Rhombencephalosynapsis is a congenital abnormality of the cerebellum characterized by the absence of the vermis and continuity of the cerebellar hemispheres, dentate nuclei, and superior cerebellar peduncles. It either occurs as an isolated anomaly (rare) or as part of a wider cerebral malformation and it has variable degrees of neurological impairment.
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Epidemiology
The condition is very rare and thought to be sporadic. No familial cases have been identified 9.
Clinical presentation
Clinical presentation includes truncal and/or limb ataxia, abnormal eye movements, head stereotypies, delayed motor development, and other features determined by supratentorial abnormalities (see below) 10. Overall patients tend to die in childhood or early adult life 9.
Associations
Rhombencephalosynapsis has been seen to be associated with:
Gomez-Lopez-Hernandez syndrome: comprises rhombencephalosynapsis, developmental delay, scalp alopecia, and trigeminal anesthesia 5
vertebral fusion anomalies
Pathology
Embryologically rhombencephalosynapsis is now considered to result from a failure of vermian differentiation. The fused development of the cerebellar hemispheres is thought to result from an insult between the 28th and 44th day of gestation 6,8. The classic appearances is of a transversely oriented single lobed cerebellum.
Rhombencephalosynapsis was traditionally thought to be a complete fusion of cerebellar hemispheres. However, off late cases of partial fusion have also been reported 6,8.
Radiographic features
MRI
Rhombencephalosynapsis is best seen on MRI brain, and is characterized by 1-4,7,8:
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vermian abnormalities
absent anterior (rostral) vermis
deficient posterior (caudal) vermis
nodulus tends to be formed
fusion of the cerebellar hemispheres, with transversely oriented inferior cerebellar folia
fusion of dentate nuclei
fusion of the superior cerebellar peduncles
This usually results in an abnormally small posterior fossa, and an abnormally shaped fourth ventricle, which takes on a diamond (or key hole) shape 7,8,10.
Additional frequently associated abnormalities include:
fusion of the thalami
fusion of inferior colliculi
absent septum pellucidum
ventriculomegaly: often from aqueductal stenosis
sutural synostosis
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other supratentorial abnormalities 8
hypoplasia of the commissural system (corpus callosum, anterior commissure)
hypoplasia of the anterior visual pathway
agenesis of the posterior lobe of the pituitary
Treatment and prognosis
Variable clinical progression ranging from early death to varying degrees of cerebellar dysfunction and neurodevelopmental delay. Some patients may reach adulthood 3.
History and etymology
It was first reported by Heinrich Obersteiner (1847-1922), an Austrian neurologist, in 1914 3.
Differential diagnosis
Imaging differential considerations include: