Clinical characteristics and preferential location of intracranial mirror aneurysms: a comparison with non-mirror multiple and single aneurysms.

Introduction: The purpose of our study was to compare the clinical characteristics and preferential localization of aneurysms in three patient groups: single aneurysm, non-mirror multiple aneurysms, and mirror aneurysms.
Methods: We retrospectively reviewed the clinical and radiological data of 2223 consecutive patients harboring 3068 aneurysms registered at the Toronto Western Hospital between May 1994 and November 2010. The patients were divided into single, non-mirror multiple, or mirror aneurysm groups. Expected incidences of mirror aneurysms at each location were calculated on the basis of the single aneurysm incidences at each location.
Results: Patients with mirror aneurysms (n = 197) did not differ from patients with non-mirror multiple aneurysms (n = 392) in having female predominance (81.7 vs. 76.3 %) or a family history of intracranial aneurysm (20.5 vs. 17.6 %). When compared with expected incidences at each location, mirror aneurysms were more frequently found at the cavernous internal carotid artery (30 vs. 11.5 %) (p < 0.0001). Mirror aneurysms involving the posterior circulation were less frequent (6.7 %) than aneurysms in the single (19.6 %) or non-mirror multiple aneurysm groups (18.9 %) (p < 0.05).
Conclusion: Patients with mirror aneurysms had similar clinical characteristics to non-mirror multiple aneurysm patients. Mirror aneurysms showed a predilection for the cavernous carotid artery, whereas they were comparatively rare in the posterior circulation.