Lobar holoprosencephaly

Last revised by Calum Worsley on 24 Dec 2023

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.

Radiographic features

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:

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.

Differential diagnosis

  • semilobar holoprosencephaly

    • fusion of the anterior aspects of the hemispheres

    • falx and interhemispheric fissure incomplete

    • partial fusion of the thalami

  • septo-optic dysplasia

    • 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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Cases and figures

  • Case 1
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  • Case 3
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