segmental arterial mediolysis
Segmental arterial mediolysis (SAM) is an increasingly recognized vascular disease of the middle-aged and elderly and a leading cause of spontanoeus intra-abdominal hemorrhage. It is characterized by fusiform aneurysms, stenoses, dissections and occlusions within splanchnic arterial branches. Imaging is not only the key to diagnosis of this condition, but can also facilitate life-saving endovascular therapy.
SAM was first described as a discrete entity in 1976 , and was initially thought to be very rare . Only 14 cases were reported over 20 years to 1997, but in the last decade reports have become much more frequent , and this is thought to be a combination of increased use of thin slice computed tomographic angiography (CTA), and increased awareness in the radiological community. The incidence may be as high as 1 per 100 000 per year.
SAM most commonly presents as spontaneous intra-abdominal hemorrhage in patients aged 50 to 80. Bleeding may occur into the mesentery or peritoneum, or less commonly, into the bowel lumen. Abdominal pain, distension, shock, falling hematocrit are typical. Bowel ischemia, hematuria, or hemobilia are also described . For middle-aged patients with non traumatic spontaneous mesenteric hemorrhage, SAM is the most likely underlying cause. Mortality has been described as high as 50% .
SAM is an uncommon arteriopathy, which is not atherosclerotic or inflammatory. The underlying histological process is lysis of the smooth muscle of the outer media of the arterial wall , resulting in intramural hemorrhage, saccular or dissecting aneurysms, thrombosis and hemorrhage. It affects the visceral arteries of the abdomen in a skip pattern , most commonly affecting the medium size branches of the superior mesenteric artery. The etiology is unknown, but an association with episodes of splanchnic vasoconstriction has been observed (e.g. shock, hypoxia, recent major operation, vasopressor infusion) . There is some histological similarity to fibromuscular dysplasia, which is a differential diagnosis, but the clinical features and lesion distribution are usually characteristic.
The hallmark of the disorder is multiple abdominal splanchnic artery aneurysms. CT is the modality of choice .
Patients presenting with SAM typically have CT scans showing mesenteric or intraperitoneal hemorrhage, and CT angiography reveals a range of arteriographic abnormalities of the branches of the visceral arteries including:
- fusiform aneurysms
A pattern of aneurysms and stenoses in series is characteristic, the ‘string-of-beads’ appearance. Dissections are typical, and otherwise rare in splanchnic branches, and the distribution tends to spare bifurcations, in contrast to mycotic aneurysms .
Treatment and prognosis
SAM is variable in the severity of presenting illness, and conservative therapy may be appropriate. Importantly, immunosupression by steroids or other drugs is thought to be counterproductive, as the arteriopathy is not inflammatory or autoimmune.
For incidentally discovered lesions, the natural history of SAM is poorly understood, but most authorities recommend treating aneurysms which have not bled, especially if more than 10 mm in size .
Significant abdominal hemorrhage often requires urgent treatment. Coil embolization is most frequently advocated, and this is usually straightforward where the affected artery can be sacrificed without causing significant distal ischemia. As usual in the visceral arteries complete exclusion often requires vessels to be occluded both proximal and distal to the bleeding site. Glue embolization has been described .
If embolization is not available or contraindicated, surgical exploration, ligation, and resection of affected gut segments and viscera may be necessary.
Cases of SAM have been described in which the onset of arteriographic abnormalities is abrupt, and then enter a symptomatic stage, during which hemorrhage carries a significant mortality. After this the arteriopathy has been shown to resolve, to remain in status quo, to fluctuate, and to move from some segments to others. This unpredictable natural history leads most authors to recommend continued surveillance over several years, with prophylactic endovascular treatment for aneurysms which are enlarging . Generally the prognosis after the presenting episode appears to be good.
Conditions to consider include :
- inflammatory arteritides
- mycotic aneurysms
- different in distribution from SAM, which affects random segments in a ‘skip’ pattern, and does not favor bifurcations
- cystic medial necrosis
- rarely affects the visceral segments, and the patient usually has stigmata of Marfan syndrome
- fibromuscular dysplasia
- rare in vessels other than the carotid and renal arteries, is not painful, affects a younger age range, and is rarely associated with hemorrhage