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Ruptured fusiform aneurysm of the superior third of the basilar artery associated with the absence of the midbasilar artery. Case report.

A case involving the absence of the midthird portion of the basilar artery (BA) associated with a ruptured fusiform aneurysm of the superior third of the basilar artery discovered after a subarachnoid hemorrhage is reported. Surgical clipping was precluded by the anatomical conditions. The aneurysm was treated by occlusion (surgical clipping and balloon occlusion) of both posterior communicating arteries to decrease the hemodynamic stress on the aneurysm wall. The pericerebellar arterial network was allowed to supply the distal BA and its collateral vessels indirectly. This treatment proved to be efficient; angiography and magnetic resonance imaging demonstrated shrinkage of the aneurysm cavity. The absence of the midthird of the BA is usually associated with a persisting trigeminal artery (nonexistent in this case) or disclosed in cases of acute BA occlusion in dramatic clinical conditions. A similar anatomical feature has been described only once before. There may be a segmental maldevelopment of the longitudinal neural arteries during embryogenesis or a defect in fusion of these paired structures during the development of the BA itself.