This has been defined as a unilateral impairment of visual acuity in the absence of any other demonstrable pathology in the eye or visual pathway. This rather negative definition fails to explain that there is a defect in nerve conduction because of inadequate usage of the eye in early childhood. The word "amblyopia" means blindness and tends to be used rather loosely by ophthalmologists. It is most commonly used to refer to amblyopia of disuse ("lazy eye") but it is also used to refer to loss of sight caused by drugs. Amblyopia of disuse is common and some patients even seem unaware that they have any problem until they suffer damage to their sound eye. This weakness of one eye results when the image on the retina is out of focus or out of position for more than a few days or months in early childhood or, more specifically, below the age of eight years. Amblyopia of disuse, therefore, arises as the result of a squint or a one-sided anomaly of refraction, or it can occur as the result of opacities in the optical media of the eye. A corneal ulcer in the centre of the cornea of a young child can rapidly lead to amblyopia. Once a clear image has been produced on the retina, either by the wearing of spectacles or other treatment, the vision in the weak eye can be greatly improved by occluding the sound eye. The younger the patient, the better are the chances of improving the vision by occlusion. Beyond the age of eight years it is unlikely that any significant improvement can be achieved by this treatment and, by the same token, it is unlikely that amblyopia will appear after the age of eight years. An adult could suffer total occlusion of one eye for several months without experiencing any visual loss in the occluded eye.
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