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.
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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 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 best characterized by MRI.
Ultrasound
- monoventricle
- fused thalami
- absent corpus callosum
- 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:
- 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:
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
-
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
Related Radiopaedia articles
Malformations of the central nervous system
-
malformations of cortical development
- abnormal cell proliferation or apoptosis
- abnormal brain size
-
microcephaly
- with normal to simplified cortical pattern
- microcephaly with lissencephaly
- microcephaly with extensive polymicrogyria
- macrocephalies (megalencephaly/macrocephaly)
-
microcephaly
- abnormal cell proliferation
- non-neoplastic
- cortical hamartomas of tuberous sclerosis
- hemimegalencephaly
-
focal cortical dysplasia (Type I and Type IIb)
- Palmini classification (2004)
- Barkovich classification (2005)
- Blumcke classification (2011)
- neoplastic
- non-neoplastic
- abnormal neuronal migration
- lissencephaly
- lissencephaly type I: subcortical band heterotopia spectrum (band heterotopia): undermigration
- lissencephaly type II (cobblestone complex)
- heterotopia: ectopic migration
- subependymal heterotopia
- subcortical heterotopia (not including band heterotopia)
- marginal glioneuronal heterotopia
- lissencephaly
- abnormal cortical organization
- mild malformations of cortical development (previously microdysgenesis)
-
polymicrogyria and schizencephaly
- bilateral polymicrogyria syndromes
- schizencephaly
- focal cortical dysplasia (Type IIa)
- abnormal brain size
- not otherwise classified
- malformations secondary to inborn errors of metabolism
- mitochondrial and pyruvate metabolic disorders
- peroxisomal disorders
- other unclassified malformations
- malformations secondary to inborn errors of metabolism
- abnormal cell proliferation or apoptosis
-
midline abnormalities of the brain
- absent septum pellucidum
- cephaloceles
-
midline nasal region lesions
- nasal dermoid
- nasal glioma
- nasal dermal sinus
- cerebral hemispheres
-
holoprosencephaly/septo-optic dysplasia spectrum
- septo-optic dysplasia
- lobar holoprosencephaly
- semilobar holoprosencephaly
- alobar holoprosencephaly
- middle interhemispheric variant/syntelencephaly
-
holoprosencephaly/septo-optic dysplasia spectrum
- corpus callosum
- intracranial lipoma
-
malformations of the cerebellum
- cerebellar hypoplasia
- focal hypoplasia
- generalized hypoplasia
- with enlarged fourth ventricle
- normal fourth ventricle
- with normal pons
- with small pons
- normal foliation
- pontocerebellar hypoplasias of Barth, types I and II
- cerebellar hypoplasias, not otherwise specified
- normal foliation
- cerebellar dysplasia
- focal dysplasia
- isolated vermian dysplasia
- molar tooth malformations including Joubert syndrome
- rhombencephalosynapsis
- isolated hemispheric dysplasia
- focal cerebellar cortical dysplasias/heterotopia
- Lhermitte-Duclos-Cowden syndrome
- isolated vermian dysplasia
- generalized dysplasia
- congenital muscular dystrophies
- cytomegalovirus
- lissencephaly with RELN mutation
- lissencephaly with agenesis of corpus callosum and cerebellar dysplasia
- associated with diffuse cerebral polymicrogyria
- diffusely abnormal foliation
- focal dysplasia
- cerebellar hypoplasia
- malformations of the brainstem