OBJECTIVEThe aim of this study is to classify nonatherosclerotic aneurysms unrelated to the branching zones (including fusiform aneurysms and dissecting aneurysms).METHODSDamage to the internal elastic lamina (IEL) is often an associated factor in the pathogenesis of aneurysm formation. In this study, 85 nonatherosclerotic aneurysms arising from an arterial trunk unrelated to the branching zones were classified into four different types, based on the lesional patterns of the IEL and the state of the intima. Type 1 corresponded to classic dissecting aneurysms, the pathogenesis of which was characterized by acute widespread disruption of the IEL without intimal thickening. Patients with Type 1 aneurysms had an ominous clinical course, and many presented with sudden subarachnoid hemorrhage with frequent rebleeding. Type 2 aneurysms were segmental ectasias, which had an extended and/or fragmented IEL with intimal thickening. Weakness of the arterial wall caused by the damaged IEL was assumed to be compensated by the intimal thickening. The luminal surface of the thickened intima was smooth without thrombus formation. The patients with Type 2 aneurysms had a placid clinical course. Type 3 aneurysms were dolichoectatic dissecting aneurysms, pathologically characterized by fragmentation of the IEL, multiple dissections of thickened intima, and organized thrombus in the lumen. Most of them were symptomatic and progressively enlarged over time. Type 4 aneurysms were saccular aneurysms unrelated to the branching zones. They arose in areas with minimally disrupted IEL without intimal thickening, and there was a risk of rupture.CONCLUSIONThere was a strong relationship between the pathological features of the aneurysms and their clinical courses. This classification may provide a rationale for modes of treatment.