Rett syndrome--a review and discussion of syndrome delineation and syndrome definition.

The current status of clinical genetic and pathogenetic knowledge in Rett syndrome is reviewed and updated. Some of the concepts which are highlighted include among others, casual homogeneity vs heterogeneity; the difficulty of dealing with potential variability while the possibility of heterogeneity still exists; progress in examining genetic hypotheses. We review our experience with 8 new patients, note the evidence for the presence of congenital hypotonia in all and several instances of minor anomalies and again raise the issue of whether girls with the Rett syndrome are in fact normal from the time of birth and in early infancy until the characteristic phenotype is recognizable. Previous recommendations for standardized international history, examination and investigation protocols are strongly reinforced, and the question as to who should study brains of Rett syndrome patients is raised by a parent. As is the case in most conditions now being studied by molecular geneticists, finding a marker will be a major etiologic breakthrough, but the work of delineating the pathogenesis will remain to be done.