= COMPLETE DYSGENESIS OF CORPUS CALLOSUM

= failure of formation of corpus callosum originating at lamina terminalis at 7–13 weeks from where a phalanx of callosal tissue extends backward arching over the diencephalon; usually developed by 20 weeks EGA

Incidence: 0.7–5.3%

Cause: congenital / acquired (← infarction of ACA)

Histo: axons from cerebral hemispheres that would normally cross continue along medial walls of lateral ventricles as longitudinal callosal bundles of Probst that terminate randomly in occipital + temporal lobes

Associated with:

1. CNS anomalies (85%):
   1. Dandy-Walker cyst (11%)
   2. Interhemispheric arachnoid cyst may be continuous with 3^rd and lateral ventricles
   3. Hydrocephalus (30%)
   4. Midline intracerebral lipoma of corpus callosum often surrounded with ring of calcium (10%)
   5. Arnold-Chiari II malformation (7%)
   6. Midline encephalocele
   7. Porencephaly
   8. Holoprosencephaly
   9. Hypertelorism median cleft syndrome
   10. Polymicrogyria, gray-matter heterotopia
2. Cardiovascular, gastrointestinal, genitourinary anomalies (62%)
3. Abnormal karyotype (trisomy 13, 15, 18)

• normal brain function in isolated agenesis
• intellectual impairment; seizures
• absence of septum pellucidum + corpus callosum + cavum septi pellucidi
• longitudinal bundles of Probst create crescentic lateral ventricles:
  • colpocephaly (= dilatation of trigones + occipital horns + posterior temporal horns in the absence of splenium
  • “bat-wing” appearance of lateral ventricles (= wide separation of lateral ventricles with straight parallel parasagittal orientation with absent callosal body)
  • laterally convex frontal horns in case of absent genu of corpus callosum
• “high-riding third ventricle” = upward displacement of widened 3^rd ventricle often to level of bodies of lateral ventricle
• anterior interhemispheric fissure adjoins elevated 3^rd ventricle ± communication (PATHOGNOMONIC)
• “interhemispheric cyst” = interhemispheric CSF collection as an upward extension of 3^rd ventricle
• enlarged foramina of Monro
• “sunburst gyral pattern” = dysgenesis of cingulate gyrus with characteristic radial orientation of cerebral sulci from the roof of the 3^rd ventricle (on sagittal images)
• failure of normal convergence of calcarine + parietooccipital sulci
• persistent eversion of cingulate gyrus (rotated inferiorly + laterally) with absence on midsagittal images
• incomplete formation of Ammon's horn in the hippocampus

OB-US (>22 weeks GA):

• absence of septum pellucidum
• “teardrop” ventriculomegaly = disproportionate enlargement of occipital horns = colpocephaly
• dilated + elevated 3^rd ventricle
• radial array pattern of medial cerebral sulci

Angio:

• wandering straight posterior course of pericallosal arteries (lateral view)
• wide separation of pericallosal arteries ← intervening 3^rd ventricle (anterior view)
• separation of internal cerebral veins
• loss of U-shape in vein of Galen

DDx:

1. Prominent cavum septi pellucidi + cavum vergae (should not be mistaken for 3^rd ventricle)
2. Arachnoid cyst in midline (suprasellar, collicular plate) raising and deforming the 3^rd ventricle and causing hydrocephalus