[Holoprosencephaly accompanied with dysgenesis of the cerebellum].

We have encountered 9 cases of holoprosencephaly, four of them with abnormalities of the posterior fossa. Three cases of semilobar type had severe cerebellar hypoplasia and a large cyst continuing to the fourth ventricle. They were diagnosed as Dandy-Walker syndrome. A case of lobar type had a relatively mild form of cerebellar hypoplasia and it was thought to be a Dandy-Walker variant. MRI was able to clarify the anatomical relationship of the intracranial construction, for example monoventricle, dorsal sac or large cyst in the posterior fossa, and it was very useful for the diagnosis of these congenital anomalies. All of them showed hydrocephalus and required treatment with ventriculoperitoneal or cyst-peritoneal shunt. With the availability of MRI, easy diagnosis of the mild forms of holoprosencephaly has been facilitated, and accompanying anomalies are also able to be diagnosed. Up to now, the structure of the posterior fossa in holoprosencephaly was thought to be normal. However, several cases of holoprosencephaly accompanied with cerebellar hypoplasia and cystic lesion of the posterior fossa, revealed by MRI or autopsy, have been reported. It was suggested that holoprosencephaly with anomalies of the posterior fossa may be less rare than has been thought, so far.