Amblyopia is a visual loss with no apparent gross pathology. There is no cloudiness of the cornea or lens, or apparent retinal lesions. Amblyopia is not corrected by glasses. It is a microscopic defect in the wiring of the retina-to-brain connections that results from disuse of one eye at an early age (generally before age 7). An adult who does not use an eye will not develop amblyopia. A child below age 7 who does not use an eye will develop amblyopia, and the condition may become irreversible if not corrected quickly. Amblyopia occurs in only one eye. It is proposed that connections in the brain from one eye do not develop well if that eye is not used. Instead, connection sites are usurped by nerve fibers from the good eye. After maturity, correcting the cause of amblyopia will not restore function, as the connection (synaptic) sites are already filled by permanent connections from the normal eye.
There are three main causes of amblyopia—physical occlusion (as by cataract or ptosis), refractive errors, and strabismus. Strabismus, which affects about 2-3% of the population, is an abnormal turning of the eye either inward (crossed eyes; esotropia) or outward (walleyes; exotropia). If a child's eye has a refractive error, in effect that eye suffers from disuse and develops amblyopia. The treatment for refractive amblyopia is early correction of visual acuity.
Amblyopia caused by strabismus is also the product of disuse. A child with crossed eyes does not see double. Rather, she learns to suppress the vision in one eye to avoid seeing double. The suppressed eye, in effect, experiences "disuse" and becomes amblyopic. The initial treatment for strabismic amblyopia is to force the use of the eye by covering the normal eye. Correction of the strabismus is also part of the treatment. This sometimes requires surgery. At other times, particularly when the basis for strabismus is an overconvergence of the eyes on attempted accommodation (the eyes normally converge when accommodating), treatment may be accomplished by accommodating for the patient without demanding extraocular muscle movement. That is, glasses, or special eye drops (e.g. phospholine iodide, a long-acting anticholinesterase) that constrict the ciliary muscles, are sometimes employed effectively. An adult who develops strabismus (e.g. from trauma) does not suppress vision in one eye, but sees double for the rest of his life.
When children with amblyopia wear a patch over the good eye, the eye with poor vision is forced into seeing and may improve. Surgery for strabismus generally has better results when both eyes see well. It important to alternate the patch from one eye to the other, as amblyopia may be induced in the good eye by covering it for too many days!
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