Alobar holoprosencephaly: ultrasonographic prenatal diagnosis.

Modern ultrasound imaging devices have increasingly allowed sophisticated prenatal diagnoses of fetal disorders. Embryopathologic and ultrasound findings are reported in five cases of alobar holoprosencephaly that were diagnosed prenatally, two cases as early as 23 menstrual weeks. Specific sonographic findings included a large central cranial fluid collection (a monoventricular cavity lacking ventricular horns and midline structures), a fused thalamus at the floor of this cavity, and characteristic facial features (proboscis, single orbit, single nostril, or severe hypotelorism). Differential diagnosis of the fetus with a large intracranial fluid collection is discussed and illustrated.