Characteristics of Peri-Internal Carotid Artery Bifurcation Aneurysms According to a New Anatomic Classification: How to Overcome Difficulties in the Microsurgical Treatment of Posteroinferiorly Projecting Carotid-A1 Junctional Aneurysms

OBJECTIVE: Based on our clinical experience, posteroinferiorly projecting carotid-A1 junctional aneurysms are often difficult to treat microsurgically. Our objective was to classify peri-internal carotid artery (ICA) bifurcation aneurysms according to their location and analyze their characteristics. METHODS: From January 2008 to October 2017, microsurgical or endovascular treatment of 6777 aneurysms were performed at our hospital. We identified 199 peri-ICA bifurcation aneurysms (2.94%) classified into true ICA bifurcation aneurysm, carotid-A1 junctional aneurysm, and carotid-M1 junctional aneurysm according to the anatomic location. Medical records including patient characteristics, aneurysm location, surgical method, any neurologic deficits, clinical outcomes, medical history, and radiologic findings were retrospectively reviewed. The anatomic position of the aneurysm was defined from the virtual surgical, anteroposterior, and lateral views, and the degree of agreement was calculated. RESULTS: There were 103 true ICA bifurcation aneurysms, 92 carotid-A1 junctional aneurysms, and 4 carotid-M1 junctional aneurysms. Carotid-A1 junctional aneurysms tended to be smaller, elongated, and more often posteroinferiorly projecting than true ICA bifurcation aneurysms. Posteroinferiorly projecting carotid-A1 junctional aneurysms tended to require complex aneurysm surgery. The virtual surgical view had an almost perfect degree of agreement with the actual surgical view. CONCLUSIONS: The characteristics of carotid-A1 junctional aneurysms and true ICA bifurcation aneurysms differ. In particular, carotid-A1 junctional aneurysms tend to have a posteroinferior projection and that causes difficulty in surgical treatment. We recommend the virtual surgical view for preoperative planning. Furthermore, an adequate Sylvian fissure opening and a strategic approach using appropriate devices to inspect blind spots should be considered for a successful treatment outcome..