Amblyopia or lazy eyes, by definition, refers to a partial reversible loss of vision in one or both eyes. In which no cause can be found by physical examination of the eye, i.e., there is absence of any organic disease of ocular media, retina and visual pathway.
Pathogenesis of Amblyopia or Lazy Eyes
Amblyopia is produced by certain amblyogenic factors operating during the critical period of visual development (birth to 6-7 years of age). During this period, the visual pathway continues to develop and brain learns to interpret the signals that come from the eye. If, for any reason, a young child cannot use one or both eyes normally, then the vision is not developed completely and the condition is called amblyopia. The most sensitive period for development of amblyopia is first six months of life and it usually does not develop after the age of 6-7 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 of Amblyopia or Lazy Eyes
Types. Depending upon the cause, amblyopia is of following types:
- Strabismic amblyopia results from prolonged uniocular suppression in children with unilateral constant squint who fixate with normal eye. Squint is the most common cause of amblyopia.
- Stimulus deprivation amblyopia (old term: amblyopia ex anopsia) develops when one eye excludes totally from seeing early in life as in congenital or traumatic cataract, complete ptosis and dense central corneal opacity.
- 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 1-2 D hypermetropic anisometropia may cause amblyopia while upto 3 D myopic anisometropia usually does not cause amblyopia.
- Isoametropic amblyopia is bilateral amblyopia occurring in children with bilateral uncorrected high refractive error.
- Meridional amblyopia occurs in children with uncorrected astigmatic refractive error. It is a selective amblyopia for a specific visual meridian.
Clinical characteristics of Amblyopia or Lazy Eyes
Clinical characteristics of an amblyopic eye are:
- Reduced visual acuity. Recognition acuity is more affected than resolution acuity.
- 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.
- Crowding phenomenon is present in amblyopics Le.. visual acuity is less when tested with multiple letter charts (e.g.. Snellen’s chart) than when tested with single charts (optotype).
- 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
- Colour vision is usually normal, but affects in deep amblyopia with vision below 6/36.
Treatment of Amblyopia or Lazy Eyes
Treatment of amblyopia or lazy eyes should be started as early as possible (younger the child, better the prognosis). Amblyopia therapy works best when initiated in young children under 3 years of age.
Occlusion of the sound (normal) 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, needs to have full refractive correction. Simplified schedule for occlusion therapy depending 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
- When the child is 4 years, the ratio is 4:1,
- By 5 it is 5:1, and
- 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.
Penalization i.e., blurring of vision of normal eye either by using atropine (atropine penalization) or by using over plus lenses. In spectacles (optical penalization) can be used as alternative when occlusion is not possible.
Pleoptic exercises were recommended in the past to re-establish foveal fixation especially in young children.
Pharmacologic manipulation using levodopa/ carbidopa has been studied as an adjunct to occlusion therapy.
Perceptual learning is an alternative as an adjunct to occlusion therapy.
Computerized vision therapy
Computerized vision therapy using specially designed software has come into vogue for the treatment of amblyopia with controversial results. Vision therapy works on the concept of operant conditioning (a form of psychological learning). Computerized Home Vision Therapy (CHVT) can be as supplementary treatment to the occlusion therapy.
Image Source : American Optometric Association