Rhombencephalosynapsis

Rhombencephalosynapsis

-Vermian agenesis
-Fusion of cerebellar hemispheres
-Fusion of dentate nuclei

The rhombencephalon is the most caudal of the early three primary brain vesicles and eventually divides to form the adult pons, medulla, and cerebellum. Embryonic development of the cerebellum is traditionally explained to begin with proliferating neuroblasts from the alar plates forming paired rhombic lips. These structures thicken and extends progressively toward the midline. At the end of the 12th week of gestation, the fusion of these rhombic lips at the midline begin rostrally, with the anterior vermis forming prior to the posterior vermis.

Rhombencephalosynapsis is a cerebellar anomaly caused by vermian maldevelopment, showing fusion of the cerebellar hemispheres, middle cerebellar peduncles, and the dentate nuclei. This anomaly is characterized by an absence of the anterior vermis and a deficiency of the posterior vermis, with the nodulus likely formed. A distinction from other pathologies such as Dandy-Walker malformation, Joubert syndrome, or tectocerebellar dysgraphia lies in the fact that this vermian hypogenesis or agenesis is not associated with disconnected cerebellar hemispheres.

Clinical manifestations depend on the extent of posterior fossa findings. Some patients may be asymptomatic if rhombencephalosynapsis is an isolated finding. However, supratentorial abnormalities which occur at the time the cerebellum is developing often can dominate clinical presentation. General symptoms include mental retardation, ataxia, spasticity, and epilepsy. Congenital hydrocephalus secondary to aqueductal stenosis is a known association with this anomaly.

Instead of a normal crescent-shaped fourth ventricle, a characteristic finding of rhombencephalosynapsis is a diamond-shaped fourth ventricle on axial MRI. Other features revolve around fused cerebellar hemispheres, which include a flat, uninterrupted base of the cerebellar hemispheres; transverse orientation of folia in the inferior cerebellum; and a large corpus medullare. Features that may be present are deficiency or absence of septum pellucidum, dysgenesis of the corpus callosum and the anterior commissure, fused fornices, and fused thalami.

Utsunomiya H, et al. Rhombencephalosynapis: Cerebellar Embryogenesis. AJNR 19:547-549 (1998)

Andrew Nguyen