Distal superior cerebellar artery aneurysm located at the newly formed anastomotic site with the long circumferential artery of the posterior cerebral artery: A case report.

We describe a case of a distal superior cerebellar artery (SCA) aneurysm that arose from a unique collateral pathway between the SCA and long circumferential artery (LCA) of the posterior cerebral artery (PCA). The patient was a 69-year-old male who was admitted to our facility for an asymptomatic and incidentally identified cerebellar aneurysm. Magnetic resonance imaging showed a saccular aneurysm arising from the right SCA in the quadrigeminal cistern. Digital subtraction angiography revealed an unusually dilated branch from the aneurysmal sac. Furthermore, this branch had retrograde flow from the quadrigeminal segment to the anterior pontomesencephalic segment, was connected to the PCA at the P1 segment, and exited from the P2 segment. We attributed this unusual angioarchitecture to collateral circulation secondary to severe P1 stenosis. Thus, the dilated unusual branch is an LCA of the PCA for supplying the distal PCA with blood flow. As a result, the aneurysm is probably formed at the junction between the SCA and LCA. Endovascular coiling for the aneurysm was successfully performed with preserved collateral system.