Prevalence and natural history of arachnoid cysts in adults

Clinical article

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Arachnoid cysts are a frequent finding on intracranial imaging. The prevalence and natural history of these cysts in adults are not well defined.


We retrospectively reviewed the electronic medical records of a consecutive series of adults who underwent brain MRI over a 12-year interval to identify those with arachnoid cysts. The MRI studies were reviewed to confirm the diagnosis. For those patients with arachnoid cysts, we evaluated presenting symptoms, cyst size, and cyst location. Patients with more than 6 months' clinical and imaging follow-up were included in a natural history analysis.


A total of 48,417 patients underwent brain MRI over the study period. Arachnoid cysts were identified in 661 patients (1.4%). Men had a higher prevalence than women (p < 0.0001). Multiple arachnoid cysts occurred in 30 patients. The most common locations were middle fossa (34%), retrocerebellar (33%), and convexity (14%). Middle fossa cysts were predominantly left-sided (70%, p < 0.001). Thirty-five patients were considered symptomatic and 24 underwent surgical treatment. Sellar and suprasellar cysts were more likely to be considered symptomatic (p < 0.0001). Middle fossa cysts were less likely to be considered symptomatic (p = 0.01. The criteria for natural history analysis were met in 203 patients with a total of 213 cysts. After a mean follow-up of 3.8 ± 2.8 years (for this subgroup), 5 cysts (2.3%) increased in size and 2 cysts decreased in size (0.9%). Only 2 patients developed new or worsening symptoms over the follow-up period.


Arachnoid cysts are a common incidental finding on intracranial imaging in all age groups. Although arachnoid cysts are symptomatic in a small number of patients, they are associated with a benign natural history for those presenting without symptoms.

Abbreviation used in this paper:CPA = cerebellopontine angle.

Article Information

Address correspondence to: Cormac O. Maher, M.D., Department of Neurosurgery, University of Michigan, 1500 East Medical Center Drive, Room 3552 Taubman Center, Ann Arbor, Michigan 48109-5338. email:

Please include this information when citing this paper: published online November 9, 2012; DOI: 10.3171/2012.10.JNS12548.

© AANS, except where prohibited by US copyright law.



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    Bar graph illustrating arachnoid cyst prevalence in patients undergoing MRI stratified by sex and age group. Imaging prevalence of arachnoid cysts in males is greater than in females (p < 0.00001). The prevalence of arachnoid cysts does not change significantly with advancing age in adulthood.



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