How often do chronic extra-cerebral haematomas occur in patients with intracranial arachnoid cysts?

Background: Chronic subdural (CSDH) or intracystic haematomas may occur as a complication in patients with arachnoid cysts, even in children and young adults. The aim of this study was to investigate how often an arachnoid cyst is complicated by such haematomas, as this information may be of importance when considering decompressive cyst surgery. Materials and methods: This study includes a total of 241 consecutive, adult and paediatric patients ( 157 males and 84 females), that were referred to our department for treatment for an arachnoid cyst during the period January 1987-September 2004. Our department is the only neurosurgical department in a well-defined geographial region, with a population of 930 000. Results: A chronic haematoma was found in a total of 11 patients (4.6%), all harbouring a temporal cyst. For the temporal cysts, the haematoma frequency was 6.5%. Haematomas occurred equally frequently in individuals with small, middle-sized and large temporal cysts, and in the age groups studied and in both genders. Conclusion: As also reflected in a literature review, intracranial chronic haematomas occur most frequently as a complication of middle fossa cysts. Possible mechanisms behind the formation of haematomas, and the strong associations with temporal fossa cysts, are discussed, with emphasis on the loose attachment of the membrane of the arachnoid cyst to the dura in the middle fossa, and its possible role as an extra wall'' covering easy-bleeding vascular structures in the dura.