10% of these occur in the posterior fossa.In a posterior fossa arachnoid cyst, unlike in DWM:
The cyst is clearly localised in a specific location separate from the fourth ventricle outlets. The cerebellum is not hypoplastic, though it may be compressed by the cyst (mass effect). The CSF flow in the cyst is not continuous with that of the fourth ventricle. Hydrocephalus, if it occurs, is due to the cyst pressing on the cerebellum and compressing the cerebral aqueduct or fourth ventricle outlets. Treatment
The main immediate goal of treatment is the control of hydrocephalus and the enlarged posterior fossa cyst, as these can lead to increased intracranial pressure and brain damage. A minority of those affected do not develop hydrocephalus and are treated based on any associated symptoms or condition. Hydrocephalus/cyst
For hydrocephalus or the posterior fossa cyst, shunts are the mainstay of treatment. However, those with DWM have a higher rate of shunt-related complications than other patients with hydrocephalus (mainly due to the unconventional anatomy). One explanation for a failure of a shunt to reduce intracranial pressure in DWM has been that the cyst may herniate into the foramen magnum and form a scarring adhesion at the cervical junction, preventing it from shrinking again.
MRI is the preferred imaging study. An abnormal MRI is associated with a worse visual outcome. References


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