acute superior mesenteric artery occlusion

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Arterial occlusive mesenteric ischemia can be a life-threatening event related to obstruction of the mesenteric arteries, most commonly the superior mesenteric artery (SMA), supplying the small bowel and colon. It is the most common cause of mesenteric ischemia.

Epidemiology

An acute occlusion is an uncommon event that typically affects elderly patients, who are at an increased risk of other cardiovascular events.

Risk factors

advanced age
smoking
prothrombotic tendency
- antiphospholipid antibodies, etc.
valvular/cardiac abnormalities
- mechanical heart valve
- atrial fibrillation
- acute myocardial ischemia
- ventricular aneurysm
right-to-left shunt
- patent foramen ovale/atrial septal defect with paradoxical embolism

Clinical presentation

Clinical presentation is variable and unfortunately often non-specific such that the diagnosis is not made for some time. It may be dramatic with acute onset severe abdominal pain or can be less well-defined .

Pathology

Acute arterial occlusion can be due to a number of causes :

embolic event: ~50% (range 40-60%)
acute in situ thrombosis superimposed on atherosclerosis: 15-30%
aortic dissection with the involvement of the SMA origin
slow flow or idiopathic

Radiographic features

Ultrasound

Ultrasound is able to demonstrate normal flow in both the SMA and SMV but is incapable of assessing side branches or the bowel wall. It has little role in the acute management of this condition.

CT

Computed tomography is widely accepted as the first-line imaging technique for evaluation due to its speed, widespread availability and ability to diagnose alternative causes of acute abdominal pain.

Technique

For a discussion on CT technique, refer to the mesenteric ischemia article.

Findings

Findings in acute SMA occlusion include :

arterial changes: lack of enhancement of the lumen of the SMA and/or its branches
- embolism location varies
  - 15% at the origin of the SMA
  - 50% immediately distal to the origin of the middle colic artery
- thrombosis usually occurs in the proximal 2 cm of the SMA
bowel changes: reflecting reduction/obliteration of blood supply
- mucosal/serosal enhancement absent
- thickness
  - variable
  - in pure arterial occlusion, the wall may be thinned (a.k.a. paper-thin wall) due to loss of intestinal muscular tone and absence of blood supply
  - a thickened wall may also be present but does not correlate with severity; reperfusion can cause a thickened wall
- ileus / dilated loops of bowel: >3 cm in diameter
- air-fluid or blood-fluid levels: due to dysfunctional peristalsis
- necrotic mural gas may be present: pneumatosis intestinalis
other changes
- mesenteric edema
- free fluid
- intrahepatic portal venous gas: due to pneumatosis intestinalis
- free intra-abdominal gas
causes: e.g. intracardiac thrombus in a dilated left ventricle

Angiography (DSA)

Once the gold standard for diagnosis, now reserved for patients who may benefit from endovascular intervention.

Treatment and prognosis

An acute SMA occlusion carries a mortality of 75-90% despite treatment . Treatment options include :

endovascular thrombectomy
intraluminal papaverine
surgical thrombectomy with resection of non-salvageable infarcted bowel

Differential diagnosis

mesenteric arteritis
splanchnic venous occlusion: superior mesenteric vein or portal vein thrombosis rather than arterial occlusion
chronic arterial occlusion with an alternative cause for abdominal pain: identification of well-formed collaterals may suggest that the occlusion is chronic
small bowel obstruction
Crohn disease: in most cases, a significantly different patient group