Congenital anomaly of the posterior fossa characterized by underdeveloped cerebellar vermis, cystic dilatation of 4th ventricle, and enlarged posterior fossa forcing lateral venous sinuses and tentorium upward. Present at birth or typically by the age of 12 months. May be diagnosed antenatally, particularly in familial cases. Common signs and symptoms include hypotonia, ataxia, cranial nerve palsy, developmental delay, and hydrocephalus. Nystagmus, upward gaze palsy, seizures, and spastic paraplegia have been observed. Sudden increased intracranial pressure due to hydrocephalus can present a life-threatening situation. More recently, psychiatric findings have been associated with this cerebellar malformation.

Management includes possible shunting as well as various forms of therapy (physical, occupational). Pharmacological therapy is under study.