Amblyopia

Definition. Amblyopia, by definition, refers to a partial
loss of vision in one or both eyes, in the absence of
any organic disease of ocular media, retina and visual
pathway.
Pathogenesis. Amblyopia is produced by certain
amblyogeneic factors operating during the critical
period of visual development (birth to 6 years of age).
The most sensitive period for development of
amblyopia is first six months of life and it usually
does not develop after the age of 6 years.
Amblyogenic factors include :
Visual (form sense) deprivation as occurs in
anisometropia,
Light deprivation e.g., due to congenital cataract,
and
Abnormal binocular interaction e.g., in strabismus.
Types. Depending upon the cause, amblyopia is of
following types:
1. Strabismic amblyopia results from prolonged
uniocular suppression in children with unilateral
constant squint who fixate with normal eye.
2. Stimulus deprivation amblyopia (old term:
amblyopia ex anopsia) develops when one eye is
totally excluded from seeing early in life as, in
congenital or traumatic cataract, complete ptosis
and dense central corneal opacity.
3. Anisometropic amblyopia occurs in an eye
having higher degree of refractive error than the
fellow eye. It is more common in anisohypermetropic
than the anisomyopic children.
Even 1-2D hypermetropic anisometropia may
cause amblyopia while upto 3D myopic
anisometropia usually does not cause amblyopia.
4. Isoametropic amblyopia is bilateral amblyopia
occurring in children with bilateral uncorrected
high refractive error.
5. Meridional amblyopia occurs in children with
uncorrected astigmatic refractive error. It is a
selective amblyopia for a specific visual meridian.
Clinical characteristics of an amblyopic eye are:
1. Visual acuity is reduced. Recognition acuity is
more affected than resolution acuity.
2. Effect of neutral density filter. Visual acuity when
tested through neutral density filter improves by
one or two lines in amblyopia and decreases in
patients with organic lesions.
3. Crowding phenomenon is present in amblyopics
i.e., visual acuity is less when tested with multiple
letter charts (e.g., Snellen’s chart) than when
tested with single charts (optotype).
4. Fixation pattern may be central or eccentric.
Degree of amblyopia in eccentric fixation is
proportionate to the distance of the eccentric
point from the fovea.
5. Colour vision is usually normal, may be affected
in deep amblyopia with vision below 6/36.
Treatment of amblyopia should be started as early
as possible (younger the child, better the prognosis).
Occlusion therapy i.e., occlusion of the sound eye,
to force use of amblyopic eye is the main stay in the
treatment of amblyopia. However, before the
occlusion therapy is started, it should be ensured
that:
Opacity in the media (e.g., cataract), if any, should
be removed first, and
Refractive error, if any, should be fully corrected.
Simplified schedule for occlusion therapy depending
up on the age is as below:
Upto 2 years, the occlusion should be done in
2:1, i.e., 2 days in sound eye and one day in
amblyopic eye.
At the age of 3 years, 3:1,
At the age of 4 years, 4:1,
At the age of 5 years, 5:1, and
After the age of 6 years, 6:1
Duration of occlusion should be until the visual
acuity develops fully, or there is no further
improvement of vision for 3 months