Dandy-Walker Malformation

DEFINITION & CLASSIC TRIAD

Dandy-Walker Malformation is a congenital posterior fossa anomaly characterized by a developmental failure of the roof of the fourth ventricle. The classic "Dandy-Walker Triad" includes:

1. Complete or partial agenesis of the cerebellar vermis.
2. Cystic dilatation of the fourth ventricle.
3. Enlarged posterior fossa with upward displacement of the tentorium, lateral sinuses, and torcular herophili (torcular-lambdoid inversion).

ETIOLOGY & PATHOGENESIS

• Embryology: Result of an insult to the development of the rhombencephalon (hindbrain) occurring before the 6th–7th week of gestation.
• Mechanisms: Likely involves a failure of the foramen of Magendie and Luschka to canalize (though this is debated) and abnormal development of the anterior and posterior membranous areas (AMA/PMA).
• Genetics: Frequently sporadic; however, associations exist with Trisomy 13, 18, 21, and Triploidy. Linked to deletions on 3q24 (ZIC1 and ZIC4 genes).

CLASSIFICATION (DANDY-WALKER COMPLEX)

1. Classic DWM: As defined above (Vermis agenesis + huge cyst + enlarged fossa).
2. Dandy-Walker Variant: Partial vermian hypoplasia with variable cystic communication, without significant posterior fossa enlargement.
3. Mega Cisterna Magna: Enlarged posterior fossa with intact vermis and fourth ventricle.
4. Posterior Fossa Arachnoid Cyst: Normal vermis and fourth ventricle but displaced by an extrinsic cyst.

CLINICAL FEATURES

• Neonatal/Infancy:
  • Rapidly increasing head circumference (macrocephaly).
  • Bulging anterior fontanelle and split sutures.
  • Prominent occiput (dolichocephaly).
  • Transillumination of the posterior fossa may be positive.
• Older Children:
  • Signs of raised intracranial pressure (ICP): Headache, vomiting, papilledema.
  • Cerebellar signs: Truncal ataxia, nystagmus, dysmetria.
  • Cranial nerve palsies (VI, VII).
  • Developmental delay and intellectual disability (in 40-70%).
• Associated Anomalies (Found in 50-70%):
  • CNS: Agenesis of corpus callosum (25%), holoprosencephaly, neuronal migration defects.
  • Non-CNS: PHACE syndrome, cleft palate, polydactyly, cardiac defects.

INVESTIGATIONS

1. Neuroimaging (Gold Standard):
   • MRI Brain: Best for assessing vermian anatomy, brainstem hypoplasia, and associated cortical malformations. Sagittal sections show "elevation of the torcular."
   • CT Scan: Shows the "Cyst-like" posterior fossa and hydrocephalus (present in 80% of cases).
   • Antenatal USG: Can detect DWM in the second trimester (look for "keyhole" appearance of the 4th ventricle).
2. Genetic Workup: Karyotyping or Chromosomal Microarray if syndromic features are present.

MANAGEMENT

• Surgical Management: Primary goal is treating hydrocephalus and reducing cyst pressure.
  • Cystoperitoneal Shunt (CP Shunt): Shunting the posterior fossa cyst directly into the peritoneum.
  • Ventriculoperitoneal Shunt (VP Shunt): Shunting the lateral ventricles (often required if aqueductal stenosis is present).
  • Combined Shunting: Linking the cyst and ventricle to a single peritoneal limb to prevent upward/downward herniation.
  • Endoscopic Third Ventriculostomy (ETV): Emerging as an alternative to shunting in selected cases.
• Medical/Supportive:
  • Management of seizures.
  • Early intervention programs (PT/OT/Speech therapy) for developmental delays.

PROGNOSIS

• Depends on the severity of vermian hypoplasia and the presence of associated supratentorial malformations.
• Mortality is approximately 10-20%.
• Intellectual outcome is better if the vermis is only partially affected and if no other CNS malformations coexist.