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.