Hypoplasia or agenesis of cerebellar hemispheres

**Definition:** Cerebellar hypoplasia corresponds to underdevelopment of cerebellar structures that can involve the vermis and/or the cerebellar hemispheres from partial to total agenesis. It has been described in the context of various clinical entities: chromosomal anomalies, in utero exposure to toxins and infectious agents, metabolic disorders (disorders of glycosylation and CoQ10 deficiencies), and a wide variety of rare genetic neurological diseases. It can be confined to the cerebellum, or affect other CNS structures: the midbrain (molar tooth syndromes), pons and medulla (ponto-cerebellar hypoplasia), cerebral cortex (lissencephaly cerebellar hypoplasia syndromes).

**Exclusions:** - PHACE syndrome