[A Rare Case of an Unruptured Fusiform Aneurysm Arising from the Lateral Limb of the Fenestration of the Anterior Medullary Segment of the Posterior Inferior Cerebellar Artery].

We report an extremely rare case of fenestration of the posterior inferior cerebellar artery(PICA)with an aneurysm at the fenestrated portion. A 60-year-old man with a medical history of hemophilia was referred to our hospital for further examination of unruptured aneurysms at the left PICA and vertebral artery(VA)that were incidentally discovered by computed tomography angiography 5 years ago during an examination of spontaneous intracerebral hemorrhage. A 3D-rotational angiography revealed a fenestration(4.7×1.4mm)in the anterior medullary segment of the left PICA associated with an unruptured fusiform aneurysm(7.2×6.1mm)arising from the entire lateral limb of the fenestrated segment. The left VA aneurysm had a wide neck(7.2×6.8mm)and was located at the portion just distal to the bifurcation of the left PICA. The patient requested conservative management of both aneurysms because no significant change in the size or shape of either had been observed during the last 5 years. An extensive review of the literature revealed that nine cases of PICA fenestration have been reported. Among these, detailed medical records were described in seven cases, and an aneurysm at the fenestrated artery had been detected in only 2 cases. To our knowledge, this is the first report of a PICA fenestration associated with an unruptured aneurysm at the fenestrated segment. Embryologically, the PICA fenestration may represent a remnant of plexiform arterial channels between the primitive VA and primitive lateral vertebrobasilar anastomosis.