Amblyopia, also known as lazy eye, is a disorder of the visual system that is characterized by poor vision in an eye that is otherwise physically normal, or a visual deficiency out of proportion to associated structural abnormalities of the eye. It has been estimated to affect 1–5% of the population. Prevalence of amblyopia has not changed much over the years.
How does it occur It is a developmental defect of spatial visual processing that occurs in the central visual pathways of brain. Two important pathogenic mechanisms have been proposed, each contributing a different amount to specific type of amblyopia:
1) Abnormal binocular interaction or competition:
This occurs when incompatible images are formed on the fovea of either eye that cannot be fused and the distorted or blurred image from the affected eye is suppressed. Neurons from the two eyes compete for control over cortical connections during the developmental period; those from the better eye gain control at the expense of neurons from the affected eye (active inhibition of that eye by the good eye and amblyopia).
2) Deprivation of formed vision
This occurs when there is little or no stimulation of fovea of one eye with no sensory input from ganglion cells and resultant shrinkage of cells of lateral geniculate body (LGB) and cerebral cortex, corresponding to that eye. This has been confirmed by histologic studies of lateral geniculate nucleus in kittens.
Studies have confirmed that some of the changes in cell structure are reversible only if the deprived eye is forced to be used during the critical period. These time periods may help planning the treatment strategies.
The myths regarding amblyopia:
- It cannot be treated after the age of 10-11 years; in the third world countries e.g. Pakistan, no ophthalmologist embarks on treating it after the age of 6-7 years.
- The scientific literature to date shows that only partial visual recovery is possible.
- No visual improvement is possible in teenagers if a failed amblyopia therapy has been tried during childhood.
- Full-time occlusion therapy may result in occlusion (disuse) amblyopia of the good eye.
Why amblyopia should be treated: It needs to be detected and treated as early as possible because of the following reasons:
- It is the number one cause of monocular visual loss in adults (3,4).
- Person with one amblyopic eye is at a higher risk of becoming blind because of potential loss to the sound eye from other causes e.g. trauma, cataract, glaucoma etc.
- Visual loss in amblyopia is treatable provided it is diagnosed at an early age and therapy instituted appropriately.
- Visual improvement is possible in almost every patient with severe amblyopia irrespective of his/her age.
- Full-time occlusion therapy is the standard treatment for any type and severity of amblyopia.
- Occlusion amblyopia does not occur if the therapy is closely monitored.
- Reversal of amblyopia does not occur if occlusion therapy is gradually weaned.
The mainstay of a successful amblyopia therapy is patient compliance. Full visual recovery is only possible by highly motivated and inspired patients and their parents, hence strong counseling is mandatory prior to institution of therapy.