Giant fusiform aneurysms are characterized by tortuosity, elongation, distention, and partial thrombosis, and are most frequently found in the vertebrobasilar circulation. Surgical removal of these aneurysms is the treatment of choice for such lesions because they often develop mass signs. We report 14 surgically treated cases that were classified into so-called thrombosed aneurysms in the posterior fossa (VA involved: 9, VA-BA involved: 3, BA involved: 2). Four cases out of 9 VA-involved cases were treated with aneurysmal trapping and aneurysmectomy after OA-PICA anastomosis. The other 5 cases were trapped and aneurysmectomy was performed because of no involvement of PICA flow. Seven out of 9 cases showed very excellent surgical outcomes instead of preoperative pyramidal signs. However, 5 cases involving VA-BA or BA were treated with high flow bypass (VA-RA-PCA bypass), and Hunterian ligation showed very poor surgical outcomes. Only 1 case recovered well after surgery and resumed social activity. The unsatisfactory results were considered to be caused by high flow bypass flew up thrombus induced within the blind aneurysm sac into the perforators, resulting in catastrophic central pontine infarction. The enhanced reversed flow through the bypass caused the already thrombosed aneurysms to grow. We analyze the operative results and discuss the pathogenesis.
