Hereditary dystrophies of the choroid and retina

Almost all classification systems for the hereditary dystrophies of the choroid and retina have been largely descriptive. In the past, diseases which are now considered a single genetic disorder have been described as different clinical entities, e.g. X-linked choroidal sclerosis and choroideremia. Clearly different genetic diseases, e.g. choroideremia and gyrate atrophy, have been considered different expressions of the same genetic defect. Certainly, the optimal separation of these disorders would result from biochemical markers or knowledge of the basic underlying genetic enzymatic defect, a situation known only for gyrate atrophy of the choroid and retina with hyperornithinemia. Disorders which are inherited in different Mendelian fashion must surely be separate genetic entities. Disorders which have common enzymatic deficiencies but which show other differences, such as responsiveness to vitamin B₆, may be allelic. The best classification system at present should take into account inheritance, clinical and functional parameters, special tests and biochemical and enzymatic aspects as they become available.