Human malformations of the midbrain and hindbrain: review and proposed classification scheme.

Although a great deal of interest in the genetics and etiology of cerebral, particularly forebrain, malformations has been generated in the past decade, relatively little is known about the basis of congenital malformations of the structures of the posterior fossa, namely the midbrain, cerebellum, pons, and medulla. In this review, we present a classification scheme for malformations of the midbrain and hindbrain based on their embryologic derivation, highlight four of the conditions associated with such abnormalities, and describe the genetics, prognosis, and recurrence risks for each. We describe several disorders in addition to Joubert syndrome with the distinctive radiologic sign known as the "molar tooth sign," comprised of midbrain and hindbrain malformations. We discuss Dandy-Walker malformation, its classical definition, and the surprisingly good outcome in the absence of other brain malformations. We consider the heterogeneous entity of cerebellar vermis hypoplasia and describe the recently identified gene associated with an X-linked form of this condition. Finally, the pontocerebellar hypoplasias are discussed in the context of their generally progressive degenerative and severe course, and the differential diagnosis is emphasized. We anticipate that as imaging technologies improve, differentiation of the various disorders should aid in efforts to identify the causative genes.