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Abdominal pain can be difficult to diagnose, and have many possible causes. Acute bowel ischemia may be due to arterial occlusion (thrombotic, embolic or vasculitic), mesenteric venous obstruction, or "nonocclusive ischemia". These etiologies will all produce similar symptoms that vary with the duration of ischemia, or lack of blood flow. This disease has a very high mortality (death rate) without prompt diagnosis, and treatment. Irreversible bowel necrosis (death of the intestine) may occur within 12 hours of onset, although good results have been obtained when patients have had angiography and surgery within 24 hours. Since the therapeutic approach is dependent on whether or not an arterial occlusion is present, the specific cause of the ischemia should be defined as quickly and as precisely as possible.
Nonocclusive mesenteric ischemia occurs in patients with low flow states such as myocardial (cardiac) insufficiency and hypotension, and in patients receiving digitalis. This is probably the most common cause of acute mesenteric ischemia. The pathophysiological basis of nonocclusive ischemia is persistent splanchnic (abdominal) vasoconstriction in response to the low flow state or cardiac glycoside. Patients with nonocclusive ischemia usually have significant underlying disease. Consequently, the prognosis (outcome) in nonocclusive ischemia is extremely poor. Nevertheless, some patients have benefited from intraarterial infusion of vasodilators such as papaverine.
Thrombotic and embolic occlusions in the proximal 10 cm of the superior mesenteric artery (SMA) are amenable to surgery. The prognosis in these patients is somewhat better than in nonocclusive ischemia if prompt diagnosis and therapy are undertaken. However, in mesenteric embolization, multiple small branches may be obstructed rendering surgical intervention ineffective.
Mesenteric venous obstruction may occur in hypercoagulable (thickened blood) states, low flow syndromes, and as a result of torsion (twisted) intussusception and strangulating bowel obstruction. Encasement of the superior mesenteric or portal vein by malignant tumor or mass may also produce mesenteric venous obstruction.
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