Rett and Angelman's syndromes: models of arrested development.

Rett and Angelman's syndromes have emerged slowly from their initial classifications as degenerative disorders and static encephalopathies; the terms development and developmental arrest have been used for 200 years. An attempt has been made to trace the genesis and the various contexts of these seminal concepts. Both disorders give frozen-framed expression of developmental levels not easily appreciated in normal children who progress from day to day. Although close in developmental level, about 9 and 15 months with a scatter of a few months, these disorders may actually overlap in a few instances. These syndromes illustrate how a slightly more advanced brain function may appear to be a big gap and how difficult it is to differentiate between a failure to progress further, which is easily confused with regression, and an apparent loss of skills actually never acquired. Learning to see the infant brain behind the aging body and how it adjusts within the constraints of its fixed low level of neural organization is the best basis for diagnosis and treatment. Identifying shortcomings and special risks is more rewarding than ill-conceived attempts to modify anatomic destiny.