Semi-lobar holoprosencephaly

Semi-lobar holoprosencephaly is a sub type of holoprosencephaly, which is  intermediate in severity, being worse than lobar holoprosencephaly and better than alobar holoprosencephaly. 

Pathology

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 will most cerebral structural congenital abnormalities, semilobar holoprosencephaly is visible on all modalities, but in general is identified on antenatal ultrasound (if performed), and best characterised by MRI. 

The basic structure of the cerebral lobes are present, but are fused most commonly anteriorly and at the thalami and there is partial diverticulation of brain (dorsal cyst). Features include ^1-4 :

• absence of septum pellucidum
• monoventricle with partially developed occipital and temporal horns
• rudimentary falx cerebri : absent anteriorly
• incompletely formed interhemispheric fissure
• partial or complete fusion of the thalami 
• absent olfactory tracts and bulbs
• agenesis or hypoplasia of the corpus callosum
• incomplete hippocampal formation

In addition, it is associated with mild degree of facial abnormality such as hypotelorism and cleft lip.

Differential diagnosis

• lobar holoprosencephaly 
• alobar holoprosencephaly
• porencephalic cyst
• arachnoid cyst
• colpocephaly