Female, 29 years old, with abnormal prenatal ultrasound, 23 weeks gestational age. Failure of hemispheric separation is seen with a smooth band of cortex which spans the midline anteriorly. The interhemispheric falx has not developed anteriorly, though there may be a partial falx posteriorly. Neither the corpus callosum nor the septum pellucidum have developed. A single communicating ventricle is seen. The deep nuclei including the basal ganglia and thalami are at least partially if not completely fused. Partial temporal lobe development is seen. The cortical mantle thins posteriorly where there is prominence of the CSF space. The cerebellum appears to have developed normally. No definite evidence to suggest intracranial hemorrhage is seen.
Findings are seen compatible with holoprosencephaly. Based on the specific imaging features, the severity appears to fall somewhere between the semilobar and alobar variants.