What distinguishes semilobar holoprosencephaly from the alobar and lobar varieties?
As the name suggests, semilobar holoprosencephaly is an intermediate form. There is some midline separation with part of the falx and interhemispheric fissure somewhat present, as are rudimentary temporal horns of the lateral ventricles, distinguishing it from alobar holoprosencephaly. A dorsal cyst is present with colpocephaly and midline fusion is still marked, features not present in lobar holoprosencephaly.
Axial non-contrast CT of the brain (with thick slice sagittal reformat) demonstrates fused frontal lobes with absent anterior part of the falx cerebri. There is also absence of the septum pellucidum with fusion of the lateral ventricles with rudimentary occipital and temporal horns, with a prominent dorsal cyst.