Fetal Posterior Fossa Malformation MRI (Dandy-Walker Continuum)

Step-by-step decision

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1. [Decision] Posterior fossa MRI (vermis + tegmento-vermian angle)
   Posterior fossa MRI (vermis + tegmento-vermian angle, midsagittal/coronal)? (Assess the vermis on midsagittal/coronal, the hemispheres on axial/coronal. Key measurement: an increased tegmento-vermian angle (normal ~0–9 degrees) indicates vermian rotation/hypoplasia.)
   • Cisterna magna >10 mm, vermis and fourth ventricle intact → Mega cisterna magna · usually benign
   • Vermis normal/mildly rotated, inferior retrovermian pouch (tegmento-vermian angle normal or mildly increased) → Blake pouch · usually good prognosis
   • Vermian hypoplasia, normal tentorium → Vermian hypoplasia (normal tentorium)
   • Vermian hypoplasia/agenesis + cystic fourth ventricle dilation + elevated tentorium + enlarged posterior fossa → Dandy-Walker malformation · comprehensive assessment
2. [End] Mega cisterna magna · usually benign
   Mega cisterna magna (>10 mm, intact vermis and fourth ventricle, normal tegmento-vermian angle): usually a benign variant; confirm intact vermis, no hydrocephalus → good prognosis; follow up to exclude associated anomalies.
3. [End] Blake pouch · usually good prognosis
   Blake pouch (failure of fenestration of the inferior retrovermian membrane): normal vermian size, only mild rotation, mildly increased tegmento-vermian angle, usually no hydrocephalus → usually a good prognosis; fetal MRI follow-up of vermian rotation/hydrocephalus evolution, exclude associated anomalies.
4. [End] Vermian hypoplasia (normal tentorium)
   Vermian hypoplasia (normal tentorium, no significant posterior fossa enlargement): increased tegmento-vermian angle, small vermis; fetal MRI to measure vermian dimensions against gestational age + assess associated CNS anomalies/chromosomes; prognosis depends on degree of vermian development and associated anomalies.
5. [End] Dandy-Walker malformation · comprehensive assessment
   Dandy-Walker malformation (vermian hypoplasia/agenesis + cystic fourth ventricle dilation communicating with the cisterna magna + elevated tentorium/torcula + enlarged posterior fossa): often with hydrocephalus and other CNS anomalies/chromosomal abnormalities, variable prognosis; fetal MRI for comprehensive assessment + karyotype/microarray + multidisciplinary counseling.

Source guidelines & references

• Fetal posterior fossa malformation MRI (tegmento-vermian angle/Dandy-Walker continuum)

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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