= characterized CLASSICALLY by

1. Vermian hypoplasia with cephalad rotation of remnant
2. Cystlike dilatation of 4^th ventricle → enlarged posterior fossa with abnormally high position of tentorium and torcular (lambdoid-torcular inversion)

Incidence: 12% of all congenital hydrocephaly

• Most common posterior fossa malformation

Genetics: sporadic occurrence; 1–5% risk of recurrence

Path: defect in vermis connecting an ependyma-lined retrocerebellar cyst with 4^th ventricle (PATHOGNOMONIC)

Cause: dysmorphogenesis of roof of 4^th ventricle with failure to incorporate the area membranacea into developing choroid plexus

Time of origin: high insertion of tentorium suggests development before end of embryonic period! Originally proposed as congenital atresia of foramina of Luschka (lateral) + Magendie (median) – but proposal NOT likely since foramina are not patent until 4^th month of GA

Associated anomalies:

• midline CNS anomalies (in >60%)
  1. Dysgenesis of corpus callosum (20–25%), lipoma of corpus callosum
  2. Holoprosencephaly (25%)
  3. Malformation of cerebral gyri (dysplasia of cingulate gyrus) (25%)
  4. Cerebellar heterotopia + malformation of cerebellar folia (25%)
  5. Malformation of inferior olivary nucleus
  6. Hamartoma of tuber cinereum
  7. Syringomyelia
  8. Cleft palate
  9. Occipital encephalocele (<5%)
• other CNS anomalies:
  1. Polymicrogyria / gray matter heterotopia (5–10%)
  2. Schizencephaly
  3. Lumbosacral meningocele
• non-CNS anomalies (25%)
  1. Polydactyly, syndactyly
  2. Klippel-Feil syndrome
  3. Cornelia de Lange syndrome
  4. Cleft palate
  5. Facial angioma
  6. Cardiac anomalies

Age: majority <1 year

• macrocephaly (90%)

Skull film:

• large skull ← hydrocephalus + dolichocephaly
• diastatic lambdoid suture
• disproportionately large expanded posterior fossa
• torcular-lambdoid inversion = upward displacement of tentorium + torcular herophili + transverse and lateral dural sinuses high above lambdoid angle

CT / US / MR:

• absence / hypoplasia of cerebellar vermis:
  total (25%), partial (75%)
• superiorly displaced superior vermis cerebelli
• small widely separated ± hypoplastic cerebellar hemispheres ← anterolateral displacement by cystic 4^th ventricle
• large posterior fossa cyst with extension through foramen magnum = diverticulum of roofless 4^th ventricle
• elevated insertion of tentorium cerebelli
• cerebellar hemispheres in apposition without intervening vermis following shunt procedure
• absence of falx cerebelli
• scalloping of petrous pyramids
• ventriculomegaly (in 72% open communication with 3^rd ventricle; in 39% patent 4^th ventricle; in 28% aqueductal stenosis; in 11% incisural obstruction); present prenatally in 30%, by 3 months of age in 75%
• anterior displacement of pons

Angio:

• high position of transverse sinus
• elevated great vein of Galen
• elevated posterior cerebral vessels
• anterosuperiorly displaced superior cerebellar arteries above the posterior cerebral arteries
• small / absent PICA with high tonsillar loop

Cx: trapping of cyst above tentorium = “keyhole configuration”

Prognosis: fetal demise in 66%; 22–50% mortality during 1^st year of life

DDx:

1. Posterior fossa extra-axial cyst
2. Arachnoid cyst (normal 4^th ventricle, patent foramina, intact vermis)
3. Isolated 4^th ventricle
4. Megacisterna magna
5. Porencephaly

It is not recommended to use terms like Dandy-Walker variant, Dandy-Walker complex, Dandy-Walker spectrum because they lack specificity and add confusion.