We aimed to identify anatomic factors favoring intra-aneurysmal clot embolization complicating coil embolization of basilar tip aneurysms. Thirty basilar tip aneurysm cases were classified angiographically into three types according to branching pattern of the superior cerebellar artery (SCA) and coil embolization complications were analyzed. The SCA may arise from the basilar artery (BA) just proximal to the origin of the posterior cerebral artery (PCA), initially coursing at an angle (more than 60°) relative to the BA, (type A). Alternatively the SCA may originate directly from the PCA at a sharp angle less than 30° relative to the BA (type C). Type B includes patterns intermediate between types A and C. Behavior of particles chosen to simulate intra-aneurysmal clots was also observed in a plastic tube model with pulsatile water flow simulating configurations A and C. Type C branching was seen in 35% (21/60) of SCA, being dominant on the left side and associated with large aneurysms and broad necks. All 3 of 24 coil embolization patients with ischemic complications in the SCA territory had large aneurysms and type C SCA branching, 2 aneurysms having broad necks. In the plastic model embolized “clots” more frequently lodged in type C than in type A SCA. “Clots” close to the orifice migrated more easily than those in the dome of the plastic aneurysm. Large basilar tip aneurysms with broad necks carry a risk of intra-aneurysmal clot migration into the SCA, during and after the embolization, especially in type C configurations, because pulsatile blood flow in the basilar artery may disperse clots between the coils and carry them into the sharply angulated SCA. Avoiding this complication requires meticulous coil packing to interrupt inflow into the aneurysm as well as appropriate anti-coagulation therapy.
