Myopia

Source:  Myopia    Tag:  myopic macular degeneration treatment

Myopia or short-sightedness is a type of refractive
error in which parallel rays of light coming from infinity
are focused in front of the retina when
accommodation is at rest

Etiological classification
1. Axial myopia results from increase in anteroposterior
length of the eyeball. It is the
commonest form.
2. Curvatural myopia occurs due to increased
curvature of the cornea, lens or both.
3. Positional myopia is produced by anterior
placement of crystalline lens in the eye.
4. Index myopia results from increase in the refractive
index of crystalline lens associated with nuclear
sclerosis.
5. Myopia due to excessive accommodation occurs
in patients with spasm of accommodation.
Clinical varieties of myopia
1. Congenital myopia
2. Simple or developmental myopia
3. Pathological or degenerative myopia
4. Acquired myopia which may be: (i) post-traumatic;
(ii) post-keratitic; (iii) drug-induced, (iv)
pseudomyopia; (v) space myopia; (vii) night
myopia; and (viii) consecutive myopia.
1. Congenital myopia
Congenital myopia is present since birth, however, it
is usually diagnosed by the age of 2-3 years. Most of
the time the error is unilateral and manifests as
anisometropia. Rarely, it may be bilateral. Usually
the error is of about 8 to 10 which mostly remains
constant. The child may develop convergent squint
in order to preferentially see clear at its far point

(which is about 10-12 cms). Congenital myopia may
sometimes be associated with other congenital
anomalies such as cataract, microphthalmos, aniridia,
megalocornea, and congenital separation of retina.
Early correction of congenital myopia is desirable.
2. Simple myopia
Simple or developmental myopia is the commonest
variety. It is considered as a physiological error not
associated with any disease of the eye. Its prevalence
increases from 2% at 5 years to 14% at 15 years of
age. Since the sharpest rise occurs at school going
age i.e., between 8 year to 12 years so, it is also called
school myopia.
Etiology. It results from normal biological variation
in the development of eye which may or may not be
genetically determined. Some factors associated with
simple myopia are as follows:
Axial type of simple myopia may signify just a
physiological variation in the length of the eyeball
or it may be associated with precocious
neurological growth during childhood.
Curvatural type of simple myopia is considered
to be due to underdevelopment of the eyeball.
Role of diet in early childhood has also been
reported without any conclusive results.
Role of genetics. Genetics plays some role in the
biological variation of the development of eye, as
prevelance of myopia is more in children with
both parents myopic (20%) than the children with
one parent myopic (10%) and children with no
parent myopic (5%).
Theory of excessive near work in childhood was
also put forward, but did not gain much
importance. In fact, there is no truth in the folklore
that myopia is aggravated by close work, watching
television and by not using glasses.
Clinical picture
Symptoms
Poor vision for distance (short-sightedness) is
the main symptom of myopia.
Asthenopic symptoms may occur in patients with
small degree of myopia.
Half shutting of the eyes may be complained by
parents of the child. The child does so to achieve
the greater clarity of stenopaeic vision.
Signs
Prominent eyeballs. The myopic eyes typically
are large and somewhat prominent.
Anterior chamber is slightly deeper than normal.
Pupils are somewhat large and a bit sluggishly
reacting.
Fundus is normal; rarely temporal myopic crescent
may be seen.
Magnitude of refractive errror. Simple myopia
usually occur between 5 and 10 year of age and
it keeps on increasing till about 18-20 years of
age at a rate of about –0.5 ± 0.30 every year. In
simple myopia, usually the error does not exceed
6 to 8.
Diagnosis is confirmed by performing retinoscopy
(page 547).
3. Pathological myopia
Pathological/degenerative/progressive myopia, as the
name indicates, is a rapidly progressive error which
starts in childhood at 5-10 years of age and results in
high myopia during early adult life which is usually
associated with degenerative changes in the eye.
Etiology. It is unequivocal that the pathological
myopia results from a rapid axial growth of the eyeball
which is outside the normal biological variations of
development. To explain this spurt in axial growth
various theories have been put forward. So far no
satisfactory hypothesis has emerged to explain the
etiology of pathological myopia. However, it is
definitely linked with (i) heredity and (ii) general
growth process.
1. Role of heredity. It is now confirmed that genetic
factors play a major role in the etiology, as the
progressive myopia is (i) familial; (ii) more common in
certain races like Chinese, Japanese, Arabs and Jews,
and (iii) uncommon among Negroes, Nubians and
Sudanese. It is presumed that heredity-linked growth
of retina is the determinant in the development of
myopia. The sclera due to its distensibility follows
the retinal growth but the choroid undergoes
degeneration due to stretching, which in turn causes
degeneration of retina.
2. Role of general growth process, though minor,
cannot be denied on the progress of myopia.
Lengthening of the posterior segment of the globe
commences only during the period of active growth
and probably ends with the termination of the active
growth. Therefore, the factors (such as nutritional

deficiency, debilitating diseases, endocrinal
disturbances and indifferent general health) which
affect the general growth process will also influence
the progress of myopia.
The etiological hypothesis for pathological myopia
is summarised in Figure 3.25:
Genetic factors General growth process
(play major role) (plays minor role)

More growth of retina

Stretching of sclera
↓ ↓
Increased axial length

Degeneration of choroid Features of
↓ pathological
Degeneration of retina myopia

Degeneration of vitreous
Fig. 3.25. Etiological hypothesis for pathological myopia.
Clinical picture
Symptoms
1. Defective vision. There is considerable failure in
visual function as the error is usually high.
Further, due to progressive degenerative changes,
an uncorrectable loss of vision may occur.
2. Muscae volitantes i.e., floating black opacities in
front of the eyes are also complained of by many
patients. These occur due to degenerated liquified
vitreous.
3. Night blindness may be complained by very high
myopes having marked degenerative changes.
Signs
1. Prominent eye balls. The eyes are often
prominent, appearing elongated and even
simulating an exophthalmos, especially in unilateral
cases. The elongation of the eyeball mainly affects
the posterior pole and surrounding area; the part
of the eye anterior to the equator may be normal
(Fig. 3.26).
2. Cornea is large.
3. Anterior chamber is deep.
4. Pupils are slightly large and react sluggishly to
light.
5. Fundus examination reveals following
characteristic signs :
(a) Optic disc appears large and pale and at its
temporal edge a characteristic myopic crescent
is present (Fig. 3.27). Sometimes peripapillary
crescent encircling the disc may be present,
where the choroid and retina is distracted
away from the disc margin. A super-traction
crescent (where the retina is pulled over the
disc margin) may be present on the nasal
side.
(b) Degenerative changes in retina and choroid
are common in progressive myopia (Fig. 3.28).
These are characterised by white atrophic
patches at the macula with a little heaping up
of pigment around them. Foster-Fuchs' spot
(dark red circular patch due to sub-retinal
neovas-cularization and choroidal
haemorrhage) may be present at the macula.
Cystoid degeneration may be seen at the
periphery. In an advanced case there occurs
total retinal atrophy, particularly in the central
area.
(c) Posterior staphyloma due to ectasia of sclera
at posterior pole may be apparent as an
excavation with the vessels bending backward
over its margins.
(d) Degenerative changes in vitreous include:
liquefaction, vitreous opacities, and posterior
vitreous detachment (PVD) appearing as
Weiss' reflex.

6. Visual fields show contraction and in some cases
ring scotoma may be seen.
7. ERG reveals subnormal electroretinogram due to
chorioretinal atrophy.
Complications
(i) Retinal detachment; (ii) complicated cataract; (iii)
vitreous haemorrhage; (iv) choroidal haemorrhage (v)
Strabismus fixus convergence.
Treatment of myopia
1. Optical treatment of myopia constitutes
prescription of appropriate concave lenses, so
that clear image is formed on the retina (Fig. 3.29).
The basic rule of correcting myopia is converse
of that in hypermetropia, i.e., the minimum
acceptance providing maximum vision should be
prescribed. In very high myopia undercorrection
is always better to avoid the problem of near
vision and that of minification of images.

Modes of prescribing concave lenses are
spectacles and contact lenses. Their advantages
and disadvantages over each other are the same
as described for hypermetropia. Contact lenses
are particularly justified in cases of high myopia
as they avoid peripheral distortion and minification
produced by strong concave spectacle lens.
2. Surgical treatment of myopia is becoming very
popular now-a-days. For details see page 46.
3. General measures empirically believed to effect
the progress of myopia (unproven usefulness)

include balanced diet rich in vitamins and proteins
and early management of associated debilitating
disease.
4. Low vision aids (LVA) are indicated in patients
of progressive myopia with advanced
degenerative changes, where useful vision cannot
be obtained with spectacles and contact lenses.
5. Prophylaxis (genetic counselling). As the
pathological myopia has a strong genetic basis,
the hereditary transfer of disease may be
decreased by advising against marriage between
two individuals with progressive myopia.
However, if they do marry, they should not
produce children.

Refractive surgery of myopia
1. Radial keratotomy (RK) refers to making deep
(90 percent of corneal thickness) radial incisions in
the peripheral part of cornea leaving the central 4 mm
optical zone (Fig 3.37). These incisions on healing;
flatten the central cornea thereby reducing its
refractive power. This procedure gives very good
correction in low to moderate myopia (2 to 6 D).
Disadvantages. Note: Because of its disadvantages
RK is not recommended presently. (i) Cornea is
weakened, so chances of globe rupture following
trauma are more after RK than after PRK. This point is
particularly important for patients who are at high
risk of blunt trauma, e.g., sports persons, athletes
and military personnel. (ii) Rarely, uneven healing may
lead to irregular astigmatism. (iii) Patients may feel
glare at night.
2. Photorefractive keratectomy (PRK). In this
technique, to correct myopia a central optical zone of
anterior corneal stroma is photoablated using

excimer laser (193-nm UV flash) to cause flattening
of the central cornea (Fig. 3.38). Like RK, the PRK
also gives very good correction for –2 to –6 D of
myopia.
Disadvantages. Note: Because of its disadvantages
PRK is not recommended presently: (i) Postoperative
recovery is slow. Healing of the epithelial defect may
delay return of good vision and patient may
experience pain or discomfort for several weeks. (ii)
There may occur some residual corneal haze in the
centre affecting vision. (iii) PRK is more expensive
than RK.
3. Laser in-situ keratomileusis (LASIK). In this
technique first a flap of 130-160 micron thickness of
anterior corneal tissue is raised. After creating a
corneal flap midstromal tissue is ablated directly with
an excimer laser beam, ultimately flattening the cornea
(Fig. 3.39). Currently this procedure is being
considered the refractive surgery of choice for myopia
of up to – 12 D.

Patient selection criteria are:
Patients above 20 years of age,
Stable refraction for at least 12 months.
Motivated patient.
Absence of corneal pathology. Presence of ectasia
or any other corneal pathology and a corneal
thickness less than 450 mm is an absolute
contraindication for LASIK.
Advances in LASIK. Recently many advances have
been made in LASIK surgery. Some of the important
advances are:
Customized (C) LASIK. C-LASIK is based on the
wave front technology. This technique, in addition
to spherical and cylindrical correction, also
corrects the aberrations present in the eye and
gives vision beyond 6/6 i.e., 6/5 or 6/4
Epi-(E) LASIK. In this technique instead of
corneal stromal flap only the epithelial sheet is
separated mechanically with the use of a
customized device (Epiedge Epikeratome). Being
an advanced surface ablation procedure, it is
devoid of complications related to corneal stromal
flap.
Advantages of LASIK. (i) Minimal or no postoperative
pain. (ii) Recovery of vision is very early as compared
to PRK. (iii) No risk of perforation during surgery and
later rupture of globe due to trauma unlike RK.
(iv) No residual haze unlike PRK where subepithelial
scarring may occur. (v) LASIK is effective in
correcting myopia of – 12 D.

Disadvantages. 1. LASIK is much more expensive.
2. It requires greater surgical skill than RK and PRK.
3. There is potential risk of flap related complications
which include (i) intraoperative flap amputation,
(ii) wrinkling of the flap on repositioning,
(iii) postoperative flap dislocation/subluxation,
(iv) epithelization of flap-bed interface, and
(v) irregular astigmatism.
4. Extraction of clear crystalline lens (Fucala's
operation) has been advocated for myopia of –16 to
–18 D, especially in unilateral cases. Recently, clear
lens extraction with intraocular lens implantion of
appropriate power is being recommended as the
refractive surgery for myopia of more than 12 D.
5. Phakic intraocular lens or intraocular contact lens
(ICL) implantation is also being considered for
correction of myopia of >12D. In this technique, a
special type of intraocular lens is implanted in the
anterior chamber or posterior chamber anterior to the
natural crystalline lens.
6. Intercorneal ring (ICR) implantation into the
peripheral cornea at approximately 2/3 stromal depth
is being considered. It results in a vaulting effect that
flattens the central cornea, decreasing myopia. The
ICR procedure has the advantage of being reversible.
7. Orthokeratology a non-surgical reversible method
of molding the cornea with overnight wear unique
rigid gas permeable contact lenses, is also being
considered for correction of myopia upto –5D. It can
be used even in the patients below 18 year of age.