Segmental arterial mediolysis (SAM) is a rare arterial disease that presents with massive hemorrhage in various parts of the body, including the abdominal cavity, retroperitoneum, heart or the brain.^1–5 It is nonatherosclerotic, noninflammatory vascular disease involving large- to medium-sized arteries and occasionally adjacent veins.^6–8 It affects the media of arteries, thus leading to smooth-muscle-cell vacuolar degeneration.^9,10

SAM is one of the disease entities that may be difficult to diagnose and the exact incidence of the disease is unknown. This can be attributed to the nonspecific clinical presentations including subclinical presentations, multiple organ involvement, common radiologic findings with other types of vasculitis such as fibromuscular dysplasia (FMD) and lack of routine angiography performed for patients with abdominal pain.^1,9,11,12 The increased awareness of SAM led to its increased reporting and the technical advances in computed tomography (CT) and magnetic resonance (MR) imaging enabled an early diagnosis before clinical presentation. In this article, we reviewed the radiologic considerations, clinical features, diagnosis and management of SAM.

Although SAM was first reported in 1976 by Slavin and Gonzalez-Vitale,¹³ the pathologic findings were then not well-recognized. According to current studies, lysis of the medial layer of the muscular arterial wall, caused by disruption of the smooth muscle cell membrane, is the primary pathologic finding.^9,11 This seems to be a dynamic process with a temporal sequence of damage in the media, and thus injuring and stimulating a robust reparative response in the muscular arteries.^6,10 Slavin^1,6 suggested both the initial injurious phase and the subsequent reparative phase. In the initial phase, vaculolar degeneration is seen in the medial layer, and in the reparative phase, proliferation of granulation tissue filling and expansion of arterial wall defects, which subsequently forms plaques, can be observed.^1,6,12

These pathologic findings are related to the pathogenesis of the disease. Repeated vasoconstrictive stimuli causing degeneration of smooth muscle cells in the media seems to be the key pathogenesis.¹⁰ Exposure to α-1 adrenergic receptor agonists or β-2 receptor agonists causing release of norepinephrine from the peripheral nervous system, is also thought to be the possible cause of SAM. The recent animal study by Slavin and Yaeger¹⁴ supports this theory in which the exogenous ractopamine, the β-adrenergic receptor agonist, injected into dogs leads to SAM.^6,10,14 Considering this hypothesis, the use of norepinephrine antagonists for the treatment of SAM can be a possible option, although it is as yet untested.⁶

SAM was originally reported to affect middle-aged and older adults; however, more recently, this disease has been seen in all age groups, and with a slight male predominance (male:female ratio of 1.5:1).^1,10,11,15 Cerebral arterial involvement is reported to be more common in younger patients.³

The symptoms vary according to the involved organ as SAM can affect almost every part of the body. As the most commonly involved site is the celiac trunk, and which we will discuss later, the most common symptom of SAM is abdominal pain, accounting for approximately 66% of the reported symptoms.¹¹ Hemoperitoneum and hypovolemic shock occur in less than 1/3 of the patients.¹⁰

End-organ damage, including bowel ischemia, hematuria, and neurologic deficits, have also been reported.¹⁰ And according to the systematic review by Shenouda et al,¹¹ six of their 85 patients were incidentally found without specific symptoms. Asymptomatic patients are being more frequently diagnosed with SAM due to the increasing use of CT and MR for other abdominal symptoms. In symptomatic patients, varying symptoms and involved sites can be obscure and, therefore, make the early and exact diagnosis of the disease more difficult.

Because the pathologic diagnosis is not available in many patients with SAM, the radiologic findings have a critical role in the diagnosis of this disease. The most commonly involved sites are the celiac artery and its branches, including the splenic artery, which account for 70% to 80% of the cases.^10,11,16 Renal arteries are the most commonly affected non-visceral arteries, and which account for seven to 24% of the cases.^10,11,16

Six types of angiographic findings were suggested by Slavin.^1,6 They are arterial dilatation, single aneurysm, multiple aneurysms, dissecting hematomas, arterial stenosis, and arterial occlusions. Lysis of media of the arterial wall can be seen as arterial dilatation or aneurysm on angiography (Fig. 1A, 2B).¹ It can involve multiple areas with an intervening, intact arterial segment, and causing the typical “string of beads” appearance (Fig. 3, 4).¹¹ Dissecting aneurysms can be seen when transmedial mediolysis results in “arterial gaps”.¹¹ Unlike mycotic aneurysms, arterial dilatation and aneurysm formation rarely involve the branching sites.10 There may be multiple aneurysms in one-third of the cases.¹⁷ Isolated peripheral arterial dissection unrelated to the aorta is characteristic of SAM.^11,17–19 Arterial stenosis and occlusions result from luminal narrowing caused by reparative granulation tissue forming plaques.^1,20,21

A dedicated CT or MR angiogram can reveal the characteristic arterial abnormalities of SAM, as previously described, as well as ancillary findings of intra-abdominal hematoma (Fig. 1B, 2A, 4A), active bleeding (Fig. 4A), bowel ischemia, and renal and splenic infarcts. CT and MR angiograms can be used for follow-up of patients with SAM.

Although the diagnosis of SAM requires pathologic confirmation, in many cases the diagnosis is based on the clinical and radiologic features.^9,15 There is a lack of consensus regarding the noninvasive diagnostic criteria for the diagnosis of SAM.¹⁸ An institutional guideline for diagnosing SAM has been suggested by Kalva et al,¹⁸ and which was based on the clinical features, laboratory values, and imaging findings as well as the exclusion of other vascular disorders.

As mentioned above, the radiologic diagnosis is confounded by a variety of vasculopathies that have similar imaging findings; however, it can be diagnosed after excluding other similar diseases. According to the institutional guidelines suggested by Kalva et al,¹⁸ the absence of a congenital predisposition for dissections, e.g., Ehlers-Danlos syndrome, Marfan syndrome, the absence of a more plausible diagnosis such as FMD or collagen vascular disorder, the absence of associated contiguous aortic dissection or atherosclerosis, and the absence of inflammatory markers can be included in the diagnostic criteria.¹⁸

The differential diagnosis of SAM includes: 1) Systemic vasculitis, such as polyarteritis nodosa, Takayasu’s arteritis, and Behcet’s syndrome; 2) Mycotic aneurysms; 3) Collagen vascular diseases such as Ehlers-Danlos syndrome; and 4) Degenerative vasculopathies such as cystic adventitial artery disease or cystic medial necrosis. The characteristic clinical features can help to distinguish systemic vasculitis, collagen vascular diseases, and degenerative vasculopathies. Increased acute-phase reactants and specific auto-antibodies present in many types of vasculitis are not seen in SAM. Mycotic aneurysms more commonly involve arterial bifurcation.

One important disease, which was previously considered to be related to SAM, is FMD. Previously, SAM was thought to be a variant or a precursor of FMD.^1,3,17,22 The morphologic appearance of FMD resembles that of the reparative phase of SAM.^1,6,8 However, a recent study shows that FMD is not solely caused by SAM but represents a group of arterial disorders with diverse etiologies.⁶ FMD typically occurs in young females and has a predisposition for the renal arteries.⁷ The clinical presentation also differs in the two diseases, i.e., SAM presents with profuse bleeding from abdominal intestinal arteries, while FMD presents with ischemic changes causing hypertension derived from renal artery alterations.⁶

In the past, the mortality rate of SAM was known to approach nearly 50%.^11,19 According to the recent study by Shenouda et al,¹¹ it is now estimated to be 26%, and which is attributed to the advanced techniques used for the early diagnosis of symptomatic and asymptomatic patients and as well as for their treatment. The management of SAM hinges on its clinical presentation, the vessels involved, and the presence of end-organ ischemia.

Although some studies have shown successful conservative management of SAM patients, the majority of these patients require an interventional procedure.^9,11,23 Since the first successful management of SAM using coil embolization was reported in 2000, minimally invasive techniques, especially catheter-based endovascular techniques, are coming into the spotlight.²¹ With the proper selection of patients and the continuous advancement of endovascular devices, catheter-based endovascular treatment can be a successful option for the treatment of SAM (Fig. 1C, 2C, 2D), as well as open surgical procedures.

Surgery is reserved for the management of occlusive symptoms, such as bowel ischemia, and for vascular lesions which cannot be accessed endovascularly. In angiographically inaccessible cases, endovascular management can be used to ameliorate the situation using balloon occlusion before surgical intervention.¹⁰

SAM is a rare arterial disease caused by degeneration of medial, smooth-muscle cells, and which can bring about catastrophic massive hemorrhage in various parts of the body.⁶ Because the pathologic diagnosis is not available in many patients with SAM, the radiologic findings have a critical role in the diagnosis of this disease. Minimally invasive techniques, especially catheter-based endovascular techniques, can be a successful option in certain patients, as well as use of the conventional, open surgical approach.