Chronic occlusion most often stems from arteriosclerosis and less often from fibromuscular hyperplasia or external compression by mass lesions. Vomiting and urgent evacuation of the bowels are common at the outset. Although the abdomen may be tender, the pain is usually disproportional to the tenderness, which is widespread and poorly localized. There may be slight distention. Sluggish bowel sounds rapidly progress to ileus.
Stools show occult blood early and occasionally become frankly bloody.
Acidosis, hypotension, and shock rapidly develop as the ischemic bowel becomes necrotic. Plain films of the abdomen may show scattered loops of dilated bowel with edema and even air in the wall of the ischemic intestine. Survival of the intestine and the patient depends on immediate reestablishment of perfusion in the occluded superior mesenteric artery. Although the diagnosis is often made by mesenteric angiography, valuable time may be lost in performing this procedure. Exploratory laparotomy without arteriography to restore mesenteric flow may be justified in critically sick patients in whom clinical suspicion is high.