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Lobar holoprosencephaly is the least severe of the classical subtypes of holoprosencephaly (HPE), characterized by the presence of the interhemispheric fissure along almost the entire midline, and with the thalami not being fused 5.
As with all types of holoprosencephaly, 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.
As with 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 holoprosencephaly the cerebral hemispheres are present. Features include 1-4:
fusion of the frontal horns of the lateral ventricles
wide communication of this fused segment with the third ventricle
partial fusion of the fornices 4 and anterior frontal lobes
Unlike semilobar holoprosencephaly, the falx is present, the interhemispheric fissure is fully formed and the thalami are not fused. Thalamic fusion should be distinguished from the interthalamic adhesion, which is a normal structure present in the majority of brains.
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.
fusion of the anterior aspects of the hemispheres
falx and interhemispheric fissure incomplete
partial fusion of the thalami
can appear similar due to absent septum pellucidum
fusion of the frontal lobes and fornices is not a feature
hypoplastic optic nerves/chiasm