Holoprosencephaly

KEY FACTS

Terminology

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Alobar holoprosencephaly (HPE) implies complete or nearly complete lack of cerebral hemisphere separation

Imaging

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1st trimester: Absent butterfly sign
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  Both choroids normally seen as “butterfly wings”
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2nd/3rd trimester
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  Absent midline structures, fused thalami
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  Monoventricle: Crescent-shaped, centered on midline
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  Dorsal cyst: Distended 3rd ventricle as fused thalami prevent normal CSF flow

Top Differential Diagnoses

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Hydranencephaly (falx is present)
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Aqueductal stenosis (falx is present)

Pathology

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Chromosomal abnormalities in 25-50%, mostly trisomy 13
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Infants of diabetic mothers have 1% risk

Scanning Tips

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Be vigilant at time of nuchal translucency screening as can make diagnosis of alobar HPE
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Use vaginal ultrasound if limited brain visibility on abdominal imaging
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Use 3D US to characterize facial anomalies
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  Facial malformations of any kind should trigger very careful evaluation of brain
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  Cyclopia, hypotelorism, absent nose, proboscis, midline cleft all associated with HPE, trisomy 13
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Look at head shape; normal head is oval in cross section
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  If head is round in all scan planes, look for brain malformation
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Look for additional findings of trisomy 13
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  Echogenic kidneys, polydactyly, cardiac anomalies, omphalocele
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  Growth restriction, which is often early onset

Axial ultrasound at 20 weeks shows absent midline structures, a monoventricle

, and a continuous anterior rind of cerebral tissue

. These are characteristic findings of alobar holoprosencephaly (HPE). Absence of the falx differentiates alobar HPE from other causes of a fluid-filled cranium.

Axial transabdominal ultrasound in a different case at 17 weeks shows the characteristic monoventricle

, continuous mantle of brain

, and fused thalami

Coronal transvaginal ultrasound in the 3rd trimester shows a continuous mantle of noncleaved cerebral cortex

above the fused thalami

. These are characteristic findings in alobar HPE.

Coronal section through the brain of similar case of alobar HPE demonstrates no evidence of an interhemispheric fissure with fusion of the rudimentary hemispheres

across the midline. The central monoventricle

is horseshoe-shaped. (From Osborn’s Brain.)

Sagittal ultrasound of a 2nd-trimester fetus shows the abnormal brain tissue

in alobar HPE “cupping” a dorsal cyst

. The dorsal cyst is thought to be an expansion of the posterior and dorsal portions of the 3rd ventricle because the fused thalami block normal egress of cerebrospinal fluid. Note the abnormal, flat profile