With better control of nutrition and of maternal and childhood infections (such as smallpox and maternal rubella), inherited disorders account for a large fraction of congenital and childhood visual impairment. Estimates are that half of all visual impairment before the age of 45 years has a genetic etiology; all inheritance patterns are represented including single gene, mitochondrial, and chromosomal (Robinson and Linden, 1993). Obtaining medical documentation of the type and degree of visual disturbance is absolutely essential to provide genetic counseling. Retinal dystrophies are the most common form of genetic blindness (accounting for more than 50%) (Aldred et al., 1994). More than 200 inherited disorders involve retinal degeneration (usually presenting as night blindness and eventual loss of central vision) (Heckenlively and Daiger, 1997). Conversely, 10% of all inherited disorders involve the retina directly or indirectly (Heckenlively and Daiger, 1997). The important medical-family history features to document when an individual has congenital blindness or later-onset visual loss are reviewed in Table 4.8.
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