This defect, which varies in severity (Gabriel 1980) is one of failure of the primary cerebral vesicle (telencephalon) to divide and expand bilaterally. In its most severe form there is a single large cerebral ventricular cavity within an undivided prosencephalic vesicle (alobar holoprosencephaly) and there is a single median eye (cyclopia).
Less severe forms are associated (semilobar and lobar) with hypoplastic olfactory bulbs and tracts (arhinencephaly) and various midline facial malformations, including hypertelorism and/or cleft lip and palate. The most common associated intracerebral abnormality is hydrocephalus.
The less severely affected infants show severe mental retardation, rigidity, seizures, and attacks of apnoea. Facial abnormality is not invariable. Prenatal diagnosis is often possible (Nyberg et al. 1987).
Origin: Brain's Diseases of the Nervous System