Arachnoid cysts are a congenital abnormality that arise from splitting of the arachnoid membrane during development and contain CSF-like fluid. They account for approximately 1% of all intracranial masses and are most commonly located in the middle fossa, cerebellopontine angle (CPA), suprasellar region, and the posterior fossa, although they may also occur in the spinal canal. Intracranial arachnoid cysts are most often asymptomatic, unless in the suprasellar region. The incidence is 5/1000. Men are 4 times as likely to be affected as women. As many are discovered incidentally on imaging, current recommendations are to follow-up with imaging in 6-8 months to rule out size expansion. They are followed over time and if stable, imaging can be repeated with large intervals in asymptomatic patients unless the lesion becomes symptomatic. Should symptoms occur, they are most commonly associated with signs of intracranial hypertension, including headache, nausea and/or vomiting, and lethargy. Symptomatic arachnoid cysts are more common in children. The Galassi classification scheme classifies arachnoid cysts in the middle cranial fossa based on size and degree of displacement of adjacent brain. Type I are small and spindle-shaped, limited to the anterior portion of the middle cranial fossa below the sphenoid ridge, and have free communication with the subarachnoid space. Type II have a superior extent located along the sylvian fissue, displace the temporal lobe, and have slow communication with the subarachnoid space. Type III are large and fill the whole middle cranial fossa, displace the temporal, frontal, and parietal lobes often causing midline shift, and have little communication with the subarachnoid space.

Treatment of symptomatic arachnoid cysts involves surgery. The preferred intervention is craniotomy and excision of the cyst wall with fenestration into the basal cisterns (marsupialization), or endoscopic cyst fenestration through a burr hole where the wall of the shunt is opened towards one of the subarachnoid cisterns. Scarring can impair flow through the subarachnoid space resulting in shunt dependency. It may also lead to cyst reaccumulation by blocking the fenestration created intraoperatively. Shunting the cysts is another option which is falling out of favor or if other options fail. This has a low morbidity and mortality and a low rate of recurrence. However, the patient becomes shunt dependent, with associated risk of shunt-malfunction and shunt-infection.