Alobar holoprosencephaly is a subtype of holoprosencephaly and is the most severe of the classical three subtypes, with both semilobar and lobar holoprosencephaly having less severe clinical manifestations.
For a general discussion of epidemiology, clinical presentation, and pathology, please refer to the main article on holoprosencephaly.
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Pathology
As with all types of holoprosencephaly, it is a rare congenital brain malformation in which there is a failure of complete separation of the two hemispheres (which usually occurs around the 4th to 6th gestational weeks) and failure of transverse cleavage into the diencephalon and telencephalon.
Radiographic features
As with most cerebral structural congenital abnormalities, alobar holoprosencephaly is visible on all modalities, but in general, is identified on antenatal ultrasound (if performed), and is best characterized by MRI.
Ultrasound
monoventricle
fused thalami
absent interhemispheric fissure
absent cavum septi pellucidi
absence of 3rd ventricle
middle and anterior cerebral arteries may be replaced by tangled branches of internal carotid and basilar vessels
severe facial malformations
MRI
The basic structure of the cerebral hemispheres is lost, with variable amounts of residual cortex. Features include 1,2:
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single midline monoventricle (or holosphere)
lateral and third ventricles are absent
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absent midline structures
absent septum pellucidum
absent interhemispheric fissure and falx cerebri
absent olfactory tract
absent, fused or normal optic nerves
middle and anterior cerebral arteries may be replaced by tangled branches of internal carotid and basilar vessels
Associated craniofacial features may also be present which include:
The fused cortex can take on one of three basic shapes 2:
pancake: cerebral tissue is confined to the anterior basicranium
cup: cerebral tissue lines variable amounts of the anterior cranium with a dorsal cyst present posteriorly
ball: a complete rim of tissue surrounds the monoventricle without dorsal cyst
Treatment and prognosis
This is the most severe type of the holoprosencephaly spectrum and often tends to be fatal in the neonatal period.
Differential diagnosis
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partial separation into hemispheres
rudimentary occipital and temporal horns
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thalami are often visible and are not fused
falx cerebri usually present
not associated with midline facial abnormalities
no cortex present, or sometimes small islands of tissue
-
severe hydrocephalus
falx cerebri usually present, but may be absent due to severe long-standing hydrocephalus
bilateral choroid plexus
thalami not fused
not associated with midline facial abnormalities
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