Holoprosencephaly

Holoprosencephaly derives from failed or incomplete cleavage of the prosencephalon; it is featured by varying degrees of fusion of the cerebral hemispheres and has a very severe prognosis in most cases. The alobar form shows a great distortion of normal brain anatomy with a single cerebral ventricle. Its prenatal sonographic diagnosis is straightforward since early gestation, and it is frequently associated with other anomalies or genetic conditions. Lesser degrees of anatomic alteration are associated with more subtle ultrasound findings and may be difficult to recognize or categorize unless a careful evaluation of both cavum septi pellucidi and frontal horns as well is conducted. In semilobar and lobar forms, the septum pellucidum is typically absent in axial planes, with fused hypoplastic frontal horns, while posterior fossa is usually normal. A recently described entity is the interhemispheric variant, in which a separation of the lateral ventricles anteriorly and posteriorly is seen, with various degree of fusion in their middle part. At multiplanar neurosonogram, anomalies involving corpus callosum and cortex development can be detected.