Name two posterior fossa malformations that can occur in the CNS.

Name two posterior fossa malformations that can occur in the CNS.

• A. Arnold-Chiari malformation
• B. Holoprosencephaly
• C. Dandy-Walker malformation and Joubert syndrome
• D. Medullary cystic disease

Answer: (C)

Posterior fossa malformations involve the hindbrain structures—the cerebellum and brainstem—and are often recognized by distinctive imaging patterns. Dandy-Walker malformation is a classic posterior fossa malformation characterized by an enlarged posterior fossa, cystic dilation of the fourth ventricle, and underdevelopment or absence of the cerebellar vermis. This combination disrupts normal drainage of CSF and cerebellar development, leading to hydrocephalus in many cases. Imaging typically shows a large posterior fossa with a fourth ventricular cyst that communicates with the cisterna magna and vermian hypoplasia. Joubert syndrome is another posterior fossa malformation marked by hypoplasia or agenesis of the cerebellar vermis and brainstem anomalies. A key imaging feature is the molar tooth sign, caused by abnormally oriented midbrain-hindbrain structures (thickened, elongated superior cerebellar peduncles with a deep interpeduncular fossa). Clinically, infants present with hypotonia and ataxia, breathing abnormalities, and developmental delays, with possible retinal and renal involvement in some cases. Holoprosencephaly involves forebrain development, not the posterior fossa, so it doesn’t fit the category. Medullary cystic disease is a renal condition, not a CNS malformation. Arnold-Chiari malformation is another posterior fossa disorder, but the pair in question specifically names Dandy-Walker malformation and Joubert syndrome.

Posterior fossa malformations involve the hindbrain structures—the cerebellum and brainstem—and are often recognized by distinctive imaging patterns.

Dandy-Walker malformation is a classic posterior fossa malformation characterized by an enlarged posterior fossa, cystic dilation of the fourth ventricle, and underdevelopment or absence of the cerebellar vermis. This combination disrupts normal drainage of CSF and cerebellar development, leading to hydrocephalus in many cases. Imaging typically shows a large posterior fossa with a fourth ventricular cyst that communicates with the cisterna magna and vermian hypoplasia.

Joubert syndrome is another posterior fossa malformation marked by hypoplasia or agenesis of the cerebellar vermis and brainstem anomalies. A key imaging feature is the molar tooth sign, caused by abnormally oriented midbrain-hindbrain structures (thickened, elongated superior cerebellar peduncles with a deep interpeduncular fossa). Clinically, infants present with hypotonia and ataxia, breathing abnormalities, and developmental delays, with possible retinal and renal involvement in some cases.

Holoprosencephaly involves forebrain development, not the posterior fossa, so it doesn’t fit the category. Medullary cystic disease is a renal condition, not a CNS malformation. Arnold-Chiari malformation is another posterior fossa disorder, but the pair in question specifically names Dandy-Walker malformation and Joubert syndrome.