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Ocular anatomy
and optical
components of the
eye
Types of refractive
error
Accommodation
and presbyopia
Refractive surgery
Assessing the
patient
Results and
post-surgery
management
The author
DR DAYA SHARMA
corneal, cataract and refractive
surgeon, Eye and Laser Surgeons,
Bondi Junction; clinical senior
lecturer, University of
Wollongong, NSW.
Refractive eye disorders
Background
MYOPIA (short-sightedness), hypermetropia (long-sightedness), astigmatism and presbyopia (loss of
accommodative power with ageing)
are common and have significant
effects on patients’ quality of life.
Although usually corrected with
either spectacles or contact lenses,
they may restrict participation or
enjoyment of work, and social,
sporting or other activities. A review
of the public health significance of
refractive error for primary care was
recently published.1
Because of the importance of
vision to a person’s quality of life,
and the ways in which refractive disorders may affect patients, the GP
— as a trusted regular point of contact for medical care — may be
asked about various refractive surgical procedures. The primary aim
of this article is to educate GPs about
the types of refractive disorders, their
surgical management and the bene-
fits and risks of various surgical
options. It will also address how GPs
should assess, manage and refer
patients who present with problems
after having refractive surgery.
This article will focus on the general
principles of refractive disorders and
their surgical management, the aim of
which is to reduce or eliminate the
need for spectacles or contact lenses.
Having such surgery therefore may
have significant positive impacts on
quality of life.2-4
The range of refractive surgical
options has evolved rapidly. Treatment selection needs to be individually tailored to the patient’s needs
and appropriateness for treatment,
with an understanding of the benefits
and risks of each technique.
The number of patients having
corneal laser refractive surgery for
correction of refractive errors has
grown significantly, as technology
has improved to make the procedure
safer and the outcomes more predictable. Alternatives to corneal laser
refractive surgery that do not involve
reshaping of the cornea have also
become more common. These
include implantation of phakic
intraocular lenses (IOL), whereby a
phakic IOL is implanted into the eye
without removing the crystalline
(natural) lens.
Refractive lens exchange (RLE),
on the other hand, involves removal
of the non-cataractous crystalline
lens and its replacement with an IOL
to correct for refractive error and/or
presbyopia. The surgical procedure
itself is essentially the same as
modern cataract surgery, with small
modifications for removal of a soft
crystalline lens. However, in patients
considering RLE, vision would be
correctable with spectacles or contact lenses, whereas in patients with
cataract, best-corrected vision is
reduced as a result of the cataract
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(ie, there is a reduction in vision
despite optimal refractive correction
with spectacles or contact lenses).
Although cataract surgery has historically not been considered a
refractive surgical procedure,
improvements in preoperative measurement and planning, surgical techniques and IOL design have led to
better refractive outcomes, and hence
patients’ expectations of better
unaided vision after cataract surgery
have also increased.
Some refractive disorders are a
result of corneal disease, such as keratoconus, in which refractive error
occurs because of steepening and
irregularity of the cornea. New management options are now available,
which aim to halt disease progression, avoiding the need for a corneal
graft — and in some cases, surgically
improving uncorrected vision to a
functional level.
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HOW TO TREAT Refractive eye disorders
Ocular anatomy and optical components of the eye
IT is important to review the relevant anatomy of the eye (figure 1)
to understand refractive disorders
and the principles of refractive surgery. From front to back, light rays
pass through the precorneal tear
film and cornea, aqueous humour
(within the anterior chamber of the
eye), crystalline lens, and vitreous
humour, before stimulating the
photoreceptors of the retina. Stray
light or scattered light is absorbed
by the retinal pigment epithelium
to help reduce image degradation.
To produce a clearly focused
image on the retina, light rays must
be refracted by the two principal
optical components of the eye: the
cornea and the lens.
Figure 1: Ocular anatomy. Horizontal section of the right globe.
Cornea
Pigment
epithelium
Sclera
The cornea typically contributes
about two-thirds of the eye’s refractive power. Its average central thickness is 540μm (standard deviation of
31μm). Disease states affecting
corneal thickness can result in an
apparently normal corneal thickness
— such as keratoconus (which produces corneal thinning), and corneal
endothelial dysfunction (which causes
corneal oedema). The cornea is also a
key structural component of the eye,
making up one-sixth of the eye wall,
with the other five-sixths being made
up of the opaque, vascularised sclera.
Cornea
Iris
Lens
Rectus
tendon
Zonule fibres
Ora serrata
Optic axis
Visual axis
Vitreous humour
Retina
Disc
Choroid
Descemet’s membrane
Lens
This is the basement membrane of
the endothelium, and becomes
thicker with age.
The crystalline lens is a biconvex
structure of higher refractive index
than the aqueous anterior to it, or
the vitreous posterior to it. Thus, it
causes further convergence of light
rays, which in ideal conditions will
focus on the retina.
The crystalline lens is contained
within a very thin capsular bag, and is
held in position by zonular ligaments
(zonules). It is a common misconception that the crystalline lens is the
most powerful refractive component.
It contributes only about one-third of
the refractive power of the eye, but it
is remarkable for its ability to adjust
focus from distant to near objects
(accommodation).
The Bowman’s layer is a thin but
strong condensed collagenous layer
overlying the stroma.
Stroma
Endothelium
Fovea
Lamina
cribosa
Macula lutea
Optic nerve
The corneal stroma accounts for
about 90% of the corneal thickness and consists of multiple layers
of collagen fibrils. It is sparsely
populated with keratocytes, which
can become activated in healing
and inflammation.
Bowman’s layer
Conjunctiva
Aqueous humour
Ciliary body
eye. Because there is a large difference in refractive index between air
and the cornea (light rays slow
down and converge as they enter
the curved anterior surface of the
tear film/cornea), the anterior
corneal surface is a critical refractive component of the eye. The posterior corneal surface has negative
refractive power (light rays diverge
on entering the aqueous), but is a
less important refractive component
than the anterior corneal surface,
because there is less difference
between the corneal refractive index
and that of the aqueous.
and helps maintain the precorneal
tear film.
Sheath
There are five main corneal layers.
Epithelium
The epithelial layer is a non-keratinised, stratified, squamous epithelium that is turned over frequently
The corneal endothelium is a monolayer of hexagonal cells, which are
metabolically active and pump fluid
out of the cornea to maintain corneal
clarity. Endothelial cells slide and
enlarge to cover a defect or loss of
cells, and are essentially non-regenerative in vivo. Significant endothelial cell
loss or dysfunction leads to corneal
oedema and resultant blurred vision.
The cornea (with its precorneal
tear film) produces the first optical
interface for light rays entering the
Types of refractive error
Myopia, hypermetropia and
astigmatism
FIGURE 2 and 3 (page 32) show
the different types of refractive error.
An eye with no refractive error is
termed emmetropic, and is wellfocused for clear distance vision.
Light rays from a distant target are
parallel when they enter the eye, and
are then focused on the retina. In
an eye with good accommodative
power (a prepresbyopic eye) and no
pathology, emmetropia results in
both excellent unaided distance and
near vision.
Ametropia is the term for an eye
that has refractive error for a distance target. The most visually significant refractive errors are produced by lower order aberrations
known as defocus and astigmatism,
commonly referred to as ‘sphere’
and ‘cylinder’.
The term ‘lower order’ refers to
the degree of complexity of the
abnormality, but these abnormalities can range from mild to severe.
For example, low myopia alone is
associated with excellent reading
vision, whereas extreme myopia
will result in very poor vision
except at an extremely close and
impractical viewing distance.
Also, low astigmatism (eg, <1D
[dioptres], see below) may result in
good unaided vision, whereas high
astigmatism (eg, >5D) may not
even be correctable with spectacles.
Having a spectacle test (refraction) involves measuring only
these two lower order aberrations
(defocus and astigmatism), which
are commonly corrected with
either prescription spectacles or
soft contact lenses (depending on
the degree of error). Correction
of defocus and astigmatism alone
will result in excellent vision for
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Figure 2: Diagrams demonstrating different types of refractive error. A: Emmetropia. Light rays from a distant target
converge to a focal point on the retina. B: Myopia. Light rays from a distant target converge to a focal point in front of
the retina. C: Hypermetropia. Light rays from a distant target converge to a focal point behind the retina. D: Types of
astigmatism. Light rays from a distant target are focused in two different focal planes. The two lines show the focal
planes, which are oriented perpendicular to each other.
A
B
C
D
Compound myopic
Simple hyperopic
most patients with no ocular
comorbidity.
Refractive power is measured in
dioptres (D), which relate to the
strength of a lens to refract light
rays; positive lenses cause convergence, whereas negative lenses
cause divergence.
A dioptre is the reciprocal of the
focal length of the lens (in metres).
Hence, a positive lens of 2D will
have a focal length of 50cm, meaning parallel light rays will converge
to a point 50cm behind the lens. A
negative lens of -5D will have a
focal length of -20cm (the focal
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Simple myopic
Mixed
Compound hyperopic
point exists in front of the lens;
parallel light rays will continue to
diverge further after passing
through the lens).
Defocus is produced when the
spherical refractive power is too
high for the eye (myopia) or too
low for the eye (hypermetropia).
In a myopic eye, light rays converge to form an image in front of
the retina, and objects are seen
more clearly as they are brought
closer to the eye (hence the term
short-sighted). This is most commonly due to axial myopia, where
the axial length of the globe is
greater than normal, but can also
be from refractive myopia, where
the cornea is steep. Myopia can be
corrected with a negative lens that
causes light rays to diverge before
reaching the cornea.
Conversely, in a hypermetropic
eye, light rays converge to form an
image behind the retina. There is
not enough refractive power,
because either the eyeball is short
(axial hypermetropia) or the cornea
is flat (refractive hypermetropia).
Distant objects are less blurred
than near objects, and objects
become progressively more blurred
as they are brought closer to the
eye (hence the term long-sighted).
To compensate, a low hypermetrope (a person with mild hypermetropia) will often use some extra
accommodative effort (focusing
power) to cause extra convergence
of light rays, and move the focal
point forward onto the retina. In
adulthood, high hypermetropia
cannot be corrected with accommodative effort, and requires a corrective positive lens.
Astigmatism generally produces
an image where objects are elongated and blurred in one direction,
but clearer perpendicular to this.
Astigmatism occurs when an optical component is steeper in one
meridian, resulting in greater
refractive power in one axis. In
most cases astigmatism arises from
the cornea, but it can also be due
to the lens or even tilting of the
retina.
A simple explanation for corneal
astigmatism is that the cornea is
shaped more like a rugby ball than
a soccer ball. Rather than a single
sharp focal point, the two axes
create two separate focal lines in
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HOW TO TREAT Refractive eye disorders
from page 28
separate planes in different orientations. Between the two focal
lines, a shape called a ‘conoid of
Sturm’ is formed. Cross sections
between the two focal lines will
demonstrate elliptical shapes,
except at one point, where a ‘circle
of least confusion’ exists. When
assessing and correcting for refractive error, correction of defocus
places the circle of least confusion
on the retina, and correction of
astigmatism collapses the conoid
down to a single focal point.
Myopia commonly presents in
school-aged children with difficulty
reading the blackboard or street
signs. It slowly, but progressively
worsens and then stabilises by the
late teens or early 20s.
In childhood, hypermetropia
may result in a (secondary) convergent squint (accommodative
esotropia); as the eyes accommodate excessively to produce a clear
image, there is more drive for the
eyes to converge. It may present
In childhood,
hypermetropia may
result in a (secondary)
convergent squint.
Figure 3: Images showing simulated blur with different types of refractive error. A: Emmetropia. Distant target is in
clear focus. B: Myopia. Distant targets are blurred, but nearer objects are clearer. C: Hypermetropia. Distant and near
objects are both blurred, but distant targets are clearer. D: Astigmatism. Objects appear distorted and elongated in
one axis (in this example, objects are elongated vertically).
B
A
later in life, manifesting in a low
hypermetrope as they lose accommodative power (presbyopia).
Some refractive errors will be
detected with appropriate school
screening programs.
Without access to trial lenses, an
inexpensive and rapid test that can
be performed to detect refractive
error is to check visual acuity (VA)
with and without a pinhole
occluder. A pinhole cuts out peripheral light rays to improve depth of
focus, and can correct up to 4D of
refractive error. Thus, improvement
with pinhole suggests a refractive
cause of reduced vision. However,
it is important to stress that
improvement to 6/6 vision does not
exclude serious comorbid ocular
pathology, particularly glaucoma,
which causes loss of visual field.
Regular vs irregular
astigmatism
C
D
Figure 4: A: The topographer is a machine that analyses astigmatism. B: Topographic patterns of astigmatism. 1)
Regular astigmatism (orthogonal symmetric). The opposing hemimeridians are of equal power, and their axes are
aligned. This type of astigmatism corrects well with spectacles. 2) Orthogonal, non-symmetric. The inferior cornea
has greater refractive power (indicated by a warmer red colour), but the axes of the hemimeridians are aligned.
3) Non-orthogonal, symmetric. The hemimeridians have equal power, but their axes are misaligned by more than 20°.
4) Non-orthogonal, non-symmetric. The hemimeridians have unequal power, and the axes are misaligned. C: Corneal
topography (anterior curvature maps) of a patient with bilateral corneal ectasia, worse in the right eye (OD), than the
left eye (OS). Both eyes show non-orthogonal, non-symmetric astigmatism, with the inferior cornea being steeper.
The changes are more subtle in the left eye.
Higher order aberrations
B
A
In addition to using trial lenses in
performing a spectacle test (refraction), astigmatism can be further
characterised by performing
corneal topography, which produces coloured corneal maps that
help to determine the shape of the
cornea. Figure 4 shows patterns of
regular and irregular astigmatism.
Regular astigmatism can be corrected with spectacles, and is
common and naturally occurring
in the cornea.
Irregular astigmatism is produced when the axes of astigmatism are not perpendicular (nonorthogonal), or there is asymmetry
in the refractive power across the
axes. Common causes of irregular
astigmatism are keratoconus, penetrating corneal trauma, and severe
corneal infections.
Mild irregular astigmatism can
be partially corrected with spectacles. However, moderate or severe
irregular astigmatism may require
hard contact lenses (which create
a regular optical surface on top of
the irregular corneal surface) or
corneal surgery for correction.
Higher order aberrations (HOA)
describe complex optical irregularities that account for the residual
1
2
3
optical aberrations after correcting for the lower order aberrations
of defocus (sphere) and astigmatism (cylinder).
For many patients, HOA are
small in magnitude and not visually significant, resulting in excellent spectacle-corrected vision.
Although they are overshadowed by the profound effects of
uncorrected lower order aberrations, greater degrees of HOA can
produce significant degradation in
visual quality and such aberrations
are usually increased with a larger
pupil size. They can produce visual
symptoms such as starbursts,
ghosting of images, halos and
monocular diplopia. Tilting a pair
of spectacles to look through the
edges of the lenses can simulate
some of the effects of HOA.
One example of HOA is coma,
which is named because it produces
a comet-like shape instead of a
point focus of light. It is experienced by patients with increased in
keratoconus and corneal grafts.
Another example of HOA is spherical aberration, which results from
light rays being refracted more (or
less) from the periphery of an optical component than the centre. In
some cases, spherical aberration can
be beneficial to patients by increasing the depth of focus. Figure 5
shows examples of HOAs.
HOA can be measured by a
variety of different clinical instruments. They are then processed by
mathematical systems (either
Zernike or Fourier analysis) to
quantify and display these aberrations. Depending on the purpose,
HOA can be measured from the
anterior and posterior corneal surface, from the whole cornea, or
from the whole eye.
Modern corneal laser refractive
surgery can take these HOA into
account, and lasers can be programmed to either treat these
HOA (wavefront-guided surgery)
or to minimise the HOA (wavefront-optimised surgery).5
HOA may also be measured
when planning intraocular refractive surgery and intraocular lens
choice.
cont’d page 32
Figure 5: Examples and diagrams of higher order aberrations (spherical
aberration and coma). A: Example of effect of total higher order abberations
on a point source of light in a keratoconic eye (point spread function). B:
Point spread function for the same keratoconic eye demonstrating the effect
only of coma (horizontal and vertical), a third-order abberation. C: Point
spread function showing the effect of spherical abberation in a keratoconic
eye.
A
4
C
E
B
C
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HOW TO TREAT Refractive eye disorders
Accommodation and presbyopia
ACCOMMODATION is the
change in focus from a distant to a
near target as a result of a change
in the curvature of the crystalline
(natural) lens. It is a key component of the triad of responses to a
near visual stimulus (the near
response), which also includes convergence of the visual axes and
pupil constriction (miosis).
Miosis creates a pinhole-type
effect of cutting out peripheral light
rays, and increases the depth of
focus to aid near vision. Although
linked to miosis in the near
response, accommodation does not
refer to changes in pupil size, but
rather to the increase in refractive
power of the eye associated with
changes in the shape of the crystalline lens.
The zonules (ligaments) are
under tension when the ciliary
muscle is in a relaxed state for distance focus. Contraction of the ciliary muscle causes relaxation of the
zonules, causing the lens to become
thicker and increase its refractive
power, to allow clear viewing of
near objects.
Progressive loss of accommodative power follows a relatively predictable course and leads to presbyopia, which is the reduced ability to
change focus for near objects
(figure 6).
Figure 6: Diagram showing refractive effect of presbyopia (distant and near object). A: Effect of uncorrected presbyopia demonstrating blurred vision for near
tasks such as reading. B: Presbyopia with appropriate near correction demonstrating clear vision for reading.
A
B
Children have about 20D of
accommodative power (5cm near
point), and this diminishes to
0.5D by age 60 (2m near point).
Typical symptoms of presbyopia include asthenopia (eyestrain), headache, blurred near
vision (and holding reading material further away), and difficulty
reading in poor light. Symptoms
typically become worse with sustained near activities as a result
of accommodative fatigue.
Most emmetropes will become
symptomatic of presbyopia in
their 40s. However, hypermetropes will become symptomatic
earlier. Low myopes experience
presbyopia differently, for example, a -2.5D myope is focused for
40cm, and will likely remove their
distance glasses to read.
Importantly, many patients are
unaware their symptoms are a
result of presbyopia, even if they
understand the concept of presbyopia. A simple and inexpensive
way to differentiate presbyopic
symptoms in a patient with good
unaided distance vision is to test
reading with and without a pair
of readymade reading glasses.
Although there is no reliable or
proven method to restore the
accommodative power of the
crystalline lens, or replicate its
accommodative function with an
IOL, surgical options for presbyopia are expanding rapidly and
can greatly benefit patients,
although they need to be tailored
to the individual.
but become contact lens intolerant
may be unhappy with their spectacle-corrected vision. For example, high myopes may become
aware of reduced image size and
an edge effect from their spectacles. Specific occupational requirements, such as acting, or sporting
activities, may prevent spectacle
wear as an option.
Contact lens-related microbial
keratitis (corneal infection),
although occurring in a minority
of contact lens users (about one in
2000 annually), is also a significant
trigger for patients desiring refractive surgery. Many patients who
have severe keratitis do not want to
go back to contact lens wear. A
broken pair of spectacles, for
example after a child pulls it off
their parent’s face, or a lost contact lens may be the final straw for
patients, that triggers them to seek
refractive surgery.
Some patients — particularly
those with irregular astigmatism
from corneal disorders, such as keratoconus or astigmatism after
corneal graft — may have difficulty
wearing hard contact lenses, and
therefore, may require a therapeutic surgical option to improve
vision.
be used in patients with thinner
corneas, eliminates the possibility
of flap-related complications, and
has less tendency to produce dryeye symptoms.
Surface ablation programmed
from corneal topography (topographic guided PRK) can also be
used to treat irregular corneas.
Figure 7 (page 34) demonstrates
the difference between LASIK and
surface ablation.
surgery to treat extreme refractive
error (bioptics). Unlike corneal
laser surgery, the refractive effect
of a phakic IOL can be reversed by
removing the lens.
Phakic IOLs are available in
different designs and can be
placed in the posterior chamber
(between the iris and the crystalline lens), fixated to the anterior surface of the iris, or placed
in the anterior chamber. Each lens
design has different advantages
and potential complications.
Why do patients seek refractive surgery?
A COMMON misconception
about refractive surgery is that all
patients seeking it are young, active
people who want refractive surgery
to pursue their outdoors lifestyle.
Although this is definitely true of
some, the range of patients desiring
refractive surgery is a lot broader,
and their motivating factors vary,
depending on their age, work environment, sports, hobbies and visual
requirements. Many patients who
seek refractive surgery want to be
independent of spectacles or contact lenses.
Correcting a prepresbyopic
patient to emmetropia can achieve
this outcome, although they need
to understand they will develop
presbyopic symptoms years later.
It is not uncommon for presbyopic patients to present with difficulty using bifocal, trifocal or progressive multifocal spectacles
(‘varifocals’), and desire an
improvement in their unaided distance vision, so they can use singlevision reading glasses.
Common difficulties with bifocal or varifocal glasses include difficulty walking on uneven surfaces
or climbing up and down stairs
(because these activities require
looking through the ‘add’ of the
lower segment, which is focused
for near).
Although some patients consider
refractive surgery for cosmetic reasons, especially if they have a high
prescription with thick lenses, successful contact lens wear does
allow patients to be less dependent
on spectacles. However, difficulty
with contact lens wear, or inconvenience of maintaining good contact lens hygiene is a frequent
reason for presentation.
There may be contact lens intolerance, ocular surface irritation,
dryness or redness. Patients who
are habituated to contact lens wear,
Overview of types of refractive surgery
REFRACTIVE surgical procedures may be performed on the
cornea (incisional techniques,
laser ablation, or implantation of
intracorneal ring segments), or
may involve implanting an IOL.
This can be done leaving the crystalline lens intact (phakic IOL),
or by replacing the crystalline lens
(RLE or cataract surgery).
Incisional corneal surgery for
myopia (eg, radial keratotomy) or
hypermetropia (hexagonal keratotomy) has essentially been abandoned because of better results
with laser refractive surgery. Incisional corneal surgery for astigmatism is still commonly performed (eg, after corneal graft
surgery). It is possible to correct
astigmatism at the time of
cataract surgery with corneal incisions. However, astigmatism-correcting intraocular lenses (toric
IOLs) may be the preferred
approach.
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| Australian Doctor | 18 May 2012
Corneal laser refractive surgery
The excimer laser very accurately
ablates corneal stroma to allow
reshaping of the cornea to treat
refractive error. For myopia, the
central cornea is flattened by ablation to reduce its power, and for
hypermetropia, the central cornea
is made steeper by ablating more
peripherally. Astigmatism is treated
by differentially removing tissue in
perpendicular planes.
Excimer laser ablation can be
performed in two ways: either on
the corneal stromal surface (variations of this technique are grouped
under the term ‘surface ablation’),
or under a stromal flap (laserassisted in situ keratomileusis,
[LASIK]).
Surface ablation was the first
type of excimer laser corneal surgery to be performed. There are
multiple methods of removal of the
corneal epithelium before the laser
ablation, which account for most
of the variation in technique
names. These include:
1. Using a rotating brush or a
blade (photorefractive keratectomy [PRK]).
2. Application of alcohol to loosen
the epithelium (laser subepithelial keratomileusis; LASEK).
3. Motorised blade (epi-LASIK).
In LASIK, either a mechanical
blade (microkeratome) or a femtosecond laser is used to create a
corneal flap. The flap is lifted so
laser ablation can be performed on
the underlying stroma, and then it
is replaced back into position.
Hence, the epithelium is intact
postoperatively, except at the edges
of the flap.
LASIK is the most commonly
performed laser refractive procedure because visual recovery is
rapid, and there is significantly less
postoperative discomfort, allowing
an earlier return to normal activities. However, surface ablation can
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Phakic IOL
Phakic IOL implantation is an
alternative to corneal laser refractive surgery, both of which preserve
the accommodating function of the
crystalline lens. However, phakic
IOL implantation has the advantage of being able to treat a wider
range of refractive error than laser
procedures (eg, up to -20D of
myopia and 6D of astigmatism).
It is also possible to combine a
phakic IOL with laser refractive
Refractive lens exchange
RLE or ‘clear lens extraction’
involves removal of the clear crystalline lens and replacement with
an IOL. Laser refractive surgery
and phakic IOLs preserve accommodation. Hence RLE is more
commonly performed in presbyopic
patients where accommodation is
already compromised, or those not
suitable for other refractive proce-
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dures (eg, in patients with high
hypermetropia and an anterior
chamber too shallow for a phakic
IOL).
The optical quality of IOLs is
excellent, but they cannot exactly
replicate the accommodative function of the crystalline lens. There
are multiple strategies that can be
adopted to address presbyopia,
reducing, or in some cases, eliminating the need for spectacles for
near correction.
In general, each strategy involves
some level of compromise between
quality of vision, degree of spectacle independence, possibility of
visual effects such as glare and
halos, and potential for reversibility.
‘Monovision’ or ‘blended vision’
is where monofocal IOLs are used
with one eye focused for distance
and the other for a nearer target,
resulting in an increased range of
focus with binocular viewing.
‘Presbyopia-correcting’ intraocular lenses, are either multifocal (creating a distance and near focus,
using diffraction or refraction), or
aim to produce some accommodative effect.
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Figure 7: Diagram demonstrating difference between LASIK and surface ablation (both demonstrate myopic
treatment, where the central cornea is flattened). In LASIK (left), a stromal flap is created and then reflected, prior to
excimer laser ablation, and then repositioned. In surface ablation (right), excimer laser ablation is performed on the
stromal surface, after the corneal epithelium has been removed.
Flap is reflected
Laser treatment
Before
Laser treatment
Original curvature
New curvature
After
Original curvature
New curvature
Figure 8: Corneal collagen cross-linking. Figure shows calibration of the UV light source prior to application of UV
light to the riboflavin saturated cornea.
Secondary intraocular lens
implant
In patients who have had previous
cataract surgery, but are either not
satisfied with the refractive outcome, or want reduced spectacle
dependence, secondary intraocular
lenses (monofocal or multifocal)
may be an appropriate refractive
alternative to IOL exchange or
corneal refractive surgery.
Corneal collagen cross-linking
Corneal collagen cross-linking (CXL)
is used to strengthen the cornea, by
inducing cross-linking between collagen fibres, using a combination of
topical riboflavin and UVA light
exposure (figure 8).
The most widely used application is for keratoconus, with the
aim of stopping or reducing the
progression of the disease. CXL
does induce some refractive
changes by corneal flattening.
Although the goal of treatment is
to stabilise the cornea, it may be
combined with other refractive procedures which aim to improve
vision.
Intracorneal ring segments
Intracorneal ring segments are primarily used to treat patients with
irregular astigmatism, in particular, those whose vision corrects
well with hard contact lenses, but
who are unable to tolerate them.
These implants add structure to
the cornea and are potentially
reversible.
Assessing the patient for refractive surgery
History
Examination and investigations
IMPORTANT considerations are
the presenting complaint, motivation for and expectations of refractive surgery, along with the social
history, particularly with regards
to visual requirements of occupation, hobbies and sports, and risk
of ocular trauma. Ocular and general medical history is relevant,
particularly with regard to potential contraindications.
This will differ according to which
procedure is planned, and may
include a range of assessments such
as:
• Unaided distance and near VA,
refraction and best-corrected VA.
• Ocular dominance, motility and
pupil examination.
• Slit lamp examination and intraocular pressure measurement.
• Corneal topography, keratometry
(corneal curvature), pachymetry
(corneal thickness).
• Wavefront aberrometry (used particularly to assess higher order aberrations).
• Cycloplegic refraction and dilated
fundus examination.
• Corneal endothelial cell count.
• Axial length measurement.
Contraindications
An important contraindication for
any refractive surgical procedure is
unrealistic patient expectations —
either related to the refractive outcome, or to the effect of the surgery
on their life.
An unstable refraction (change of
>0.5D in a year), pregnancy or breastfeeding will generally preclude
patients from surgery, and refractive
surgery typically is not considered for
those under the age of 18. The box,
right, details the different contraindications for refractive surgery.
Contraindications for refractive surgery
Potential contraindications for excimer laser refractive surgery.
• Dry-eye syndrome, neurotrophic cornea
• Previous ocular Herpes simplex infection or Herpes zoster opthalmicus6
• Corneal dystrophy or ectasia; thin cornea/abnormal corneal topography
• Glaucoma7
• Medication (isotretinoin or amiodarone)
• Connective tissue diseases (rheumatoid arthritis, SLE, Sjögren syndrome,
Wegener’s granulomatosis)
References
1. Cochrane GM, et al.
Management of refractive errors.
BMJ 2010; 340:c1711.
2. Garamendi E, et al. Changes in
quality of life after laser in situ
keratomileusis for myopia.
Journal of Cataract and
Refractive Surgery 2005;
31:1537-43.
3. Ieong A, et al. Quality of life in
high myopia before and after
implantable Collamer lens
implantation. Ophthalmology
2010; 117:2295-2300.
4. Ieong A, et al. Quality of life in
high myopia: implantable
Collamer lens implantation
versus contact lens wear.
Ophthalmology 2009; 116:27580.
5. Fares U, et al. Efficacy,
predictability, and safety of
wavefront-guided refractive laser
treatment: metaanalysis. Journal
of Cataract and Refractive
Surgery 2011; 37:1465-75.
6. de Rojas Silva V, et al. Laser in
situ keratomileusis in patients
with a history of ocular herpes.
Journal of Cataract and
Refractive Surgery 2007;
33:1855-59.
7. Bashford KP, et al.
Considerations of glaucoma in
patients undergoing corneal
refractive surgery. Survey of
Ophthalmology 2005; 50:24551.
8. Nanavaty MA, Daya SM.
Refractive lens exchange versus
phakic intraocular lenses.
Current Opinion in
Ophthalmology 2012; 23:54-61.
9. Alio JL. Lens surgery (cataract
and refractive lens exchange) and
retinal detachment risk in
myopes: still an issue? British
Journal of Ophthalmology 2011;
95:301-03.
10. Barsam A, Allan BD. Metaanalysis of randomized
controlled trials comparing
excimer laser and phakic
intraocular lenses for myopia
between 6.0 and 20.0 diopters.
Cornea 2012; 31:454-61.
11. Huang D, et al. Phakic
intraocular lens implantation
for the correction of myopia: a
report by the American
Academy of Ophthalmology.
Ophthalmology 2009;
116:2244-58.
12. de Vries NE, et al.
Dissatisfaction after
implantation of multifocal
intraocular lenses. Journal of
Cataract and Refractive Surgery
2011; 37:859-65.
13. Sharma DP, et al. Microbial
keratitis after corneal laser
refractive surgery. Future
Microbiology 2011; 6:819-31.
Online resources
• American Academy of
Opthalmology, Eyecare America
— information on refractive
surgery: www.geteyesmart.
org/eyesmart/ glasses-contactslasik/index.cfm
• Poorly controlled diabetes
• Thyroid eye disease
Potential contraindications for phakic IOL implantation.
• Ocular disease such as cataract, glaucoma, uveitis and iris abnormalities
• Corneal endothelial dysfunction, or low endothelial cell count
• Anterior segment anatomy not suitable for phakic IOL
Potential contraindications for refractive lens exchange.
• Suitability for alternative refractive surgical procedure8
• High axial myopia, absence of a posterior vitreous detachment (PVD)
(increases risk of retinal detachment)9 . Note that presence of a PVD results in
a lower risk of retinal detachment.
• Comorbid ocular disease
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HOW TO TREAT Refractive eye disorders
Results of refractive surgery
IT is important that patients have
realistic expectations of visual outcomes; refractive surgery is very successful in reducing spectacle dependence, especially for sporting or social
activities, but patients should not
expect that a refractive procedure will
guarantee a perfect optical result.
A small proportion of patients may
require a second procedure to optimise the refractive outcome. Except in
the case of refractive cataract surgery,
patients should generally not expect
surgery will improve on their bestcorrected preoperative vision, (ie, the
outcome will generally not be better
than that achieved with optimal use
of spectacles or contact lenses). How-
ever, gains in visual acuity are becoming more frequently reported in trials.5
Efficacy and safety of corneal laser
refractive surgery is continually
improving. Although visual recovery
after LASIK is faster, and refractive
stability occurs earlier, final visual
results of LASIK and surface ablation
are similar.
Contemporary LASIK results show
67-100% of patients achieve unaided
vision of 6/6 or better, depending on
the series.5 For myopia >8D, phakic
IOL implantation may offer better
predictability, stability and visual
quality than corneal laser refractive
surgery.10,11
Refractive lens exchange involves
an expanding array of IOL designs
and strategies to manage presbyopia,
and can offer patients good unaided
distance and near vision.8 However,
good patient selection, counselling
and postoperative management is
important in achieving patient satisfaction.12
Complications
Two potentially serious, but rare
complications after corneal laser
refractive surgery are microbial keratitis, and corneal ectasia (thinning
and irregularity of the cornea resulting in bulging of the cornea and progressive myopic astigmatism).13
LASIK flap-related complications
are less common since the introduction of the femtosecond laser. However, the flap can be dislocated by
trauma even in the late postoperative period. Interface inflammation
after LASIK (diffuse lamellar keratitis) and haze after surface ablation
are other potential complications.
Dry eye after LASIK is relatively
common, but is usually transient.
The risk profile of phakic intraocular lenses is different because it is an
intraocular procedure, and therefore
has a low risk of endophthalmitis
(about one in 6000). Other risks are
endothelial cell loss, cataract formation, secondary glaucoma, iris atrophy and traumatic dislocation.11
Refractive lens exchange has higher
risks of cystoid macular oedema and
retinal detachment. However, it obviates the potential requirement for
later cataract surgery.8
Management and referral of post-refractive surgery patients
IN assessing ocular or visual complaints, a past history of refractive
surgery is important to elicit. This
may not be volunteered after routine refractive surgery (eg, prior
uncomplicated LASIK).
Prior refractive surgery can certainly alter management. For
example, ocular trauma resulting
in dislocation or folds in the LASIK
flap (rather than a corneal abra-
sion) can occur even years after
successful surgery. Early referral for
flap repositioning would be appropriate in this situation.
Refractive surgery patients will
usually notify their surgeon if they
are experiencing early postoperative problems. However, they may
present with an acute problem to
their GP or an ED, especially after
trauma.
In the early postoperative period,
infection (microbial keratitis after
laser refractive surgery, or endophthalmitis after intraocular surgery),
although rare, is a potentially serious
complication which needs urgent
referral. This may be indicated by
worsening vision, pain, photosensitivity and redness.
Patients may present much later
with visual complaints related to
How to Treat Quiz
Refractive eye disorders
— 18 May 2012
1. Which THREE statements regarding
refraction in the eye are correct?
a) The lens is the most powerful refractive
component of the eye
b) The cornea contributes about two-thirds of
the refractive power of the eye
c) The anterior surface of the cornea has greater
refractive power than the posterior surface
d) The crystalline (natural) lens has a biconvex
shape and higher refractive index than the
vitreous, causing light rays to converge on the
retina
2. Which TWO statements regarding types of
refractive errors are correct?
a) Most refractive errors are due to lower order
abnormalities, defocus (sphere) and
astigmatism (cylinder)
b) Uncorrected lower order aberrations of
refraction have minimal effects on visual
acuity
c) A spectacle test (refraction) assesses both
lower order and higher order aberrations of
refraction
d) In correcting refractive errors, a positive lens
causes convergence of light
3. In regard to myopia which TWO
statements are correct?
a) In a myopic eye, light rays form an image
behind the retina
b) The usual cause of myopia is an eye that is
their refractive surgery. One example
is the development of posterior capsular opacification (caused by proliferation of remnant crystalline lens
cells behind the IOL) after refractive
lens exchange, which can be treated
with a YAG laser capsulotomy.
Another example is corneal ectasia
after LASIK, which results in increasing myopic astigmatism, characterised by gradual blurring of vision.
Examination of the patient
should include unaided and pinhole VA in each eye, and if available, slit lamp exam with fluorescein staining if presenting after
ocular trauma. If in any doubt
about how the patient should be
managed, the GP should call the
refractive surgeon for advice,
especially if there is severe pain
or reduction in vision.
INSTRUCTIONS
Complete this quiz online and fill in the GP evaluation form to earn 2 CPD or PDP points. We no longer accept
quizzes by post or fax.
The minimum mark required to obtain points is 80%.
ONLINE ONLY
www.australiandoctor.com.au/cpd/ for immediate feedback
longer than normal
c) Having a flat cornea can also cause myopia
d) Myopic eyes need a negative lens for
correction
4. Which THREE statements regarding
astigmatism are correct?
a) Astigmatism can be compensated for by extra
accommodative effort
b) Astigmatism is due to greater refractive power
in one axis of the eye
c) Most cases of astigmatism are due to the
shape of the cornea
d) Regular astigmatism can be corrected with
glasses
5. Which TWO statements about irregular
astigmatism are correct?
a) It is due to the axes of astigmatism being
perpendicular to each other, with symmetrical
power across the axes
b) Causes include keratoconus and corneal
infections
c) Glasses cannot provide any correction
d) For moderate and severe cases, hard contact
lenses may be of assistance
6. John, 47, presents for an eye check. He has
never worn glasses, but reports gradual
deterioration in his vision over the past few
years. Which THREE statements are
correct?
a) If you find John’s visual acuity improves with
a pinhole occluder, this suggests he has a
refractive problem
b) If John’s vision corrects to 6/6 with a
pinhole occluder, you can reassure him
glasses will be all that is required
c) If John’s distance vision is good, but his
near vision is deteriorating, this is most
likely due to presbyopia
d) Testing John’s reading with and without the
use of off-the-shelf reading glasses can help
determine whether John’s symptoms relate
to age-related changes in the eye
7. Which THREE features of presbyopia are
correct?
a) Earlier onset in hypermetropes
b) A patient with low myopia may find they
need to remove their glasses in order to read
c) It is due to the thickening of the lens with age
d) Symptoms are worse with prolonged near
activities
8. Which TWO statements regarding laserassisted in situ keratomileusis (LASIK)
surgery are correct?
a) LASIK is the most frequently performed
laser refractive procedure
b) LASIK involves excimer laser ablation
on the corneal surface
c) A patient with a thin cornea would
be advised to have LASIK rather than
surface ablation
d) LASIK has the advantage of rapid visual
recovery
9. Which THREE statements regarding
refractive surgery are correct?
a) Vision is generally not expected to improve
beyond what the patient can achieve with
optimal use of glasses or contact lens
b) Laser refractive surgery and phakic
intraocular lenses (IOL) preserve
accommodation
c) Refractive surgery should not be undertaken
until refraction is stable
d) Visual outcomes of LASIK are superior to
those of phakic intraocular lenses in high
myopia
10. Which THREE statements regarding the
complications of refractive surgery are
correct?
a) Watery eyes are a common side effect of
LASIK surgery
b) Possible complications of corneal laser
refractive surgery include microbial keratitis
and corneal ectasia
c) Dislocation of the LASIK flap can occur long
after surgery due to ocular trauma
d) In the early postoperative period, a patient
with worsening vision, pain and
photosensitivity requires urgent assessment
for ocular infection
CPD QUIZ UPDATE
The RACGP requires that a brief GP evaluation form be completed with every quiz to obtain category 2 CPD or PDP points for the 2011-13 triennium. You can
complete this online along with the quiz at www.australiandoctor.com.au. Because this is a requirement, we are no longer able to accept the quiz by post or
fax. However, we have included the quiz questions here for those who like to prepare the answers before completing the quiz online.
HOW TO TREAT Editor: Dr Marcela Cox
Co-ordinator: Agilene De Villa
Quiz: Dr Marcela Cox
NEXT WEEK The psychosexual aspects of male genital dermatoses is a frequent source of morbidity. How to Treat next weeks will focus on common skin diseases localised on the male genitalia, as well as less
common, but important genital-specific skin conditions. The authors are Dr Adrian Lim, dermatologist and phlebologist; board member of the Australasian College of Phlebology; director of training of the Australasian College of Dermatologists; consultant dermatologist to the Royal North Shore Hospital, Sydney and in private practice in Sydney and Kogarah, NSW; Associate Professor Stephen Shumack,
dermatologist; board member of the Australasian College of Phlebology, Skin and Cancer Foundation Australia, and Epiderm; consultant dermatologist to the Royal North Shore Hospital, Sydney; medical director
of the Skin and Cancer Foundation Australia; and in private practice in Sydney and Kogarah, NSW.
34
| Australian Doctor | 18 May 2012
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