Fetal Agenesis of the Corpus Callosum MRI

Step-by-step decision

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1. [Decision] Corpus callosum MRI assessment
   Corpus callosum MRI assessment (midsagittal direct visualization)? (The CC develops front-to-back at 10–18/20 weeks (genu first, splenium/rostrum last). MRI is superior to ultrasound (midsagittal directly shows the CC; ultrasound relies on indirect signs like absent cavum septi pellucidi). Suspect a CC anomaly when there is ventriculomegaly.)
   • CC normal/equivocal (need to re-check indirect signs) → CC normal
   • Partial agenesis/hypoplasia (usually missing splenium/rostrum) → Partial agenesis/hypoplasia
   • Complete agenesis (CC not seen at all on midsagittal + indirect signs) → Complete agenesis · assess associated anomalies
2. [End] CC normal
   CC normal: midsagittal shows a complete C-shaped CC, cavum septi pellucidi present; exclude indirect signs (colpocephaly/parallel lateral ventricles); if only equivocal → coronal re-check (distinguish hypoplasia from a normal variant, and ectopic hippocampal commissure).
3. [End] Partial agenesis/hypoplasia
   Partial agenesis/hypoplasia (usually missing posterior splenium/rostrum): fetal MRI to measure CC length/thickness against gestational age; assess indirect signs (colpocephaly, elevated third ventricle) and associated anomalies; karyotype/microarray; prognosis is slightly better than complete but variable, counseling needed.
4. [End] Complete agenesis · assess associated anomalies
   Complete agenesis: direct (CC not seen on midsagittal) + indirect (colpocephaly, parallel lateral ventricles racing-car/longhorn sign, elevated third ventricle ± dorsal cyst, absent cavum septi pellucidi, Probst bundles, radial gyri); focus on associated anomalies (interhemispheric cyst, migration/cortical malformation, Dandy-Walker, holoprosencephaly) + karyotype/microarray — isolated complete ACC usually has a fair prognosis but variable; multidisciplinary counseling.

Source guidelines & references

• Fetal corpus callosum anomalies MRI assessment and associated anomalies (Diagnostics 2024; Brain 2024 expert consensus)

This pathway is our own synthesis of the decision logic in the guidelines above (not the guideline verbatim); thresholds and workflows change as guidelines update — in practice follow the latest guideline, your institution's protocol and the individual patient.

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