We describe the first recorded case of segmental mediolytic arteritis (SMA) in Japan. A 71‐year‐old Japanese woman developed sudden abdominal pain and went into shock during hospitalization for treatment of valvular heart disease. Laparotomy revealed a ruptured pseudoaneurysm of the left gastric artery. Histological examination of the resected artery revealed multiple extensive areas of the dilated wall where the media had partially or totally disappeared with or without the intima. These features were typical of those described previously for SMA (1, 2). Three dimensional analysis of the arterial lesions revealed that the residual media formed branches and cavities down the long axis of the artery, indicating that the major cause of the medial disappearance was arterial dissection. Three months later, the patient died of sepsis. Autopsy revealed pseudoaneurysm formation at three different locations in the splenic artery. Histologically, these pseudoaneurysms showed destructive changes in the arterial wall similar to those described above, with organization and thrombosis, and were considered to be the end stage lesion of SMA. This case is considered to be the fifth adult case of SMA in the literature and the first one showing chronic changes. Acta Pathol Jpn 42: 201–209,
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