A.Prof Frank Gaillard et al.

Rhombencephalosynapsis (RS) is a congenital abnormality of the cerebellum characterised by the vermis absence and continuity of the cerebellar hemispheres, dentate nuclei, and superior cerebellar peduncles. It either occurs as an isolated anomaly (rare) or as part of wider cerebral malformation and it has variable degrees of neurological impairment.

The condition is very rare and thought to be sporadic. No familial cases have been identified 9.

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.

Rhombencephalosynapsis has been seen to be associated with:

Embryologically rhombencephalosynapsis is now considered as a failure of vermian differentiation with the fused development of the cerebellar hemispheres, thought to result from an insult between the 28th and 44th day of gestation 6,8. There is classically 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.

Rhombencephalosynapsis is best seen on MRI, and is characterised by 1-4,7-8:

  • 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
  • other supratentorial abnormalities 8

Variable clinical progression ranging from early death to varying degrees of cerebellar dysfunction and neurodevelopmental delay. Some patients may reach adulthood 3.

It was first reported by Obersteiner in 1914 3.

Imaging differential considerations include:

Malformations of the central nervous system
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Article information

rID: 8344
Synonyms or Alternate Spellings:
  • Rhomboencephalosynapsis

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