Atypical forms of Rett syndrome.

In the absence of any biological marker, Rett syndrome (RS) is defined by clinical criteria which have been proposed at the second Vienna conference on RS and patients who do not fulfill those criteria cannot be included. However, some patients partially fulfill the criteria but lack some of the essential characteristics. Seven such patients are reported. All patients were girls. Atypical manifestations included absence of a normal development during the first months of life (5 patients), absence of deterioration (1 patient), or presence of initial and intense seizure activity (2 patients). If such cases are indeed atypical RS, the spectrum of clinical manifestations will have to be broadened and deterioration of previously acquired skills may not be an essential requirement for its diagnosis. The exclusive occurrence of atypical and of typical cases in females suggests that both constitute a single morbid entity.