A special word is needed about this curious condition, which accounts for unilateral impairment of vision in over 2% of the population. Any eye casualty officer is familiar with the patient with a foreign body on the cornea of one eye and the other eye being amblyopic. ("How can I drive home with this patch on, doctor?") The words "lazy eye" are sometimes used but in lay terms this can also mean squint.
The eye suffering from amblyopia of disuse shows certain features:
• Impaired Snellen visual acuity but usually able to decipher vertical lines of letters better than horizontal ones.
• Small residual squint or, if not, the affected eye relatively hypermetropic.
• An indefinite central scotoma, which is difficult to assess by routine visual field testing.
• History of poor vision in one eye since childhood.
The diagnosis of amblyopia can be by exclusion but it must never be reached without a careful examination of the eyes. In recent years, there has been a considerable research interest in this subject and there appear to be nerve conduction anomalies in the occipital cortex, which can be induced by visual deprivation.
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