Segmental mediolytic arteritis. A clinicopathologic and ultrastructural study of two cases.

We describe the histopathologic and ultrastructural changes in two cases of segmental mediolytic arteritis (SMA) and summarize the clinical and pathologic findings in previous reports. SMA is initiated by the transformation of the arterial smooth-muscle cytoplasmic contents into a maze of dilated vacuoles containing edema-like fluid. With vacuolar rupture, the smooth-muscle cells are disrupted and the mediolytic process completed. Mediolysis is accompanied by fibrin deposition and hemorrhages at the adventitio-medial junction and within the media. Inflammation is inconstant and limited to the periadventitial tissues. Transmural mediolysis leads to the formation of arterial wall gaps--defects in the vascular wall bridged by a serofibrinous layer. The serosal and intramural arteries and arterioles of the jejunum and the epicardial coronary arteries were the targets of SMA in this report. SMA occurs in two clinical settings: (a) in abdominal muscular arteries and arterioles of predominantly elderly patients presenting either with ischemic bowel disease or shock, and (b) in the coronary arteries of neonates in conditions associated with severe hypoxemia. We conclude that SMA is the result of an inappropriate vasospastic response expressed in a splanchnic vascular bed undergoing vasoconstriction as a response to shock or severe hypoxemia.