, horizontal
transverse scans (i.e., transverse scans of the 12- or 6o’clock meridians) are performed with the marker oriented toward the nose.
Therefore the upper part of
the echogram always represents the nasal portion of the
globe. On the other hand,
vertical transverse scans (i.e.,
3- or 9-o’clock meridians) are
performed with the marker
directed superiorly, so the
top of the echogram represents the upper portion of the
globe. Oblique transverse
scans (transverse scans of the
1:30, 4:30, 7:30, or 10:30-meridians) are performed with
the marker directed toward
the upper portion of the
globe.
parallel) to the limbus. The
sound beam sweeps along
the meridian opposite the
probe rather than across the
meridian as does the transverse scan. In this way, the
longitudinal scan shows the
anteroposterior extent of a lesion rather than the lateral
extent Another way to think
of this orientation is as a radial section (like the spoke
of a wheel) with the sound
is being examined. For example, if the probe is placed
on the 6-o’clock meridian,
the sound beam sweeps
along the 12-o’clock meridian; this is designated as a
longitudinal scan of the 12o’clock meridian (Fig 4c).
The use of the longitudinal
probe orientation greatly facilitates three-dimensional
thinking and promotes a better understanding of B-scan
Axial scans
The axial orientation is the
third probe position. This is
performed with the probe
face centered on the cornea;
the sound beam is directed
through the center of the lens
and the optic nerve . As mentioned previously, this scan
is
the
easiest
for
echographers to understand
because it displays the lens
and optic nerve in the center
of the echogram, thus simplifying orientation of the
pathologic condition. Unfortunately, however, sound
attenuation and refraction
from the lens often hinder
resolution of the posterior
portion of the globe, thus limiting the usefulness of this
scan (Fig. 4b). It is helpful,
though, for documenting lesions and membranes in relation to the optic nerve and
is also useful for evaluating
the macular region.
Longitudinal scans
Longitudinal scans are
also performed with the
probe placed peripheral to
the limbus. The probe is rotated 90 degrees from the
position of the transverse
scan so that the longest diameter of the oval probe face
is perpendicular (rather than
December, 2003
B-scan differs from A-scan in that it
produces a two-dimensional acoustic
section (such as a photograph) by using
both the vertical and horizontal dimensions of the screen to indicate configuration and location.
beam sweeping from the
optic disc out toward the
periphery along a given meridian. In longitudinal scans,
the marker is always directed toward the center of
the cornea, regardless of
which meridian is being examined. This produces an
echogram with the optic disc
and posterior fundus displayed on the lower portion
of the screen, whereas the peripheral globe is displayed
superiorly. The designation
of the longitudinal scan is
simply that meridian which
findings. It is often the optimal method of displaying
the posterior and peripheral
insertion of a membrane, as
well as the posterior and anterior borders of tumors. This
technique is especially helpful for evaluating those
membranes that insert into
the optic disc at steep angles
or when an anterior structure
(e.g., lens or lens implant)
produces strong sound attenuation (precluding optimal visualization with an
axial scan).
278
Topographic B-scanning
If a lesion is detected with
either the A- or B-scan basic
screening examination, topographic evaluation of the
lesion is then performed. The
lesion is first assessed with
the appropriate transverse
approach. For example, if a
lesion is detected in the superior temporal quadrant of
DOS Times - Vol.9, No.6
APPLIANCES
the right eye, a transverse
scan of the 10:30 meridian is
performed (the probe is
placed at 4:30 and the marker
is oriented superonasally).
The probe is then shifted
from limbus to fornix, thus
sweeping the sound beam
through the lesion from posterior to anterior. This allows
display of the lesion’s gross
shape and dimensions, as
well as lateral extent (i.e.,
which meridians are affected
by the lesion). The longitudinal approach is then applied, with the sound beam
oriented radially, perpendicular to the transverse
view. For a lesion located at
the 10:30 position in the right
eye, the probe face is placed
on the 4:30 meridian with the
marker directed toward the
center of the cornea. The
lesion’s anteroposterior extent (between the optic disc
and ora along the 10:30 meridian) is assessed, and its
shape and gross dimensions
are reevaluated.
apart). These various A-scan
maneuvers help to classify
the general type of lesion
(pointlike, bandlike, membranelike, and masslike) and
to evaluate exact location
and extent.
Topographic A-scanning
Once the involved meridians have been generally
determined with B-scan, Ascan is also applied. Lesion
topography is always evaluated at the tissue sensitivity
setting. The probe is first
placed at the limbus of the
meridian opposite the lesion’s center. It is then shifted
between limbus and fornix
to assess the lesion anteroposteriorly (i.e., radially).
The probe is then shifted
from side to side (parallel to
the limbus) to evaluate the
lesion laterally. Another
helpful technique is to examine the lesion from different
sound beam directions (i.e.,
with the probe placed in positions that are 90 degrees
Quantitative echography—
type II
This technique is used
solely to differentiate RD
from dense vitreous membranes. If a membrane like
lesion produces a 100% tall
spike at tissue sensitivity
during quantitative type I
and its other acoustic characteristics are equivocal,
quantitative type II may be
applied.
December, 2003
Quantitative echography—
type I
Quantitative echography
type I is used to estimate the
reflectivity (i.e., spike height)
of all detected lesions. Once
the sound beam is directed
perpendicular to a lesion, the
amplitude (i.e., height) of its
spike is observed in the
echogram. This simple technique is of great value in differentiating many types of
intraocular lesions, such as
dense posterior vitreous detachment (PVD) from retinal
detachment (RD), and melanoma from other intraocular
tumors. The determination
of reflectivity is necessary for
evaluation of a mass lesion’s
internal structure and sound
attenuation.
Kinetic echography
Kinetic echography is
used to dynamically assess
the motion of or within a
lesion.Two types of motion
(i.e., aftermovement and vascularity) can be detected
with the appropriate instrumentation. Aftermovement
indicates mobility as determined by motion of the lesion echoes following cessation of eye movement. For
example, a nonsolid lesion
(e.g., PVD or RD) displays
aftermovement, whereas a
solid lesion (e.g., tumor) does
not. Vascularity, spontaneous motion of echoes on the
screen, is indicative of blood
flow within vessels.
Anterior segment evaluation: immersion technique
Indications for anterior
segment evaluation are limited because this area is optically visible in the vast majority of cases. However,
when echographic data are
needed, they may be reliably
obtained with a simple immersion technique. This is
accomplished by inserting a
small scleral shell between
the lids. The shell is then
filled with methylcellulose.
Using this standoff technique, the cornea, anterior
chamber, iris, lens, and retrolental (or retroiridal) space
can be evaluated along the
ocular axis with B- and Ascan Immersion axial length
measurements can also be
obtained with A-scan. More
peripherally located, anterior structures (i.e., iris, anterior chamber angle, or ciliary body) can also be assessed by shifting the shell
and probe laterally over the
area to be displayed.
Recently, Pavlin and colleagues have developed a
new B-scan method for
evaluating the anterior segment. This new technique
utilizes frequencies in the
range of 50 to 100 MHz and
is referred to as Ultrasound
Biomicroscopy (UBM). The
method provides very high
resolution for examining an-
279
terior segment structures
and lesions in ciliary body region. UBM provides much
higher resolution of 25-50microns than does conventional
B-scan ultrasonography (150
microns). The clinical applications of UBM are in
glaucoma(open
versus
closed angle closure glaucoma, plateau iris syndrome), iridocorneal scars,
IOL status, post-trauma and
can be used to study pars
plana sclerotomy ports.
Role of Optical Coherence
Tomography
Optical Coherence Tomography (OCT) is a new
imaging modality the principles of which are similar to
B-mode ultrasound, that
produces high resolution
(upto 10 microns) cross sectional images of the retina
.However, OCT utilizes the
reflection of a superluminescent diode source of light
with a wavelength of 830nm
rather than sound waves
from different structures of
the eye and has applications
in macular hole, macular
edema and ARMD.
Suggested Reading
1. Mundt GH, Hughes WF: Ultrasonics in ocular diagnosis. Am J
Ophthalmol 41:488, 1956
2. Purnell EW: Intensity modulated (B-scan) ultrasonography. In
Goldberg RE, Sarin LK (eds), Ultrasonics in Ophthalmology: Diagnostic
and Therapeutic Applications, pp 102123. Philadelphia, Saunders, 1967
3. Dallow RL (ed): Ophthalmic
ultrasonography: Comparative
techniques. Int Ophthalmol Clin 19:4,
1979
4. Ossoinig KC: Quantitative
echography-the basis of tissue differentiation. J Clin Ultrasound 2:33,
1974
5. Coleman DJ: Reliability of ocular and orbital diagnosis with Bscan ultrasound. Am J Ophthalmol
74:708, 1972
DOS Times - Vol.9, No.6
DOS QUIZ NO. 6
DOS QUIZ NO. 6
1.
Most common lesion involving anterior segment of eye in AIDS..................................................................
2.
Most common symptomatic metastatic uveal tumors ....................................................................................
3.
Dilator pupillae orginates from which embryonal layer ................................................................................
4.
Most common cause of bull’s eye maculopathy ..............................................................................................
5.
Cherry red spot disappears after injury by.......................................................................................................
6.
Economic blindness is called when snellen acuity falls below ....................................................................
7.
Epithelium of canaliculus is lined by ................................................................................................................
8.
Which laser is used in IOL master .....................................................................................................................
9.
Magnification caused by direct ophthalmoscope ............................................................................................
10.
Most common systemic disease associated with necrotizing scleritis .........................................................
Rules:
l Please send your entries to the DOS office latest by 25th December, 2003.
l Prize Rs. 500/- Courtesy: Syntho Pharmaceuticals
l Quiz Trophy will be given to the member who answers maximum number of quizes in a
year during the Annual GBM of DOS.
Answers for the DOS Quiz No. 4
1. Most common lid tumour in India
Sebaceous Cell Carcinoma
2. Epidemic keratoconjuctivitis is caused by
Adenovirus 8, 19
3. Stocker line is seen in
Pterygium
4. Which Corneal dystrophy is having systemic association
Lattice II dystrophy
5. Foldable IOL was invented by
Tom Mozzoco
6. Inverse glaucoma is seen in
Sphero Phakia
7. "Headlight in the Fog" appearance of fundus is seen in
Toxoplasmosis
8. Harada – Ito procedure is indicated in
Pure Exclylotropia in Superior oblique palsy
9. Pulfrich Phenomonen is seen in
Optic Neuropathy
10. Which acid & alkaji injury is most dangerous
NH3OH & HF
December, 2003
280
DOS Times - Vol.9, No.6
JOURNAL ABSTRACTS
Comparison of scleral buckling with
combined scleral buckling and pars
plana vitrectomy in the management of
rhegmatogenous retinal detachment with
unseen retinal breaks
Tewari HK, Kedar S, Kumar A, Garg SP, Verma LK.
Clin Experiment Ophthalmol. 2003 Oct; 31(5): 403-7.
Dr Rajendra Prasad Center for Ophthalmic Sciences, AIIMS,
New Delhi, India.
Authors compared conventional scleral buckling and combined pars plana vitrectomy and scleral buckling procedures
in rhegmatogenous retinal detachments with unseen retinal
breaks. Forty-four consecutive eyes with uncomplicated, primary rhegmatogenous retinal detachments with a clear media and unseen retinal breaks were randomized to two groups.
The scleral buckling group underwent 360 degrees scleral
buckling, cryopexy and external subretinal fluid drainage. In
the combined surgery group, 360 degrees scleral buckling, pars
plana vitrectomy, air-fluid exchange, endolaser and injection
of 14% perfluoropropane gas was done. Results showed at 3
months follow up the primary reattachment rate was 80% (16/
20 cases) in the combined surgery group, and 70% (14/20 cases)
in the scleral buckling group (P = 0.716). The visual acuity
improved significantly from a preoperative median of hand
movement (HM; range: HM to 6/60; similar in both the
groups), to a median of 6/60 (range: perception of light to 6/
18) in the combined surgery group and a median of 6/36
(range: HM to 6/18) in the scleral buckling group, the difference between the two groups not being statistically significant (P = 0.4). The number of intraoperative and postoperative complications was more in the combined surgery group.
(Four cases were lost to follow up and were doing well when
last examined.) They conclude the conventional scleral buckling was found to be a safe and effective technique in the primary management of uncomplicated, rhegmatogenous retinal detachments with unseen retinal breaks when the media
is clear.
Pars plana vitrectomy, intraocular gas,
and radial neurotomy in ischaemic central retinal vein occlusion
TH Williamson, W Poon, L Whitefield, N Strothoudis and
P Jaycock
British Journal of Ophthalmology 2003;87:1126-1129
Department of Ophthalmology, St Thomas’s Hospital, London
SE1 7EH, and Department of Ophthalmology, Queen Mary’s
Hospital, Sidcup, Kent, UK
In this study pars plana vitrectomy (PPV), mild panretinal
photocoagulation, and intraocular gas injection were employed
to prevent NVG. The potential role of incision of the lamina
cribrosa (radial neurotomy) for visual recovery was examined.
Eight eyes of seven patients with ischaemic CRVO had PPV,
mild panretinal photocoagulation, and intraocular
December, 2003
perfluoropropane gas injection. Four eyes had radial neurotomies performed. The patients were examined by fundus photography, fundus fluorescein angiography, optical coherence
tomography, and Goldmann visual field analysis.
No patients suffered from neovascular glaucoma. Visual recovery was seen in patients with and without neurotomy but
some patients had cataract extraction to allow visualisation
for PPV. Fundus photography demonstrated reduced engorgement of retinal veins in two of the patients with neurotomy
and one with PPV alone. Optical coherence tomography demonstrated macular oedema in three patients with neurotomy
and all patients with PPV alone. Segmental visual field loss
was seen in one patient with neurotomy suggesting damage
to the optic nerve head.
PPV is safe in ischaemic CRVO. Combined with mild PRP
and intraocular gas injection the risk of neovascular glaucoma
is low. Neurotomy can be added to try to improve the chances
of recovery of central vision but may cause additional peripheral visual field loss.
Therapeutic contact lenses: the role of
high-Dk lenses.
Foulks GN, Harvey T, Raj CV.
Ophthalmol Clin North Am. 2003 Sep;16(3):455-61.
Department of Ophthalmology, University of Pittsburgh School
of Medicine
Currently, the armamentarium of contact lenses that can
be used for therapeutic effect provides a wider selection of
lenses than ever before. If the therapeutic goal is protection
and healing of the corneal epithelium, epithelial or stromal
edema is best avoided, and the selection of a high-Dk silicone
hydrogel (balafilcon A, lotrafilcon A) lens or a very thin membrane-type lens (crofilcon) is the best choice. If the goal is surface protection as well as stimulation of stromal wound vascularization, selection of a low-water content, thick, hydrophilic lens is the better option. If the patient is prone to lens
loss or requires frequent replacement of the therapeutic lens,
a prudent economic decision is to select a daily disposable
moderate-water content lens. Specific circumstances may mandate the selection of a specific therapeutic lens. Patients with a
prior history of active giant papillary conjunctivitis may be
better served by the use of a crofilcon glyceryl methacrylate
lens, which has a lower incidence of this complication. Patients
who have dry eye may benefit from a higher-water content
lens if adequate unpreserved tear supplementation is provided
with or without punctal occlusion. The options when selecting a therapeutic contact lens are wider than ever before. Although the new generation of high-Dk lenses promises fewer
limiting problems of vascularization and infection, one can
use the older traditional therapeutic lenses when induced vascularization of the cornea is needed or when an economic necessity exists. Not all of the available lenses are FDA approved
for therapeutic use, and such wear is an off-label use. The patient should be informed of the goal of therapy as well as the
benefits and risks of therapeutic contact lenses.
281
DOS Times - Vol.9, No.6
EVENTS
Forthcoming Events – NATIONAL
———————————————————————————————————————————————————
Event Conference
Date
Venue
Contact Person and Address
———————————————————————————————————————————————————
7th Annual Conference of
Punjab Opthalmological Society
13th-14th
Dec. 2003
Akal Eye Hospital & LASIK Contact Person: Dr. Balbir Singh Bhaura (M.S.)
Laser Centre,
Akal Eye Hospital & Lasik Laser Centre,
Jalandhar
Model Town, Jalandhar - 144 003
Telefax: 0181-2273606, 2271606, 2461606, 5073604
E-mail: [email protected]
Conference E-mail: [email protected]
62nd All India
Ophthalmological Conference
8-11
Jan. 2004
Banaras Hindu
University, Varanasi
Conference Secretariat: Prof. V. Thakur
Nataraj Eye & Laser Centre, 156B, Ravindrapuri,
Varanasi - 221 005, India
Phone: 0542-2276505, 09415201167
Fax: 0542-2276707
Email: [email protected]
XI International Congress of
Ocular Oncology
23-27th
Jan. 2004
L.V. Prasad Eye
Institute, Hyderabad
Contact Person: Dr. Santosh G. Honavar,
ICOO Secretariat, LV Prasad Eye Institute,
LV Prasad Marg, Banjara Hills, Hyderabad
Tel.:+91-40-23548267, e-mail: [email protected]
12th Annual Meeting Vitreo
Retinal Society of India
20-22nd
Feb. 2004
Corbett Claridges
Hideaway, Ramnagar
Uttaranchal
Contact Person: Mr. Shobhit Chawla,
Organising Secretary, Prakash Netra Kendra,
NH 2, Vipul Khand-4, Gomtinagar,
Lucknow (U.P.)
Annual DOS
Conference
3rd-4th
April 2004
India Habitat Centre
Lodhi Road, New Delhi
Contact Person: Dr. Jeewan S. Titiyal,
Secretariat (DOS) R.No. 476, 4th Floor,
Dr. R.P. Centre for Opthalmic Sciences,
New Delhi - 110 029 Ph.: 26589549,
Fax : 26588919, E-mail: [email protected]
Website: http://www.dosonline.org
INTERNATIONAL
Event Conference
Date
Venue
Contact Person and Address
———————————————————————————————————————————————————
Euro Asian Opthalmology Congress 11-15
Dec. 2003
Shanghai, China
8th ESCRS Winter Refractive
Surgery Meeting
23-25
Jan. 2004
Barcelona
International Symposium on
Ocular Pharmacology and
11-14
Mar. 2004
MONTE CARLO
ASCRS Annual Symposium
1-5
May 2004
18-22
Sept. 2004
SAN DIEGO, CA USA
XXII Congress of the ESCRS
December, 2003
PARIS, FRANCE
282
Contact: Euro Asian Congress Secretariat
Tel.: 86-2163-031-757, Fax: 86-2163-029-643
E-mail: [email protected]
Contact: ESCRS Temple House, Temple Road,
Blackrock, Co Dublin, Ireland.
Tel.: 3531-209-1100 Fax: 3531-209-1112
E-mail: [email protected]
Contact: Iliana Eliar, Assistant Project Manager,
Kenes International Global Congress Organizers
& Association Management Services
E-mail: <[email protected]>
Contact: ASCRS Tel.: 1703-591-2220
Fax: 1703 591 0614, Web: www.ascrs.org
Temple House, Temple Road, Blackrock,
Co Dublin, Ireland
Tel.: 3531-209-1100 Fax: 3531-209-1112
E-mail: [email protected]
DOS Times - Vol.9, No.6
DELHI OPHTHALMOLOGICAL SOCIETY
Stamp Size
2 Colour
Photograph
(LIFE MEMBERSHIP FORM)
Name (In Block Letters) _________________________________________________________________________
S/D/W/o _____________________________________________________________ Date of Birth _____________
Qualifications _________________________________________________________ Registration No. __________
Sub Speciality (if any) ___________________________________________________________________________
ADDRESS
Clinic/Hospital/Practice ______________________________________________________________________
_______________________________________________________________ Phone _________________
Residence ________________________________________________________________________________
_______________________________________________________________ Phone _________________
Correspondence ___________________________________________________________________________
_______________________________________________________________ Phone _________________
Email ___________________________________________________________ Fax No. ________________
Proposed by
Dr. _______________________________ Member Ship No. ______________ Signature _________________
Seconded by
Dr. ________________________________ Membership No. ______________ Signature _________________
[Must submit a photocopy of the MBBS/MD/DO Certificate for our records.]
I agree to become a life member of the Delhi Ophthalmological Society and shall abide by the Rules and Regulations of the Society.
(Please Note : Life membership fee Rs. 3100/- payable by DD for outstation members. Local Cheques acceptable,
payable to Delhi Ophthalmological Society)
Please find enclosed Rs.____________in words ______________________________________________________ by
Cheque/DD No.______________________ Dated____________ Drawn on_____________________________________
Three specimen signatures for I.D. Card.
Signature of Applicant
with Date
FOR OFFICIAL USE ONLY
Dr._______________________________________________________________has been admitted as Life Member of
the Delhi Ophthalmological Society by the General Body in their meeting held on________________________________
His/her membership No. is _______________. Fee received by Cheque/DD No._______________ dated__________
drawn on __________________________________________________________________.
(Secretary DOS)
December, 2003
283
DOS Times - Vol.9, No.6
INSTRUCTIONS
1. The Society reserves all rights to accepts or reject the application.
2. No reasons shall be given for any application rejected by the Society.
3. No application for membership will be accepted unless it is complete in all respects and accompanied by a Demand Draft of Rs. 3100/- in
favour of “Delhi Ophthalmological Society” payable at New Delhi.
4. Every new member is entitled to received Society’s Bulletin (DOS Times) and Annual proceedings of the Society free.
5. Every new member will initially be admitted provisionally and shall be deemed to have become a full member only after formal ratification
by the General Body and issue of Ratification order by the Society. Only then he or she will be eligible to vote, or apply for any Fellowship
propose or contest for any election of the Society.
6. Application for the membership along with the Bank Draft for the membership fee should be addressed to Dr. Jeewan S. Titiyal, Secretary,
Delhi Ophthalmological Society, R.No. 476, 4th Floor, Dr. R.P. Centre for Ophthalmic Sciences, AIIMS, Ansari Nagar, New Delhi – 110029.
7. Licence Size Coloured Photograph is to be pasted on the form in the space provided and two Stamp/ Licences Size Coloured photographs
are required to be sent along with this form for issue of Laminated Photo Identity Card (to be issued only after the Membership ratification).
DOS Credit Rating System Report Card
DCRS July 2003 – Army Hospital (R&R)
Total No. of Delegates ....................................................................................................................................................................... 121
Delegates from Out side (N) ............................................................................................................................................................. 114
Delegates from Army Hospital (n) ........................................................................................................................................................7
Overall assessment by outside delegates (M) ............................................................................................................................ 888.5
Assessment of case presentation-I (Dr. Lt. Col.A. Banarji) by outside delegates .................................................................... 803.5
Assessment of case presentation-II (Dr. Lt. Col. (Mrs.) Madhu Bhaduria) by outside delegates ............................................ 814.5
Assessment of clinical talk (Dr. D.P. Vats) by outside delegates ................................................................................................ 862.5
DCRS 30th August, 2003 – Sir Ganga Ram Hospital
Total No. of Delegates ................................................................................................................................................................. 82
Delegates from Out side (N) ....................................................................................................................................................... 66
Delegates from Sir Ganga Ram Hospital (n) ............................................................................................................................... 16
Overall assessment by outside delegates (M) ...................................................................................................................... 468.5
Assessment of case presentation-I (Dr. Jasmita Popli) by outside delegates ..................................................................... 440.5
Assessment of case presentation-II (Dr. Anita Sethi) by outside delegates ........................................................................ 476.5
Assessment of Clinical Talk (Dr. S.N. Jha) by outside delegates .................................................................................... …..450.0
DCRS 27th September, 2003 – Hindu Rao Hospital
Total No. of Delegates ................................................................................................................................................................. 70
Delegates from Out side (N) ....................................................................................................................................................... 59
Delegates from Hindu Rao Hospital (n) ...................................................................................................................................... 11
Overall assessment by outside delegates (M) ......................................................................................................................... 432
Assessment of Case Presentation-I (Dr. Ruchi Goel) by outside delegates ............................................................................ 414
Assessment of Case Presentation-II (Dr. A.K. Nagpaul) by outside delegates ................................................................... 401.5
Assessment of Clinical Talk (Dr. Ruchi Goel) by outside delegates .................................................................................... ..433.5
DCRS 1st November, 2003 – Dr. R.P. Centre for Ophthalmic Sciences
Total No. of Delegates ................................................................................................................................................................. 86
Delegates from Out side (N) ....................................................................................................................................................... 62
Delegates from Dr. R.P. Centre for Ophthalmic Sciences (n) ..................................................................................................... 24
Overall assessment by outside delegates (M) ......................................................................................................................... 473
Assessment of Case Presentation-I (Dr. Sachin Kedar) by outside delegates ....................................................................... 447
Assessment of Case Presentation-II (Dr. Murlidhar R.) by outside delegates ..................................................................... 455.5
Assessment of Clinical Talk (Prof. S. Ghose) by outside delegates .................................................................................... ..460.5
December, 2003
284
DOS Times - Vol.9, No.6
TEAR SHEET NO. 6
Newer Diagnostic Modalities in Ophthalmology
Cornea
Confocal microscope: scanning slit corneal confocal microscope
Frame size: 300X200microns with 5 microns thickness.
Uses: For detailed assessment of
1. Endothelium –
¡ high quality specular microscope
¡ endothelial count
¡ guttate, endothelial polymegathism and pleomorphism
2. Stroma
¡ anterior stroma keratocytes morphology
¡ nerve fibres especially after LASIK
¡ Flap interface morphology after LASIK.
3. Epithelium and bowman’s membrane
Lens:
IOL masters
¡ Non-contact optical device based on partial coherence interferometer
¡ Measures the AL from the corneal vertex to the retinal pigment epithelium
¡ Accurate to within ±0.02 mm or better.
¡ Five times higher accuracy in measuring AL as compared to USG.
¡ Measures: AL, Km, Anterior chamber depth, white-to-white corneal
diameter.
¡ Based on Haigis formula
¡ Provides IOL power according to desired IOL type, desired post-operative refraction
Draw back: cannot be used in dense cataract and opaque media.
Anterior Segment
Ultrasound biomicroscopy (UBM)
l High resolution ultrasound (50MHz )
l Resolution 25-50 microns.
l Used to evaluate the structural details of the anterior segment structure, iris angle ciliary body, lens zonules.
l Useful in assessing angle, iris and trabecular meshwork relationships
in eyes with corneal edema and corneal opacity
l Measurement of angle in ACG, OAG, checking patency of PI
Glaucoma
SWAP [Short wave automated perimetry (blue on yellow perimetry)]
l Evaluates the blue cones that are lost early in glaucoma
l Uses yellow background and blue stimulus to selectively stimulate
the blue cones
l Advantage: detects the glaucomatous VFD earlier than standard
white on white perimetry.
l Uses: early diagnosis of glaucoma, early detection of progression,
predict the risk of conversion of OHT to glaucoma
l Drawback: nuclear sclerosis and cataract may mimic early progression on SWAP
Frequency doubling perimetry (FDP)
Detects early loss of the Nonlinear M-cells (a subset of magnocellular
cells)
l Based on frequency doubling illusion(high temporal and low spatial
frequency)
l
December, 2003
l
l
l
Advantages: very quick test (45 sec to 6 min), portable instrumentation, no spectacle correction required upto 7D of refractive error.
Use: for screening
Drawbacks: lack of any longitudinal data, early progression and focal defects may be missed
Heidelberg retinal tomography (HRT)
Confocal Laser Scanning ophthalmoscope (CSLO).
l Gives three-dimensional images retina and optic nerve head.
l Uses a diode laser (670 nm), resolution 30 µ, field of view 10° × 10°,
15° × 15°, or 20° × 20°. Pupil dilation is not necessary.
l A three-dimensional image is acquired as 32 consecutive and equidistant optical section images, consisting of 256 × 256 picture elements.
l Uses Moorefield regression analysis to classify various disc sectors
as normal borderline or abnormal.
l Use for early diagnosis, detection of early progression and follow-up
of glaucoma.
l
GDx VCC (Retinal nerve fibre layer analyzer with variable corneal
compensation)
l NFA-GDx works on the principle of scanning laser polarimetry.
l Measures retardation of polarized light by RNFL
l GDx-VCC has a variable corneal compensator that compensates for
corneal birefringence, giving more accurate thickness of RNFL.
RETINA & Posterior Segment
HRA (Heidelberg Retinal Angiograph)
¡ Scanning laser tomography
¡ Uses argon (488 nm) laser for the fluorescein angiogram and a diode
(788 nm) laser for the ICG angiography.
¡ Simultaneous, side-by-side delivery of both fluorescein and ICG angiographies
¡ Single images can be captured at up to 20 frames per second with
512 x 512 resolutions.
¡ Acquires upto 12 frames-per-second with a resolution of 256 x 256
pixels. providing real time high speed angiography
Optical Coherence tomography (OCT)
Non contact, non invasive tool
l Uses low coherence near infrared light 830nm
l Resolution 10-15µm, field of view 300, requires pupillary dilatation
and clear media
l Based on Michaelson interferometer.
l Uses: macular thickness assessment in
v Diabetes,
v ARMD,
v Cystoid macular edema,
v Macular hole,
v CSR
l In glaucoma
v Optic disc tomography
v Retinal nerve fibre layer thickness measurement
l
Parul Sony, MD
Dr. R.P. Centre, AIIMS, New Delhi - 110029
285
DOS Times - Vol.9, No.6