Microscopic study of holoprosencephalic facial anomalies in trisomy 13 fetuses.

The cerebral and facial anatomy of four trisomy 13 fetuses was studied in order to delineate the varying degrees of expression of severity of holoprosencephaly. Fetal heads were serially sectioned and analyzed microscopically in the horizontal plane. Examples of cyclopia, cebocephaly, and a proposed new category, premaxillary dysgenesis, were studied. The last category represents the least severe end of the facial spectrum of holoprosencephaly in this series. In this condition, there are deficiencies or clefts within the premaxilla, in contrast to the usual site of clefting between the maxilla and the premaxilla. There is asymmetry of the defects in the anterior midface of all four cases.