Ruptured Saccular Aneurysm Caused by Necrotizing Arteritis of the Polyarteritis Nodosa Type in Primary Angiitis of Central Nervous System.

Primary angiitis of central nervous system (PACNS) is a rare idiopathic vasculitis that typically involves small arteries. An 18-year-old woman was operated on for resection of a ruptured aneurysm in a cerebral artery. Multiple aneurysms of cerebral arteries had been detected by neuroimaging examinations since the age of 12, and she had been administered drugs following a diagnosis of PACNS since the age of 15. The resected aneurysm was a ruptured saccular aneurysm occurring in a medium-sized artery. Histologically, necrotizing arteritis of the polyarteritis nodosa (PAN) type was noted in the aneurysmal wall. It consisted of an admixture of acute and healing stages. In the acute stage, fibrinoid necrosis in the intima and media and intense inflammatory cell infiltrate in the entire wall were present. The inflammatory cells mainly consisted of lymphocytes, including plasma cells, neutrophils, and macrophages. In the healing stage, disappearance of fibrinoid necrosis, fibrosis in the intima and media, and scarce inflammatory cells were noted. The acute stage was mainly present near the ruptured site. From these findings, the aneurysm was considered to have been caused by necrotizing arteritis of the PAN type. Although saccular aneurysmal formation in a medium-sized artery is rare in PACNS, an understanding of aneurysms produced by necrotizing arteritis of the PAN type offers useful information for the diagnosis and treatment of PACNS.