Callosal Dysgenesis

TERMINOLOGY
• Agenesis/dysgenesis corpus callosum (ACC) Synonyms
• Callosal agenesis/dysgenesis, commissural agenesis/dysgenesis

Definition
• Partial or complete absence of corpus callosum (CC), hippocampal commissure (HC), or anterior commissure (AC); can be isolated or associated with additional cerebral malformations

• Spectrum of congenital CC structural abnormalities
○ Total agenesis (absence from birth of all anatomically defined regions of CC)

○ Partial agenesis (absence from birth of at least 1, but not all, regions of CC)

○ Hypoplasia (thinner CC with normal anteroposterior extent)

○ Hyperplasia (thick CC resulting from decreased postnatal axonal pruning)

○ Dysgenesis (CC present but malformed in some way, including partial ACC and hypoplasia of CC)


• Diagnostic criteria 

1. Partially or completely absent CC on midline sagittal and coronal planes
2. Lateral ventricles separate and parallel (axial), bull's head, trident, Viking helmet, or moose head appearances (coronal)
3. Size :When present, CC remnants vary in size, extent, shape , Prior to myelin maturation, may be difficult to define (T2WI is better)
4. Atrium/occipital horns often dilated ("colpocephaly")
5. DTI: Callosal fiber tracts form Probst bundles instead of crossing, where corpus callosum is absent
6. Vertical/posterior course of anterior cerebral artery

CLINICAL ISSUES
• Any age; classically identified in early childhood, most common malformation found in fetuses
• Seizures, developmental delay, cranial deformity/hypertelorism
• Sporadic/isolated agenesis/dysgenesis corpus callosum: Normal/near normal at 3 years (75%), but subtle cognitive defects apparent with increasing complexity of school tasks
• Agenesis/dysgenesis corpus callosum with associated/syndromic anomalies = worst

DIAGNOSTIC CHECKLIST
• Look for absent/incomplete corpus callosum rather than indirect signs
• Fully assess for associated lesions

Best imaging tool
○ MR

Protocol advice
○ Multiplanar MR (look for associated malformations)
○ If MR unavailable, multiplanar CT will diagnose ACC
○ In fetuses, use ultrafast single-shot T2WI in 3 planes

Consider
• Syndromic associations common

Image Interpretation Pearls
• Look for absent/incomplete CC rather than indirect signs
• Fully assess for associated lesions

Imaging gallery

Coronal graphic shows a lack of transverse corpus callosum and separate lateral ventricles. The interhemispheric fissure extends to the 3rd ventricle. The bundles of Probst contain the parasagittally rerouted callosal fibers.

Coronal T2WI with callosal dysgenesis shows Viking helmet or moose head appearance of widely separated lateral ventricles . The very hypointense white matter tracts medial to the lateral ventricles are the Probst bundles . Note also heterotopic GM.

Sagittal T2WI in the previous patient shows absence of the corpus callosum, radially-oriented gyri converging on a high-riding 3rd ventricle. Note the azygous anterior cerebral artery (ACA) . The anterior commissure is also absent.

Axial T2WI in the previous case shows the characteristic parallel, nonconverging lateral ventricles  seen in corpus callosum agenesis. Heavily myelinated Probst bundles ſt are seen just medial to the lateral ventricles.

Sagittal T1WI MR shows pure partial agenesis/dysgenesis corpus callosum (ACC). The posterior portion of the corpus callosum is missing , but the junction with the fornix  seems preserved. Posterior coronals would show Probst bundles, whereas anterior images would appear normal. 

Coronal T2WI MR shows ACC with (shunted) interhemispheric meningeal cysts . Note the massive nodular heterotopia on the patient's right . A Probst bundle has formed on the left st but not on the right.

Axial NECT with widened windows shows ACC with colpocephaly, calcified  midline lipoma that extends through choroid fissures into lateral ventricles . 

Axial T1WI MR in ACC shows parallel ventricles, colpocephaly, and a midline lipoma . Note that the lipoma protrudes through the choroid fissure into the lateral ventricles .

Sagittal T1WI MR in a 19-year-old female with Chiari 2 is shown. ACC in Chiari 2 is always partial ; Probst bundles are never seen. Note the multiple supra- and infratentorial Chiari 2 features. 

Axial DTI of a child with complete callosal agenesis shows Probst bundles as green uncrossed callosal fibers coursing front to back.