Lobar holoprosencephaly is the least severe of the classical subtypes of holoprosencephaly (HPE), characterized by the presence of the inter-hemispheric fissure along almost the entire midline, and with the thalami being completely or nearly completely separated .
As with all types of HPE, it is a rare congenital brain malformation in which there is failure of complete separation of the two hemispheres and failure of transverse cleavage into diencephalon and telencephalon.
For a general discussion of epidemiology, clinical presentation and pathology, please refer to the article on holoprosencephaly (HPE).
As will most cerebral structural congenital abnormalities, lobar HPE is visible on all modalities, but in general is identified on antenatal ultrasound, and best characterized by MRI.
Unlike the more severe forms, in lobar HPE the cerebral hemispheres are present. Features include :
- fusion of the frontal horns of the lateral ventricles
- wide communication of this fused segment with the third ventricle
- fusion of the fornices
- absence of septum pellucidum
- normal or hypoplasia of the corpus callosum
- anterior cerebral artery may be displaced anteriorly to lie directly underneath the frontal bones (snake under the skull sign)
An easier way to assess may be looking for the presence of a fused fornix which could be seen as an anteroposteriorly aligned echogenic linear structure within the third ventricle.