Holoprosencephaly is the failure of the brain to devide into two distinct hemispheres.  There are differing levels of severity, from least to most severe: (1) middle interhemispheric variant (MIHV), (2) lobar, (3) semi-lobar, and (4) alobar.  Presentation and prognosis are dependent on the amount of separation between the two hemispheres and may be reflected in facial structures, for example cyclopia (presence of a single eye).