Rhombencephalosynapsis

Rhombencephalosynapsis (RS) is a congenital abnormality of the cerebellum, with variable degrees of neurological impairment. It either occuroccurs as an isolated anomaly (rare) or as part of wider cerebral malformation.

Epidemiology

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

Clinical presentation

Clinical presentation is predominantly determined by supratentorial abnormalities (see below) and as such is variable. Similarly the severity of neurological dysfunction is also variable, however 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 of rhombencephalosynapsis, developmental delay, scalp alopecia and trigeminal anaesthesia ⁵
• vertebral fusional anomalies

Pathology

Embryologically rhombencephalosynapsis is now considered as a failure of vermian differentiation with 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 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 brain

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.

Additional frequently associated abnormalities include:

• fusion of the thalami
• fusion of inferior colliculi
• absent septum pellucidum
• ventriculomegaly: often from aqueductal stenosis
• sutural synosotosis
• 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

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 Obersteiner in 1914 ³.

Differential diagnosis

Imaging differential considerations include:

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