Distal aneurysms of intracranial arteries: application of numerical nomenclature, predilection for cerebellar arteries, and results of surgical management.

BACKGROUND: Distal intracranial aneurysms are rare, have unclear origins, and are frequently nonsaccular. Published clinical experience with these aneurysms is limited.

OBJECTIVE: To examine differences between distal aneurysms of cerebral and cerebellar arteries and to examine results associated with surgical therapy in 140 patients.

METHODS: Distal aneurysms in the cerebral arteries were defined as outside the circle of Willis, on or beyond the A2 anterior cerebral artery, M2 middle cerebral artery, or P2 posterior cerebral segments. Distal aneurysms in the cerebellar arteries were on or beyond the s2 superior cerebellar artery, a2 anterior inferior cerebellar artery, or p2 posterior inferior cerebellar artery segments. Clinical data, microsurgical technique, and patient outcomes were reviewed.

RESULTS: The incidence of distal cerebellar artery aneurysms was 4.3 times greater than distal cerebral artery aneurysms (6.5% vs. 28.6%; P< 0.01). The A3 anterior cerebral artery segment and the p2 and p3 posterior inferior cerebellar artery segments were the most common sites. Presentation with aneurysm rupture was more frequent with cerebellar aneurysms (65% vs. 40%; P< 0.05). Distal cerebellar artery aneurysms were less likely than distal cerebral artery aneurysms to be clipable (40% vs. 72%; P< 0.01), with 42% treated with trapping alone. Overall, 14% required a bypass.

CONCLUSIONS: Distal intracranial aneurysms have a predilection for cerebellar arteries and are not as rare as the literature suggests. Application of standardized nomenclature for segmental anatomy to these lesions will increase the precision of anatomic description and clarity of clinical discourse. Although technically difficult, good clinical results can be expected with surgical management.