Lobar holoprosencephaly is the least severe form where the occipital and temporal horns are well formed but the cerebral hemispheres frontally are not completely cleaved. The septum pellucidum is absent, and the corpus callosum may be absent or dysplastic.

Septooptic dysplasia is analogous to mild holoprosencephaly and is characterized by absence of the septum pellucidum and hypoplasia of the optic nerves. The frontal horns of the lateral ventricles have a squared appearance and point inferiorly on coronal imaging.