A posterior fossa cyst in the region of the cisterna magna is a cerebrospinal space cavity within the single layer of the arachnoid space. Posterior fossa cysts may or may not compress the brainstem and the cerebellum.
The distinction between a posterior fossa cyst in the region of the cisterna magna and a megacisterna magna is not possible and is clinically unnecessary unless the cyst contains blood or is compressing the brainstem and the cerebellum. Posterior fossa cysts that compress the brainstem and the cerebellum may require surgery.

Joubert Syndrome
Joubert syndrome may present with hypotonia and decreased dynamic tone, hyperventilation alternating with apnea, and abnormal eye movements during the neonatal period. Joubert syndrome is characterized by agenesis or hypoplasia of the cerebellar vermis. The posterior fossa is normal or small in size (Figure 119.1). The tentorium is not high. Other central nervous system abnormalities that often present with Joubert syndrome are dysplasia or heterotopia of the cerebellar nuclei, absence of the pyramidal decussation, and histological abnormalities of the inferior olivary nuclei, trigeminal tract, solitary fascicle, and dorsal column nuclei.

A

B

Figure 119.1.— Schematic representation of Joubert syndrome. Agenesis [A] or hypoplasia [B] of the cerebellar vermis and small cisterna magna.