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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 being completely or nearly completely separated 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
- absence of septum pellucidum
- normal or hypoplasia of the corpus callosum
- the anterior cerebral artery may be displaced anteriorly to lie directly underneath the frontal bones (snake under the skull sign) and it may be azygous
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
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