Dandy-Walker malformation

**Definition/Triad:** (1) Complete or partial agenesis (hypoplasia) of the cerebellar vermis with upward rotation of the vermis remnant, (2) cystic dilation of the fourth ventricle that communicates with the enlarged posterior fossa, and (3) enlarged posterior fossa with high-riding tentorium (elevated torcula/"torcular-lambdoid inversion"). Hydrocephalus is common but often develops postnatally (in ~80% by 3 months), and is not required for diagnosis.

**Pathophysiology/Embryology:** Developmental failure of the roof of the fourth ventricle (rhombencephalic roof) and cerebellar vermis during weeks ~7-10 of gestation. Failure of the foramina of Magendie (and often Luschka) to open contributes to obstructive/communicating hydrocephalus. Most cases are sporadic and multifactorial. Associated factors include chromosomal abnormalities (trisomies 9, 13, 18, 21; 3q24 deletion involving ZIC1 and ZIC4; FOXC1 deletion on 6p25), teratogens (congenital CMV, rubella, toxoplasmosis, warfarin, alcohol), and maternal diabetes.

**Clinical features:** Usually diagnosed in the first year of life. Macrocephaly/enlarged head with prominent occiput, bulging fontanelle, splayed sutures, "setting-sun" sign, irritability, vomiting, and developmental delay. Cerebellar signs (hypotonia, ataxia, nystagmus, truncal instability) and delayed motor milestones. Cranial nerve palsies and respiratory irregularities can occur in severe cases. Some are diagnosed incidentally later in life.

**Associated anomalies:** Agenesis of the corpus callosum (~20-30%), occipital encephalocele, heterotopias, polymicrogyria, cardiac defects (ASD/VSD, PDA), polydactyly/syndactyly, cleft palate, and renal anomalies. Syndromic associations: PHACE syndrome (Posterior fossa malformations, Hemangiomas, Arterial anomalies, Cardiac defects, Eye abnormalities), Walker-Warburg syndrome.

**Diagnosis:** Prenatal ultrasound (can be detected as early as the second trimester) and fetal MRI. Postnatal MRI is the best modality, showing the vermian hypoplasia, posterior fossa cyst, and tentorial elevation. CT shows the enlarged posterior fossa cyst and hydrocephalus. Genetic testing (chromosomal microarray) and TORCH workup are appropriate.

**Differential (Dandy-Walker continuum):** - Dandy-Walker malformation: severe vermian hypoplasia + large posterior fossa + elevated tentorium. - Dandy-Walker variant (vermian hypoplasia): mild vermian hypoplasia, normal-sized posterior fossa. - Mega cisterna magna: enlarged cisterna magna with normal vermis and normal fourth ventricle; usually benign. - Posterior fossa arachnoid cyst: normal vermis, which may be compressed/displaced. - Chiari II malformation: small posterior fossa, associated with myelomeningocele (opposite of Dandy-Walker). - Joubert syndrome: "molar tooth sign" on MRI, vermian hypoplasia, episodic hyperpnea.

**Management:** Treat hydrocephalus with a ventriculoperitoneal (VP) shunt and/or cystoperitoneal shunt, or endoscopic third ventriculostomy. Supportive care, early intervention/physical therapy, and monitoring for neurodevelopmental deficits. Genetic counseling. Prognosis varies and depends on the degree of vermian hypoplasia and associated anomalies (the latter strongly affects intellect outcome); a significant proportion have intellectual disability, and mortality is higher with associated anomalies.

**Board pearls:** Think "infant with big head + cerebellar signs + cystic posterior fossa on imaging + absent/hypoplastic vermis" = Dandy-Walker. Contrast with Chiari II (small posterior fossa, myelomeningocele). It is a malformation of the fourth ventricle outlet region (foramina of Magendie/Luschka atresia).