Download Concept of Visual Acuity

Clinical
Concept of Visual Acuity
Dr. Arpan B Patel, DNB Resident, Aravind Eye Hospital, Madurai
Vision is the most sophisticated and highly
developed special sense. The faculty of vision is
a very complex phenomenon based on different
physiological processes which are still poorly
understood. Broadly, visual function comprises
light sense, form sense, colour sense and the sense
of contrast.
The contrast sense is that faculty of vision by
virtue of which the eye perceives slight changes
in the luminance between regions which are not
separated by definite borders. In its simplest terms,
contrast sensitivity refers to the ability of the visual
system to distinguish between an object and its
background.
For example, imagine a black cat on a white
snowy background (high contrast ) against a white
cat on a white snowy background (low contrast).
The most common cause of decreased
contrast sensitivity is ageing and the development
of cataract. Although, the improvement of visual
acuity is the major goal of any cataract surgery,
with the present state of art, and the technology
like phacoemulsification with foldable intraocular
lens implantation, it is possible to provide a
Snellens visual acuity 6/6 with almost normal
field of vision.
However, some patients are not entirely
satisfied by the quality of visual acuity achieved.
They complain of glare and “fogginess” or
“haziness” of vision. These complaints are
attributed to the reduction of contrast sensitivity.
Contrast sensitivity is considered to be the best
available measure of visual function in daily life.
Incision of the cataract surgery getting
smaller with phacoemulsification together with
the better design and material of the intraocular
lens implanted provide the best possible visual
rehabilitation to the patients with cataract in the
form of improvement in correct sensitivity.
Excellent visual acuity measured with Snellens
chart after intraocular lens implantation reflects
the visual acuity at 100% contrast levels. However
the real life situation are associated with various
levels of illumination and objects with different
shades and contrast. Contrast sensitivity, glare
sensitivity and visual acuity evaluation are the best
method of evaluating the problems of patient in
daily circumstances.
This study is undertaken to study and evaluate
the optical performance of pseudophakic subjects
with rigid PMMA, foldable spherical acrylic
and foldable aspheric acrylic intraocular lens
implantation after phacoemulsification cataract
surgery using contrast sensitivity and visual acuity.
Visual Acuity and Functional Vision
Visual environment in every-day life consists of
much more than just the simplified way in which
vision is assessed using the letters or numbers on
a standard eye chart. Visual acuity (VA) estimation
is the central part of the ophthalmic examination
and is the measure of the spatial resolution of the
eye. Visual acuity testing is the “gold standard”
for primary outcomes of clinical trials.
Although Visual acuity testing is not the
only measurement of visual function, it remains
the most commonly used test. Despite being
primarily a measure of the resolving power of the
eye, it can be diminished in many different ocular
conditions. Visual acuity is related to the angle
subtended at the eye by the smallest recognizable
optotype and is measured by the patient’s ability
progressively smaller optotypes.
The Snellens chart (Figure 2.1) introduced
in 1862 by Dutch ophthalmologist Dr.Herman
2
Snellen, has been ubiquitously adopted as the
standard for measuring visual acuity in clinical
practice, because it is readily available as well as
quick and easy to perform and is the universally
accepted tool for testing visual acuity despite its
poor reliability and reproducibility.
The chart has letters of different sizes arranged
from largest at the top to smallest at the bottom
which are read,one eye at a time, at a distance
of 6 meters (20 feet). Each letter on the chart
subtends an angle of 5 minutes (min) of arc at the
appropriate testing distance and each letter part
(eg. limbs of the letter E ) subtends an angle of 1
min of arc. (Figure 2.2 ) Thus, it is designed to
measure acuity in angular terms. In a healthy adult,
the resolution limit is between 30 seconds and
AECS Illumination
1 min of arc. The scoring method used is the line
assignment method, where a patient gets credit
for lines, not letters, read, Accepted convention
does not specify Snellen acuity in angular terms;
instead, Snellen acuities are usually expressed as a
fraction with the numerator equal to the distance
from the chart and the denominator being the size
of the smallest line that can be read.
The reciprocal of the Snellen Visual acuity
equals the angle (in minutes of arc) that the strokes
of the letter subtend at the person’s eye and is
called the Minimum Angle of Resolution (MAR),
which can also be given in log10 form,abbreviated
as logMAR. The simplest method for computing
the proper average visual acuity from any notation
is to covert the value to the logMAR equivalent
and then take the average of the logMAR values.
The formulas for going from decimal to logMAR
and then back are as follows:
logMAR = -log (decimal acuity )
decimal acuity = antilog (- logMAR ) = 10-logMAR
Figure 2.1
Figure 2.2
Standardization of the visual acuity task requires
the following :
1. A logarithmic size progression (Constant ratio
form one size to the next)
2. The same number of letters at each size level
3. Spacing between letters and between rows that
is proportional to letter size
4. Equal (or similar) average legibility for the
optotypes at each size level
Newer logMAR charts (Figure 2.3) are now
available that have negated the disadvantages of
the Snellen chart. However, these charts are not
being used regularly in daily practice.
In 1976, Bailey and Lovie published a more
standardized chart that was adapted and used
for the Early Treatment of Diabetic Retinopathy
Study (ETDRS). The ETDRS charts are now
considered as the most accurate and standard
method of VA recording, when precise data on VA
measurement is required.
The logMAR chart is based on Bailey and
Lovie’s work and incorporates recommendations
of the Committee on Vision of the American
Vol. XVI, No.3, July - September 2016
Figure : 2.3
National Academy of Sciences-National Research
Council.
The technique for Visual Acuity measurement
using a logMAR chart is very similar to the
Snellen chart in that it requires patients to read
to the lowest line possible. However, there can be
practical difficulties with incorporating logMAR
into clinical practice – many examination rooms
are designed for Snellen charts at 6 m. The test
distance for the logMAR chart is 4 m, but this
problem is avoided by a very simple conversion
factor. The regular geometric design of the
optotypes allows easy conversion for other nonstandard distances.
The change in size of the optotypes in the
Snellen chart is arbitray and there is no regular
progression from the easiest to the most difficult
lines. Therefore, at the lower end of acuity level
both over - and underestimation of the Visual
Acuity can occur.
A two-line reduction is Visual Acuity from
6/24 to 6/60 is more than a doubling of the visual
angle, but the loss of two lines from 6/6 to 6/9 (on
a chart with a 6/7.5 line ) is less than a doubling
of the visual angle, that is, loss or gain of one line
does not have the same meaning in different parts
3
of the chart. The geometric progression is size of
the optotypes that occurs with the logMAR chart
avoids this problem.
The Snellen chart becomes progressively
harder further down the chart the patient
reads: only one letter is present at the 6/60 stage
whereas upto eight letters are shown at the better
acuity levels. This difficulty is compounded
by the contour interaction or ‘visual crowding'
phenomenon – the legibility of an optotype is
less clear when presented with other optotypes
in close proximity. Contour interaction varies
throughout the Snellen chart and the 6/60 and 6/6
visual acuity are not entirely comparable.With the
logMAR chart, the only parameter that determines
visual acuity (VA) is the angular size of the letters.
The smallest line a patient can read is used
as a measure of his or her visual acuity. One or
mistakes pre line are often compensated for, but
this can have a different meaning at different
levels of acuity with the Snellen chart, a patient
must correctly identity seven out of eight letters
(89%) on the 6/6 line, whereas on the 6/36 line,
identification of only one letters (50%) may occur.
The logMAR chart has an equal number of letters
per line and each individual letter is assigned an
individual score resulting in a more accurate and
consistent measurement of visual acuity.
Different Snellen charts may have different
numbers of letters,that is,not all charts have a
6/7.5 line and there is no general standardization.
Moreover, scoring Snellen visual acuity as the
smallest line at which a majority of letters is
correctly identified has been shown to restrict the
sensitivity of the test to detecting changes over
time.
The Snellen chart is recongnizable and familiar
to the layman and medical professional alike -‘6/6
vision’ is a common place term. This however, in
the current climate of evidence-based medicine,
does not justify a delay in adopting a better chart.
Although the logMAR chart is regularly used as a
research tool, it has to become more common in
clinical settings. Indeed, for newer treatments such
as Photodynamic Therapy, logMAR visual acuity
4
measurements are mandatory in order to be able
to monitor treatment outcomes. LogMAR charts
allow standardization of vision assessment and are,
therefore, very useful in the research environment.
As the 10 letters used in logMAR charts are chosen
for their equality of readability and difficulty level,
a three-line worsening of VA is equal to a doubling
of the visual angle regardless of the intial acuity.
The logMAR chart is said to be more timeconsuming and less easy to understand than
the Snellen chart. This however, is not entirely
accurate. The potential difficulty of use would be
easily overcome by frequent use. The Snellen chart
with its long precedent is so well entrenched in
common use that this may be the single biggest
factor preventing logMAR from superseding it.
The logMAR chart also includes a conversion
measure into Snellen equivalent if this is required
for communication to those in other medical
specialities who may not be as familiar with
logMAR.
In the context of assessment of Visual Acuity,
the extra time involved is minimal and should
be negated by increased accuracy and sensitivity.
However, refraction on a logMAR chart is timeconsuming. Patients continually become ‘lost’
because of the crowding phenomenon, must read,
and re-read the chart to locate the correct letters
for fixation. Newer condensed forms of the chart
have been shown to be as reliable as and quicker to
perform than the full logMAR chart, which would
allay this problem. Further refinement of the chart
is required to overcome the impracticalities of the
chart for refraction.The logMAR is, therefore,
superior in its scientific principles, clinical
accuracy and reproducibility. Acceptability of the
LogMAR chart is less than Snellen chart because
snellens chart is very well established in current
clinical practice and ease in method of use.
On the contrary few researchers still believe
Snellens visual acuity is the best form of visual
assessment in day to day clinical settings and
practice.
AECS Illumination
LogMAR chart
This chart has been designed for 4 meters,using
high contrast lettering. It is based on the Bailey and
Lovie’s work incorporating recommendations of
the committee on vision of the American National
Academy of sciences - National Research Council.
As compared to the Snellen chart which does
not have standard size progression e.g 6/60, the
next line is 6/36 and then 6/24, as can be seen that
the size progression is not linear, this problem is
taken care of in the logMAR chart.
In the LogMAR chart the progression of each
line size is approximately 1.25 times greater than
the line below. A change of 10 increments on
this scale represents a change of exactly 10 times,
and a change of three steps represents a change of
approximately two times.
Secondarily, the number of the letters is
constant in each line. Therefore, it makes visual
acuity measurement more accurate as the patient
is able to read all letters of a particular line to be
marked with visual acuity, whereas in snellens
chart the number of letters change in each and
this may be reason for the patient to guess where
there is a single letter i.e at 6/60 and thus resulting
in the wrong measurement of the visual acuity.
Thirdly, letters are equally spaced on each
line and the distance between each line also has
a linear progression.Ten Sloan letters or pictures
are selected for easy readability. The letters are
C D H K N O R S V Z.
Concept of Contrast
Visual acuity measured using a Snellen chart has
high-contrast letters; however, the world is almost
never seen in such contrast. However, vision is not
only about visual acuity, visual function comprises
light sense, form sense,color sense and sense of
contrast.
The standard letter charts used by most
ophthalmologists are not effective in the early
detection of disease such as cataracts and glaucoma.
However, other commonly performed tests (such
as the slit lamp, intraocular pressure testing and
Vol. XVI, No.3, July - September 2016
visual fiels ) can detect certain vision abnormalities
caused by such diseases. Years of research and
clinical trials has provided a more comprehensive,
non-invasive technique for disease screening the
contrast sensitivity testing.
The standard high-contrast visual acuity chart
measure the ability to see black letters (about 1
or 2 percent reflectance) on a white background
(close to 100 percent reflectance) giving close to
100 percent contrast.The real world, however, is
far from this ideal. It consists of objects with an
average reflectance of only 18 percent, and the
contrast between objects of interest and their
background is usually much less than 100 percent.
The jet black letters on the white background
have great deal of reverse contrast so that even a
patient with severely reduced contrast sensitivity
can still read the chart. Patient perceives the letters
as gray on white rather than black on white, but
still be able to recognize them.The examiner has
no idea how letters look to the patient, so proving
the standard visual acuity charts insensitive to test
visual function.
For example, the contrast between the
pavement and the sidewalk, which is the main
clue that defines the edge of a curb, may typically
be just a few percent.
Contrast provides critical information about
edges, borders, and variations in brightness. People
with poor contrast sensitivity fail to see large, lowcontrast objects under conditions of poor visibility
(such as fog) despite normal or near normal visual
acuity. The importance of measuring contrast
sensitivtity is that it can provide information,
which cannot be obtained from visual acuity
measures, and it is often a better predictor of
performance than visual acuity.
A meaningful measure of contrast sensitivity
can provide a more complete picture of the visual
function when used in conjunction with Snellens
acuity.
5
Contrast sensitivity testing can help in the
early detection of a wide range of visual problems
including, cataracts, glaucoma, amblyopia,
Alzheimer’s, AIDS, Macular degeneration,
Diabetes, and exposure to toxic agents.
Incorporating contrast sensitivity testing during
routine eye examinations would arm clinicians
with valuable information about their patient’s
visual condition. This information can further
aid in proper patient care, referrals and treatment.
Contrast Sensitivity [CS]
It is defined as the ability to detect the presence
of minimal luminance difference between objects
or areas. Example for this would represent the
ability to read a road sign board at night in the fog.
The minimal luminance difference between the
letters on the sign and the background is minimal,
rendering the letters more difficult to see.Viewing
the same letters in the day light without the fog is
much easier by comparison.
There is some correspondence between the
expected snellens acuity may be normal in the
face of decreased CS. It has been noted that
aging process is correlated with decreased CS.
Contrast Sensitivity = (L max - L min)
(L max + L min)
L is luminance.
In a patient with loss of low frequency contrast
sensitivity may be able to read 6/6 but unable to
see in the fog. While blur due to refractive error
alone affects the lower frequencies, scatter of light
due to cataract causes loss of all frequencies.
Interestingly the patients evaluated after
intraocular lens implanation were found to
have decreased CS compared to normal eyes. A
possible explanation to this is, crystalline lens
when accommodates, will focus at its peak wave
length of the spectrum and small amout of light
is defocused which travel to retina. The implanted
6
IOL cannot accommodate thus causing the
chromatic aberration to be greater.
Measurement of contrast sensitivity : A
test for the hidden loss of vision
On routine ophthalmic examination, it is rare
that patient is tested for the other most important
functions of vision like Fields and Contrast
Sensitivity. It has to be remembered that a 6/6
result, will not always uncover a possible hidden
loss in contrast sensitivity. This will indicate a
serious loss of visual function and this loss is
usually a sign of eye disease.
The detection of the low contrast is an
important part of vision. It follows little consensus
regarding the best method to use, to measure and
to test Contrast Sensitivity. Unlike test for other
elements of vision, there is no universal standard
testing method to measure contrast. The tests
currently available are either of grating type or
optotypes as targets.
Grating Types
1. The Functional Visual Acuity Contrast Chart
Test [FACT] [Stereo Optical Company, Inc,
Chicago IL] Originally developed by Arthur
Ginsburg is available as handheld and wall
mounted and illuminated with external source.
It is forced choice test.
2. The VCTS6500 [Distich Consultans,Inc
Dayton] is a grating wall chart with five spatial
frequencies. Tested at 3 mts and it is also a
forced choice test.
3. CSV-1000 (vector vision, Arcanum, OH) The
test distance is 8 feet for CS testing. Consists
of round sine-wave grating.
Letter Optotypes
1. The small letter contrast sensitivity test
[SLCT] is printed for use at a distance of 4
meters with normal over head illumination.
2. The Bailey-Lovie Letter Chart consists of
optotypes also called as Contrast Acuity Charts.
Five per size, with logarithmical increment in
AECS Illumination
declining optotypes size. The increment in size
are usually 0.1 log units. This is essentially a
refined snellens chart, which has logrithmical
increment istead of the uneven increments
to allow for the better comparison between
patients. Typically, a high contrast setting
usually greater than 90%, is held constant and
acuity measured. A low contrast setting [13%]
can also be held constant, and the acuity is
measured. The result would be two points on
contrast sensitivity function curve.
3. The Pelli-Robson Letter – Sensitivity Chart
uses letters of only one size in the groups of
three, each group then decreasing in contrast.
The correct level of contrast is obtained by the
line of the letters corresponding to two correct
responses of the letters.
Though there is strict controversy as which
method of CS testing is the best, Grating chart are
mostly research oritented and Optotype charts are
also equally effective in contrast sensitivity testing.
Pelli-Robson CS test is a reliable and easy
to apply method and Contrast sensitivity the
pelli-Robson chart can be useful in developing
countries.
Pelli-Robson Contrast Sensitivity Chart
Method of examination of CS
• This chart utilizes letters of the same size but
with reducing contrast to provide a quick
means of assessing patient contrast sensitivity.
• There are two charts and two scoring pads.
The charts have different letter sequence but
otherwise they are similar. Each chart has six
letters in a row organized into two triples of
varying contrast, thus there are two contrast
levels in each row. The illumination of the chart
is 85cd/Mm2 and glare should be avoided.
• The test is carried out at a distance of 1 meter
(42 inches) with patient wearing the best
correction and before dilating the pupils and
with near addition if required.
Vol. XVI, No.3, July - September 2016
• Patient is asked to read the alphabet starting
from left hand upper 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 two letters are correctly
identified.
• The Log Contrast sensitivity value for this
triplet is given by the number on the scoring
pad nearest to the triplet, either on the left or
on the right side.
• Each eye is tested separately and then both the
eyes. All three measurements should not take
more than 8 minutes.
• Usually the binocular CS is higher than the
monocular vision by 0.15 units. However
development of cataract may result in poorer
binocular CS when compared to the eye been
tested separately.
Interpretation of Readings
The score a single number is a measure of the
subject log contrast sensitivity. Thus a score of 2
means the patient was able to read at least two of
the letters of the three letters with contrast of 1%
(contrast sensitivity = 100% or Log 2). A PelliRobson score of 2.0 indicates normal contrast
sensitivity of 100%. Scores less than 2.0 means
significantly poorer contrast sensitivity.
A score of less than 1.5 is consistent with
visual impairment and a score of less than 1.0
represents visual disability. This score represents an
approximately 10 folds loss of contrast sensitivity
that is the person with contrast sensitivity of
1.0 requires 10 times as much contrast to see as
compared to normal vision. Mean value of the
log CS 1.72 in a group of patients between 6070 years have been found in a study. A loss of this
magnitude would be quite disabling and will have
huge impact on one’s ability to drive or read.
Poor CS adversely affects the ability to read
the text, for example reading a newspaper. A CS
of 1.0 or better is required to read high contrast
print at a normal speed. Most people with a CS
of 1.0 or less will read text slowly. This level of
7
contrast sensitivity is disabling with regard to
walking speed. If CS is less than 1.3 the patient
will have an increased likelihood of automobile
accident. While performing typical manual tasks
the contrast between different crucial parts of the
task materials can be very low. It is likely that if
one’s ability to see under such reduced contrast
is impaired, then their ability to perform these
tasks will also be affected and Pelli-Robson test,
especially at 3m, could be a quick and reliable
clinical test for contrast sensitivity at the region
of peak sensitivity.
The Pelli-Robson test with optotypes (letters)
only measure one CPD (cycles per degree) region
at a recommended distance and the examination
must be done at different distances if more cycles
per degree are needed.
Limitation of the Human Eye
The media can scatter light as it is being
transmitted from an object. The optics of the eye
may degrade the quality of the image. Retina has
anatomical and physiological limits, and neural
image processing can later the perception of a
target and reduce or enhance contrast sensitivity.
The highest level of the acuity attainable by the
human eye is under photopic or bright light
conditions.
Clinical Significance of the Contrast
Sensitivity
While the achievement of uncorrected vision
remains a laudable target for any cataract surgery,
the goal of high quality vision increasingly reflects
understanding of the visual system as a whole.
Snellens visual acuity represent only a small
portion of the functional vision.
Contrast sensitivity has the ability to detect
the difference in functional vision when snellens
vision acuity measurement cannot. Blur images
of the refractive error affect only the high spatial
frequencies, the scatter of light due to the cataract
and corneal opacities affect at all frequencies.
Contrast testing thus offers critical information
which helps to explain the patients’ complaints.
8
Unfortunately the CS decreases with age even
with or without ocular pathology. This decline
in the CS is due to the changes in the spherical
aberration of the crystalline lens.
In addition to understanding the visual
complaints of the patients having good snellens
acuity, it also helps in early diagnosis of ocular
disease, to determine when a patient needs
treatment, and to judge the efficiency of the given
treatment.
Contrast sensitivity can be used to determine
exactly when does diminished functional vision
from a cataract warrants surgery.
It helps to explain the patients having 6/6
visual acuity and complaining about the quality of
vision and then having cataract surgery and being
happy with results.
The sine wave grating contrast sensitivity of a
pseudophakic patient with a spherical intraocular
lens (IOL) implanted is no better than that of a
phakic patient of a similar age who has no cataract.
This may be the reason of non-specific
visual complaints (‘washed-out images’), despite
normal Snellen acuity, after cataract surgery and
monofocal IOL implantation.
Aberrations of the Eye
The wave-aberrations is defined as the difference
between the perfect (spherical) and the real wave
fronts for every point over the eye’s pupil. These
aberrations are classified as monochromatic and
chromatic. Monochromatic aberrations are specific
to a particular wave-length of visible light as it
travels through optical system. Monochromatic
aberrations can be further classified into;
1. Low-order aberrations, such as defocus
(spherical refractive error) & astigmatism
2. Higher-order aberrations, such as coma,
spherical aberration, trefoil, quadrafoil, or
terafoil, secondary astigmatism, pentafoil.
In phakic eyes, the decrease in visual acuity and
in contrast sensitivity that occurs with age is
usually attributed to lens changes. These changes
AECS Illumination
develop with presbyopia and are primarily related
to increasing wavefront aberration.In young
subjects, the crystalline lens compensates with
its negative spherical aberration for the positive
spherical aberration of the cornea, resulting in
a low level of spherical aberration of the entire
eye. Unfortunately after of 40 years, the spherical
aberration of the lens progressively turns positive,
adding to the spherical aberration of the cornea
to increase the total aberrations of the eye. Thus
aging lens loss its balance with the cornea, as both
magnitude and the sign of its spherical aberration
change significantly. Thus, a loses of balance
between corneal and lenticular spherical aberration
causes the degradation of the optical quality in
the aging eye.
Current IOLs also have positive spherical
aberration, thus producing a pseudophakic
eye that in this regard is no better than an
aged eye with a transparent lens. Conventional
pseudophakic IOLs fail to restore the potential
maximal optical quality, because of imperfect
centration, tilt, and increased positive spherical
aberration due to the addition of the positive
corneal spherical aberration of the IOL to that of
the cornea. Thus, lens with optimized aspheric
design could improve the optical performance in
balancing the positive corneal spherical aberration,
provided tilt and decentration of the optic of the
IOL are controlled.
Aspheric Optics
The understanding of sophisticated optics
and its relevance to clinical ophthalmology is
credited to corneal laser refractive surgery. Vision
symptoms that were not consistent with excellent
high-contrast visual acuity rapidly led to an
understanding of the impact of large-aperture
optics and aberrations on the quality of vision.
This understanding rapidly found its way into
cataract and IOL surgery.
The natural corneal curvature has an asphericity
that averages +10.27μ RMS. In youth, this is
balanced by a lenticular asphericity averaging
Vol. XVI, No.3, July - September 2016
-0.27μ RMS, leading to minimal total spherical
aberration.
It has been suggested that the best optical
is obtained if the entire amount of spherical
aberration is corrected (i.e there is zero total
spherical aberration after surgery). The original
conventaional IOL designs all had optics designed
on elementary optics, which resulted in positive
spherical aberration in the IOL. This increased the
total spherical aberration of the eye after cataract
and IOL surgery, with resultant loss of image
quality, loss of contrast sensitivity.
IOL designs increasingly address the issue of
the spherical aberration of the eye. Aspheric optics
9
clinically improves quality of vision and contrast
sensitivity compared to spherical optics.
Aspheric lens designs have targaetd different
ranges of spherical aberration. The Alcon aspheric
IOLs (e.g. SN60WF series) are designed for -0.17μ
RMS of asphericity, with prolate posterior surface,
strking a middle ground. The prolate surface of
the aspherical IOL compensates for the spherical
aberration of the cornea. Aspheric IOLs must be
centered within 0.4mm of the pupil center to
results in optical benefit. The lower the negative
asphericity, the less benefit to a well-centered IOL
but, correspondingly, the less degradation form
decentration or tilt.