Holoprosencephalies
Holoprosencephaly refers to incomplete differentiation of the fetal prosencephalon (forebrain) into separate hemispheres. Holoprosencephaly may manifest as alobar , resulting in one posteriorly located monoventricle (versus the normally separated two lateral ventricles) and fusion of the thalami; semilobar , with two hemispheres posteriorly but not anteriorly; or lobar, with only mild frontal abnormalities. The middle interhemispheric variant has been more recently described and consists of abnormal midline hemispheric connection in the posterior frontal and parietal lobes. The mildest form along the continuum is thought to include septo-optic dysplasia, which is characterized by absence of the septum pellucidum and optic nerve hypoplasia.
