IS Case 222: Arachnoid cyst

Trushar Sarang, MD

Imaging Sciences URMC 2008
Publication Date: 2009-05-22


Patient is a 45-year-old female presenting with left facial paresthesias.


Left cerebellopontine angle fluid collection/mass characteristic of an arachnoid cyst.


Arachnoid cyst


An arachnoid cyst is a benign cystic malformation commonly occurring in the region of the cerebellar-pontine angles. As the name implies the cysts contain normal cerebrospinal fluid, but sometimes contain xanthochromic fluid. They most commonly occur as developmental anomalies, however, these cysts can occur after surgery or trauma or in association with certain neoplasms.

The cysts are usually asymptomatic; however, can sometimes cause headache, seizures, and focal neurologic symptoms related to mass effect (as in this case).

The MRI characteristics help to distinguish this mass from other common cerebellar-pontine angle masses. The mass is low on T1 and does not have appreciable enhancement. This excludes most tumors such as meningiomas and schwannomas. The high T2-signal without fat suppression excludes a lipoma. The lack of restricted diffusion helps rule out an epidermoid.

Treatment is usually conservative. If there is significant mass effect surgical exicision is indicated.


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  2. Khan AN, Turnbull I, Al-Okalli R, MacDonald S, Mahmood K. Arachnoid cyst: eMedicine. December 9, 2008.
  3. Haberkamp TJ, Monsell EM, House WF, Levine SC, Piazza L. Diagnosis and treatment of arachnoid cysts of the posterior fossa. Otolaryngol Head Neck Surg. 1990 Oct;103(4):610-4. [PMID: 2123320]

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