Rhombencephalosynapsis

Rhombencephalosynapsis is a congenital abnormality of the cerebellum characterised by 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 wider cerebral malformation and it has variable degrees of neurological impairment.

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:

• VACTERL
• Gomez-Lopez-Hernandez syndrome: comprises of rhombencephalosynapsis, developmental delay, scalp alopecia and trigeminal anaesthesia ⁵
• vertebral fusion anomalies

Pathology

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 28^th and 44^th day of gestation ^6,8. There is classically a transversely orientated 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 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 orientated 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
  • 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:

• Joubert syndrome
• Dandy-Walker spectrum
• Lhermitte-Duclos disease
• vermian hypoplasia
• tectocerebellar dysraphia