IDEAS IN PATHOLOGY - SEGMENTAL ARTERIAL MEDIOLYSIS - A PRECURSOR TO FIBROMUSCULAR DYSPLASIA

We describe five cases of segmental arterial mediolysis (SAM), formerl y known as segmental mediolytic arteritis. SAM occurs in epicardial co ronary arteries and in the abdominal splanchnic arteries. Patients wit h abdominal SAM are generally elderly, whereas coronary SAM presents i n neonates, children, and young adults. SAM is initiated by mediolysis of the outer media, which can expand to involve the mid- and inner me dia. Accompanying alterations include fibrinous linear deposits at the medial adventitial junction and replacement of the lysed muscle fiber s by fibrin, erythrocytes, and granulation tissue. Transmural mediolys is results in arterial wall gaps frequently complicated by dissecting hematomas and aneurysms. Abdominal hemorrhages stem from these complic ations. SAM involving abdominal splanchnic arteries clinically present s with abdominal pain and distension, falling hematocrit, and shock. S imultaneous involvement of more than one abdominal artery is frequent, and branches of the celiac axis are most commonly affected. Various d isease states that provoke pathologic stimuli for endothelial mediated vasoconstriction occur in the immediate clinical background of patien ts with SAM. Certain morphologic features of SAM suggest that this art erial lesion is due to vasospasm, which we putatively ascribe to focal endothelial paracrine dysfunction. Organization of uncomplicated SAM lesions could resemble certain types of fibromuscular dysplasia, sugge sting that the genesis of such arterial lesions is related to vasospas m.