(1/10/03 8:30 am)
Occlusion Of The Abdominal Aorta|
Occlusion Of The Abdominal Aorta And Its Branches
Occlusion of the abdominal aorta and its major branches (most notably the superior mesenteric, celiac axis, and renal arteries) can be acute or chronic. Acute occlusion is usually the result of embolism, acute thrombosis in a narrowed artery, or aortic dissection. Chronic occlusion most often stems from arteriosclerosis and less often from fibromuscular hyperplasia or external compression by mass lesions.
ACUTE OCCLUSION OF THE SUPERIOR MESENTERIC ARTERY
Sudden occlusion of the superior mesenteric artery is typically due to an embolism, causing an abdominal catastrophe (abdominal apoplexy). Patients appear in extremis; complain of terrible, diffuse abdominal pain; and often writhe in agony. 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. There is usually a leukocytosis (> 15,000/µL). 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.
Thrombolytic therapy with intra-arterial streptokinase or urokinase has been used in the treatment of splanchnic embolism with modest success, but it is not a good alternative to surgical reperfusion unless the surgical risk is very poor.
ACUTE OCCLUSION OF THE CELIAC AXIS
Occlusion is usually due to embolism to branches of the celiac axis. Infarction of a portion of the liver from hepatic artery embolism and splenic infarction from splenic artery embolism may occur, but they rarely require surgical intervention. Prognosis is poor.
ACUTE OCCLUSION OF THE RENAL ARTERY
Occlusion due to renal artery embolism is heralded by the sudden onset of flank pain, followed by hematuria. Early surgical removal of the embolus (within 2 to 4 h) may preserve renal function, although renal infarction is frequently present.
ACUTE OCCLUSION OF THE AORTIC BIFURCATION
Abrupt occlusion of the aortic bifurcation is usually from embolism. It is characterized by the sudden onset of pain, pallor, paralysis, and coldness in the legs. Urgent embolectomy is indicated and can usually be performed transfemorally.
CHRONIC OCCLUSION OF THE SUPERIOR MESENTERIC ARTERY AND CELIAC AXIS
Because collateral circulation between the major splanchnic trunks is extensive, chronic mesenteric vascular insufficiency almost invariably involves high-grade narrowing or occlusion of both the celiac axis and the superior mesenteric artery.
The usual cause is arteriosclerosis. However, fibromuscular hyperplasia (or dysplasia) is not uncommon, particularly in young women with renovascular hypertension.
Symptoms of mesenteric vascular insufficiency typically occur postprandially (intestinal angina) because digestion requires increased mesenteric blood flow; pain begins about 30 min to 1 h after eating and is steady, severe, and usually periumbilical. Pain may be relieved quickly by sublingual nitroglycerin, aiding diagnosis. Patients become fearful of eating, and weight loss, often severe, is the rule. Intestinal malabsorption may contribute to weight loss.
Doppler ultrasonography of the superior mesenteric artery and celiac axis may accurately detect reduced flow through these trunks. Mesenteric arteriography is crucial in demonstrating the presence and severity of the occlusion and the suitability for surgery.
Nitroglycerin sometimes provides relief. In severely symptomatic patients, surgical revascularization of the superior mesenteric artery and celiac axis can usually be achieved by bypassing from the supraceliac or infrarenal aorta to the splanchnic arteries distal to the occlusion.
CHRONIC OCCLUSION OF THE RENAL ARTERY
Chronic occlusion of one or both renal arteries may lead to renovascular hypertension. This diagnosis is suggested by the finding of elevated plasma renin levels, especially after captopril administration. IV urography usually shows a smaller kidney with delayed excretion of contrast medium on the affected side; radionuclide nephrograms may also show differences in renal perfusion and function. Renal arteriography is important in defining the anatomy of the occlusion, and differential renal vein renin levels may show high levels of renin coming from the ischemic kidney.
In carefully screened cases, revascularization of the ischemic kidneys is highly successful in relieving hypertension. In anatomically suitable cases, revascularization can be performed by percutaneous balloon angioplasty, although most cases require surgery. For more detailed discussion of renovascular hypertension, see Ch. 199.
CHRONIC OCCLUSION OF THE AORTIC BIFURCATION
Chronic occlusion of the aortic bifurcation is usually due to arteriosclerosis and is manifested by intermittent claudication in the legs and buttocks and erectile impotence (Leriche's syndrome). Femoral pulses are absent. Claudication can be relieved by surgically bypassing the occluded area of the aorta with an aortoiliac or aortofemoral graft.