Teaching Case

A 7 month-old female patient referred with hypotonia,irritability and nystagmus.

A 7 month-old baby girl was referred with hypotonia, irritability and nystagmus. MRI study of the brain was performed on a 1.5 Tesla MR scanner using SE T1, FSE T2 weighted, and coronal FLAIR sequences in three imaging planes. The MR images demonstrated absence of the vermis with resultant fusion of the cerebellar hemispheres and dentate nuclei.

Rhombencephalosynapsis was first described by Obersteiner in 1914. It is a very rare congenital anomaly of the posterior fossa, characterized mainly by vermian agenesis or severe hypogenesis, fusion of the cerebellar hemispheres and apposition or fusion of the dentate nuclei. Since MRI provides excellent anatomic details and improves our ability to assess brain development and to detect associated malformations, the identification of rhombencephalosynapsis is much easier. Rhombencephalosynapsis is frequently associated with supratentorial abnormalities such as ventriculomegaly, fusion of the inferior colliculi, deficiency or absence of the septum pellucidum, hypoplasia of anterior commissure, fused thalami, dysgenetic or deformed corpus callosum, and schizencephaly. Associated extracranial congenital anomalies have been reported in the musculoskeletal, cardiovascular, respiratory and urinary systems. The clinical manifestations and the prognosis ultimately depend on the severity of the associated abnormalities.The principal clinical signs of rhombencephalosynapsis include cerebellar dysfunction (hypotonia, nystagmus, ataxia), mental and motor retardation.MR scans of patients with rhombencephalosynapsis demonstrate hypogenesis or agenesis of the vermis, fusion of cerebellar hemispheres, fusion of cerebellar dentate nuclei, superior cerebellar peduncles.Convergence of the dentate nuclei formed a horse-shoe shaped arc across the midline posterior to the fourth ventricle. In rhombencephalosynapsis, there are vermian agenesis and a fusion crossing the midline without being separated by a cleft which is seen in Joubert’s syndrome.