Holoprosencephaly and related entities.

Holoprosencephaly is a brain anomaly of varying severity with associated extracranial, symptomatic abnormalities in only a minority of cases. The class of brain defects known as holoprosencephaly represents a continuum usually divided into three types: alobar, semilobar, and lobar. Each has both distinctive radiological characteristics and some similarities. Typical facial anomalies are found in the severe forms. Absent septum pellucidum and septo-optic dysplasia, possibly of similar embryological origin, have some resemblances to lobar holoprosencephaly, but are clinically and radiologically separate in most instances. Agenesis of the corpus callosum, which is superficially like holoprosencephaly, should not be confused with the prosencephalic defects.