Holoprosencephaly is a complex intracranial abnormality characterized by absent or incomplete cleavage of the prosencephalon. It has been roughly categorized into 3 types (from most severe to least severe): (1) alobar holoprosencephaly, or complete absence of midline forebrain division resulting in a monoventricle and fused cerebral hemispheres; (2) semilobar holoprosencephaly, or incomplete forebrain division resulting in partial separation of cerebral hemispheres, typically posteriorly; and (3) lobar holoprosencephaly, or complete ventricular separation with focal areas of incomplete cortical division or anterior falcine hypoplasia.
Alobar form is rare autosomal recessive malformation consisting of large holospheric brain. Alobar holoprosencephaly is characterized by nearly complete lack of ventricular and hemispheric cleavage. The brain is basically an undifferentiated holosphere with central monoventricle and fused thalami.
From a genetic point of view all holoprosencephalies have the same significance. Their frequency is estimated at 1 in 16,000 to 1 in 18,000 births. Holoprosencephalies have a heterogenous cause; chromosomal aberration is found in about one half of the cases. Chromosome 13 is most frequently involved. Partial deletion of chromosome 13 as well as trisomy may coexist with holoprosencephalies.