[A Rare Case of an Unruptured Aneurysm Arising from the Proximal End of the Fenestration of the Infracallosal Segment(A2)of the Anterior Cerebral Artery].

We report an extremely rare case of a fenestration in the infracallosal(A2)segment of the right anterior cerebral artery(ACA)associated with an unruptured aneurysm at the proximal end of the fenestrated portion. A 70-year-old man was referred to our hospital for further examination and treatment of unruptured aneurysms at the A1/A2 junction of the right ACA and the distal end of the right vertebral artery that were incidentally detected on MR angiography. CT angiography and 3D-rotational angiography revealed a fenestration in the proximal A2 segment of the right ACA with a small(3.6×3.8mm)aneurysm arising from the proximal end of the fenestration. The dome of the aneurysm projected to the left anterior/inferior direction. He underwent aneurysm neck clipping surgery via the right pterional approach without complications, while the vertebral artery aneurysm was managed conservatively. An extensive review of the literature revealed that only a small number of cases of A2 fenestration have been reported to date. To our knowledge, this is the first report of an A2 fenestration associated with an aneurysm at the fenestrated segment. Embryologically, the A2 fenestration may represent a remnant of plexiform arterial channels that constitute the primitive ACA system including the primitive olfactory artery, median artery of the corpus callosum and anterior communicating plexus.