The Center for Cerebellar Malformations
Rhombencephalosynapsis
General Information
Rhombencephalosynapsis is a rare
congenital abnormality characterised by dorsal fusion of the cerebellar
hemispheres, agenesis or hypogenesis of the vermis, fusion of dentate nuclei
and superior cerebellar peduncles (Toelle et al., 2002). In this sense, it may be viewed
as a condition with failure to form the midline structures of the cerebellum,
akin to holoprosencephaly, which affects primarily the cerebral cortex. There are often associated cerebral
cortical malformations, such as corpus callosum hypoplasia or cortical
dysplasia (Mendonca et al., 2004), but major features of
holoprosencephaly are not typically associated. Clinical findings range from mild truncal ataxia and normal
cognitive abilities to severe cerebral palsy and mental retardation. This broad spectrum may be a result of
the frequent association with hydrocephalus. Pathological studies have revealed
defects that include fusion of the inferior colliculi and absence of dorsal
olivary nuclei (Yachnis, 2002).
MRI Scans
Rhombencephalosynapsis: left axial T-2 MRI showing fusion of
the cerebellar hemispheres, right coronal T-2 showing absence of vermis and horizontal
orientation of posterior cerebellar folia (adapted from Boltshauser 2004).