A 4 months old male child with macrocephaly presented with progressive increase in size of head since birth. No focal neurological deficit was seen at the time of presentation. On examination, occipitofrontal circumference was significantly more than the normal range for age.
T2 weighted sagittal image of brain through midline shows narrowing of the CSF space at the level of the aqueduct with resultant non-communicating hydrocephalus. There is herniation of the cerebellar tonsil beyond the foramen magnum (Figure 1). Coronal T1WI of brain through cerebellum shows horizontally oriented foliae which are continuous across the midline with absence of the cerebellar vermis- suggesting rhombencephalosynapsis. Supratentorial ventricles are dilated (Figure2). T1W axial image of the brain at the level of the cerebellum shows fusion of the cerebellar hemispheres with horizontally directed foliae. The cerebellar vermis is not visualised. However, the cerebellum is not hypoplastic( Figure 3)
Rhombencephalosynapsis refers to congenital abnormality of absence of the vermis and continuity of the cerebellar hemispheres across the midline. There is continuity of dentate nuclei, and superior cerebellar peduncles as well. When it occurs as a part of a wider spectrum of wider cerebral malformation it is commonly supratentorial. However, it can occur as an isolated anomaly.
The clinical presentation is variable ranging from variable degrees of cerebellar dysfunction and developmental delay to premature death.
It is a sporadic condition without any familial association.
Embryologically, failure of vermian differentiation is considered the major cause of rhombencephalosynapsis. This failure of differentiation is a result of an insult between the 28th and 44th day of gestation. This results in a transversely oriented, single-lobed cerebellum.
Rhombencephalosynapsis is seen to be associated with:
• Gomez- Lopez- Hernandez Syndrome: comprises of rhombencephalosynapsis, developmental delay, scalp alopecia and trigeminal anaesthesia.
• vertebral fusion anomalies
Imaging is of utmost importance for diagnosis of condition and also to look for associations. Rhombencephalosynapsis is characterized by vermian abnormalities: anterior or rostral vermis is completely absent, posterior or caudal vermis is usually deficient. Nodulus is generally well-formed. Cerebellar hemispheres are fused with transversely oriented inferior cerebellar foliae. Dentate nuclei are fused along with fusion of superior cerebellar peduncles. This leads to an abnormally small posterior fossa and an abnormally shaped fourth ventricle, which takes on a diamond (or keyhole) shape.
Varying degrees of cerebellar dysfunction and neurodevelopmental delay is the usual outcome. Unfortunately, no specific treatment of the condition is available at present.
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 Demaerel P, Morel C, Lagae L et-al. Partial rhombencephalosynapsis. AJNR Am J Neuroradiol. 2004;25 (1): 29-31.
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