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Amblyopes can demonstrate many and various visual deficits, involving loss of spatial range, contrast and positional deficits (reviewed by Hess46), but the most widely accepted clinical definition of amblyopia is based on visual acuity. This suggests that amblyopia is a unilateral or bilateral decrease of visual acuity caused by a deprivation of form vision and/or an abnormal binocular interaction (usually early in life) for which no organic causes can be detected by physical examination of the eye and which, in appropriate cases, is reversible by therapeutic measures.117
Estimates of the prevalence of amblyopia are wide-ranging; it is said to affect between 1.6% and 3.5% of the population. This variation reflects the nature of the population studies from which the data are derived88 and the arbitrary defining of amblyopia as two or more lines interocular difference on a recognition chart or more than one octave interocular difference, and the difficulty of clinically untangling subcategories of amblyopia. Subcategories, which relate to the etiology of amblyopia,22 were introduced in an attempt to explain the varying degrees of success in amblyopia therapy.
The subcategories of functional amblyopia in which no organic lesion exists are broadly as follows:
1. Stimulus deprivation amblyopia, for example, due to media opacity (and occlusion amblyopia, the iatrogenic visual loss of the good eye after patching)
2. Strabismus amblyopia
3. Anisometropic amblyopia
4. Refractive or isometropia amblyopia with bilateral high refractive error (this includes meridional amblyopia)
5. Psychogenic amblyopia, a visual conversion reaction (treated separately in chapter 51)
Organic amblyopias, such as those due to nutritional or toxic effects, are dealt with separately in chapters 54 and 55.
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