The natural history of arachnoid cysts in adults is not well defined.  

     Our purpose was to define the natural course or arachnoid cysts.

     48,417 patients underwent brain MR imaging. 677 patients were identified with arachnoid cysts (1.4%).

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

     Men had a higher prevalence than women (1.1%) (P<0.0001). 36 patients (5.3%) had multiple arachnoid cysts. Arachnoid cyst location was most often middle fossa (34%), retrocerebellar (33%), and convexity (14%). Middle fossa cysts were predominantly left-sided (69%, p<0.001). 36 patients (5.3%) were considered symptomatic and 23 (3.4%) underwent surgical treatment. Cerebellopontine angle cysts (16%, p=0.002) and suprasellar cysts (55%, p<0.00001) were more likely to be considered symptomatic. Middle fossa cysts were less likely to be considered symptomatic (p=0.007). 195 patients met the criteria for natural history analysis. After a mean follow-up of 3.8 +/- 2.8 years, 5 cysts increased in size (2.6%) and 2 cysts decreased in size (1%). Only 1 patient (0.05%) developed new or worsening symptoms over the follow-up period.

     This is a retrospective study.

     Arachnoid cysts are a common incidental finding in brain MR imaging.

     Although they are symptomatic in a small number of patients, they are associated with a benign natural history for those presenting without symptoms.