Etiology of Arachnoid Cysts

Abstract

Postoperative reduction in the size of arachnoid cysts was examined in 48 surgical cases. In these and 11 autopsy cases, the cyst walls were examined histologically. Nine cases showed less than 20% reduction, 27 exhibited reduction of 30 to 80%, and 12 showed over 90% reduction in cyst volume. The outer walls of every cyst were found to contain arachnoid membrane. Interestingly, the walls of two cysts contained choroid plexus. One of these, initially detected by computed tomography, was in the middle cranial fossa. Because of its transparent, glistening membrane, observed at operation, it was considered to be an arachnoid cyst. Recurrence of symptoms and cyst formation rendered reoperation necessary. In the outer membrane of the wall of the cyst removed at the second operation, choroid plexus-like tissue was observed microscopically. In the second case, a small cyst resembling an arachnoid cyst was incidentally found at autopsy in the cerebellopontine angle. A small tuft containing choroid plexus was present on the floor of the cyst. Both cases were diagnosed as arachnoid cysts on the basis of histological examination of specimens of their outer membranes. The authors postulate that heterotopic choroid plexus in the subarachnoid space can secrete cerebrospinal fluid (CSF) and may split the arachnoid membrane, thus forming a growing cyst. In another interesting case, after craniotomy and total removal of a malignant choroid plexus papilloma, a large cyst resembling an arachnoid cyst appeared in the left frontal lobe along the tract of the ventricular drainage. The cyst seemed to have formed from CSF secretion from the metastatic choroid plexus. This case appears to support the authors' hypothesis.