Alobar holoprosencephaly

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.

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 (which usually occurs around the 4^th to 6^th 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 best characterized by MRI.

Ultrasound

• monoventricle
• fused thalami
• absent corpus callosum
• absent interhemispheric fissure
• absent cavum septi pellucidi
• absence of 3^rd 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:

• single midline monoventricle (or holosphere)
  • lateral and third ventricles are absent
• absent midline structures
  • absent septum pellucidum
  • agenesis or hypoplasia of the corpus callosum
  • absent interhemispheric fissure and falx cerebri
  • absent olfactory tract
• dorsal cyst of holoprosencephaly
• 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:

• proboscis
• mono-orbit/cyclopia
• mononostril
• hypotelorism
• cebocephaly

The fused cortex can take on one of three basic shapes ²:

1. pancake: cerebral tissue is confined to the anterior basicranium
2. cup: cerebral tissue lines variable amounts of the anterior cranium with a dorsal cyst present posteriorly
3. 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

• semilobar holoprosencephaly
  • partial separation into hemispheres
  • rudimentary occipital and temporal horns
• hydranencephaly
  • 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