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• Formerly called arrhinencephaly

• Incomplete separation of cerebral hemispheres

Diagnostic criteria 
• Anomaly and severity defined by degree of forebrain cleavage
○ Spectrum; no clear division between types
○ Single ventricle
○ Absent or partial hemispheric and basal cleavage with absent/incomplete interhemispheric fissure/falx
○ Azygous anterior cerebral artery (ACA)
○ ± associated facial defects

• Alobar holoprosencephaly (HPE)
○ Single "ball," "cup," or "pancake" brain around primitive monoventricle
○ ± dorsal cyst, dysmorphic facies

• Semilobar HPE
○ Frontal lobes > 50% fused
○ Thalami, hypothalamus "fused"
○ Posterior interhemispheric fissure, falx may be present
○ ± dorsal cyst, mild/absent facial malformations

• Lobar HPE
○ Interhemispheric fissure present along most of midline
○ Only most inferior frontal lobes fused
○ Thalami almost/completely separated
○ Septum pellucidum always absent
○ Rudimentary frontal horns, ± 3rd ventricle, corpus callosum normal or incomplete

Best imaging tool
○ MR (3-planar, high definition), special attention to midline

• Whenever HPE in doubt, look at basal forebrain for cleavage failure

Image Interpretation Pearls
• Not all cases of missing septum pellucidum are single ventricles

Imaging Gallery 

3D reconstruction of the facial bones in a newborn infant with a cleft lip and palate shows severe hypotelorism  and a moderately large midline defect of the maxilla extending into the hard palate and nasal septum . The 2 central incisors are absent.

Axial NECT scan in the previous patient shows a horseshoe-shaped central monoventricle with a thin rim of surrounding cortex that is fused across the midline . The basal ganglia are also fused, characteristic of alobar holoprosencephaly.

Sagittal T2WI in the previous patient at 17 months shows the posterior fossa and brainstem appear relatively normal. The supratentorial compartment is almost completely filled with the CSFcontaining monoventricle. The hypothalamus and basal ganglia  are fused and there is a thin rim of agyric cortex  around the anterior aspect of the monoventricle.

T2WI in the previous patient shows the thin cortex is fused across the midline anteriorly . No interhemispheric fissure was present and the falx is absent.

Sagittal T2WI MR shows small, single frontal lobe  with single undivided anterobasal striatum and the widely expanded tela choroidea forming the prosencephalic dorsal cyst .

Axial T1WI MR in the same patient shows lack of hemispheric division. The Sylvian fissures st are anteromedial due to frontal hypoplasia. The anterobasal triatum  and medial thalamus are not divided, and the ventricular atria open posteriorly into the dorsal cyst.

Oblique 3D reconstruction was obtaine in the same child with semilobar holoprosencephaly (HPE). The major finding is absence of anterior interhemispheric fissure. The gyral pattern is almost normal, but the sylvian fissure  is shallow and vertical (due to frontal lobe hypoplasia).

Oblique posterior view of surface rendering 3D T1 gradient-echo sequence shows the large dorsal cyst . Note that the temporal  cortex shows no normal recognizable gyral pattern. The cerebellum appears normal.

Axial T1WI in a 7 month old with central incisor shows mostly separated frontal lobes with a moderately well developed interhemispheric fissure  and falx . The septum pellucidum is absent and the frontal horns appear rudimentary.

Coronal T2WI in the same patient shows the falx cerebri  in a well-developed interhemispheric fissure. The basal ganglia are separated. The temporal horns are relatively well-formed but vertically oriented. The findings are consistent with lobar holoprosencephaly.

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