[Basilar artery aneurysm associated with agenesis of unilateral internal carotid artery: two case reports].

: Two patients presented with a basilar artery aneurysm associated with agenesis of the right internal carotid artery. In both cases, the right middle cerebral artery was supplied by the basilar artery via the enlarged right posterior communicating artery, and the right anterior cerebral artery was supplied by the left internal carotid artery via the anterior communicating artery. The first patient was a 65-year-old woman who suffered from a subarachnoid hemorrhage due to rupture of a basilar bifurcation aneurysm, which was demonstrated as a "de novo" aneurysm 13 years after successful clipping of the anterior communicating artery aneurysm. Delayed surgery was planned, but she died due to recurrent hemorrhage. The second patient was a 67-year-old woman who had had a history of progressive left sided weakness over the previous few years. Conventional angiography and 3 dimensional CT angiography showed a large aneurysm of the basilar artery trunk with a wide neck. Endovascular embolization was performed with mechanically detachable coils, and the aneurysm was incompletely occluded. The patient regained complete strength in the left arm after the treatment, but the follow-up angiography at 5 months disclosed recanalization of the treated aneurysm, associated with shift of the packed coils. Hemodynamic stress resulting from unique collateral circulation with agenesis of the internal carotid artery may cause a predisposition to "de novo" aneurysm formation or recanalization of an occluded aneurysm with coils.