The cerebellum is the part of the brain that plays an important role in balance and co-ordination. ‘Cerebellar hypoplasia’ refers to the incomplete or poor development of the cerebellum, causing it to be small. It may be isolated or be accompanied by abnormalities of other structures in the brain. Individuals with cerebellar hypoplasia may present with developmental delay, seizures, poor balance and co-ordination, involuntary eye movements (nystagmus) and learning difficulties.
It may be genetic or non-genetic in origin. Among the non-genetic causes are environmental influences such as exposure to certain medication (eg. retinoic acid), alcohol and viral infections during pregnancy. The genetic causes vary depending on what other structures are involved eg. RELN gene is implicated in lissencephaly with cerebellar hypoplasia; TSEN genes in ponto cerebellar hypoplasia (the brainstem is affected in addition to the cerebellum); OPHN1 gene in cerebellar hypoplasia with enlarged ventricles; VLDLR gene in cerebellar hypoplasia with simple cortex and small brainstem.
Cerebellar hypoplasia can also occur as a part of other genetic syndromes such as Smith Lemli Opitz syndrome, Carbohydrate Deficient Glycoprotein syndrome, and Joubert syndrome.