Diagnostic Criteria

  • Classic DWM diagnosis requires:
    • Vermian hypoplasia (underdeveloped cerebellar vermis).
    • Enlarged posterior fossa.
    • Elevation of tentorium cerebelli and torcular Herophili.
    • Cystic dilation of the fourth ventricle.
  • Hydrocephalus is common (~85% by 1 year):
    • Considered a complication, not essential for diagnosis.
    • May have delayed presentation.

Pathogenesis

  • Fundamental defect in rhombencephalic roof development.
  • Mechanism:
    • Failure of anterior membranous area (AMA) integration into developing vermis.
    • Possible delayed or failed opening of posterior membranous area (PMA) foramina.
    • Redundant AMA expands due to CSF pulsations → posterior fossa cyst formation and enlargement.

Neuropathology

  • Vermis lobules present but significantly hypoplastic or dysplastic, especially inferior lobules.
  • Vermian development arrests around 12 weeks post-conception (p/c).
  • Hypo/dysplasia severity gradient (superior → inferior) due to decreased influence of the isthmic organiser with increasing distance.
  • Affected structures originate from the rhombic lip (rhombomere 1).
  • Relatively normal development of primary ventricular neuroepithelium derivatives:
    • Purkinje cells
    • Deep cerebellar nuclei

Clinical Prognosis

  • Highly variable outcomes, largely dependent on:
    • Extent of vermian hypo/dysplasia.
    • Presence of associated cerebral or extracerebral anomalies.
  • When anomaly confined to posterior fossa:
    • Primary prognostic factor: integrity of vermian lobulation.
    • Size of cyst or posterior fossa enlargement is not prognostic.
  • Intellectual impairment:
    • Seen in approximately 50% of patients.
    • Directly correlated with disturbances in vermis lobulation.

Management Approaches

  • Generally conservative unless complicated by:
    • Significant hydrocephalus.
    • Compression from posterior fossa cyst.
  • Traditional treatment for hydrocephalus:
    • Ventriculoperitoneal shunt (VP shunt).
  • Alternative approaches:
    • Endoscopic third ventriculostomy + choroid plexus cauterisation (ETV+CPC).
    • Cyst-peritoneal shunts.
    • Placement of stents connecting third ventricle and posterior fossa cyst.