AORTIC DISEASE - AORTIC ANEURYSMS



Aortic aneurysms, localized areas of increased aortic diameter, may occur in the ascending aorta, aortic arch, descending thoracic aorta, or abdom­inal aorta, depending on the etiology. For exam­ple, aneurysms of the sinuses of Valsalva may occur in Marian’s syndrome, syphilis, as a com­plication of infective endocarditis, or as a con­genital lesion, while aneurysms of the ascending aorta or aortic arch occur in syphilis, aortic dis­section, or cystic medial necrosis with or without Marian’s syndrome. Aneurysms of the descending thoracic aorta occur from syphilis, atheroscle­rosis, or dissection. Aneurysms just distal to the take-off of the left subclavian artery are commonly due to trauma. Abdominal aortic aneurysms are usually atherosclerotic but can be secondary to syphilis or extension of a dissection from above.

Arteriosclerotic aneurysms usually occur in the abdominal aorta, most often between the renal ar­teries and the aortic bifurcation, and are most common in males older than 50 years. Abdominal aortic aneurysms may cause back or epigastric pain but usually produce no symptoms. Their first manifestion may be rupture with severe pain and exsanguinating hemorrhage. Impending rupture of an aneurysm is often heralded by increasing abdominal or back pain, usually constant, “gnaw­ing” in quality, and not affected by movement. Aneurysms may be tender to palpation. Some pa­tients seek medical attention because of fullness or abnormal pulsations in their abdomen. A prom­inent abdominal aortic pulsation may be felt in normally thin people and be mistakenly diag­nosed as an abdominal aortic aneurysm; on the other hand, an abdominal aortic aneurysm may not be palpable in patients who are obese or who have muscular abdominal walls. On occasion, manifestations of peripheral emboli secondary to debris from the aneurysm may be noted in the feet and toes. Bruits may be heard over the aneurysm, and associated occlusive peripheral vascular dis­ease is often present. Aneurysms may rapture into the inferior vena cava producing an arteriovenous fistula, into the duodenum with acute massive gastrointestinal hemorrhage, into the retroperi­toneal space manifested by flank or groin hema­tomas, or into the abdominal cavity causing ab­dominal distention.

The size of the aneurysm can be estimated from physical examination, routine chest x-ray if cal­cification of the aortic wall is present, abdominal ultrasound, CT scan, and aortography. The normal aorta is no more than 2 cm in diameter. Physical examination is the least accurate method for es­timating abdominal aortic size; abdominal ultra­sound is simple and very accurate. Angiography may be less accurate because mural clot may cause the lumen to appear smaller than it really is. If surgery is contemplated, however, aortog­raphy is necessary to assess the extent of aortic involvement and the status of the aortic branches.
Half of all aneurysms exceeding a 6-cm diam­eter rupture within one year, and surgical resec­tion with prosthetic graft replacement is usually recommended. Aneurysms less than 6 cm also may rupture. In patients with relatively high sur­gical risk, aneurysms of 4 to 6 cm should be fol­lowed closely and surgery undertaken upon signs of expansion or impending rupture.

Three-fourths of atherosclerotic aneurysms are confined to the abdominal aorta. One-fourth also involve the thoracic aorta, most commonly the de­scending aorta. Thoracic aortic aneurysms may cause tracheal deviation with pulmonary com­plications, hoarseness due to recurrent laryngeal nerve compression, dysphagia from obstruction of the esophagus, Horner’s syndrome from compres­sion of the sympathetic chain, or superior vena caval obstruction. Steady, gnawing pain may occur, and occasionally a visible aneurysmal pulsation can be noted in the sternum or in the su­prasternal notch. Rupture of the aneurysm is as­sociated with severe chest or back pain. Thoracic aortic aneurysms are usually visible on chest x-ray and must be differentiated from other me­diastinal masses. The aneurysms can be identified by CT scan, but aortography usually is necessary before surgery. Larger thoracic aortic aneurysms, especially those exceeding 6 to 7 cm in diameter and those showing evidence of expansion or caus­ing symptoms, should be resected and replaced with a vascular prosthesis if possible. Aortic valve surgery may be required if the aneurysm involves the ascending aorta, and reimplantation of the great vessels or coronary arteries is sometimes necessary. In patients who are poor surgical risks or in whom an extensive aneurysm obviates sur­gery, reduction of blood pressure and force of ven­tricular ejection with beta blockade is reasonable.

A dissecting aneurysm is caused by a tear of the aortic intima with formation of a false channel within the aortic media. Blood in the false chan­nel may re-enter the true aortic channel via a sec­ond intimal tear or rupture through the adventitia into the periaortic tissues. Aortic dissections, probably not related to atherosclerosis of the aorta, tend to occur in middle-aged men with hy­pertension, Marfan’s syndrome, cystic medial ne­crosis, and coarctation of the aorta. They are also somewhat more common during pregnancy. They may be caused by trauma and may be a compli­cation of aortic angiography. If the dissection trav­els retrogradely toward the aortic valve, severe aortic valvular regurgitation may occur. Involve­ment of the coronary, carotid, or spinal cord ar­teries may produce myocardial infarction, stroke, or paraplegia, respectively. Dissections can rup­ture into the pericardium and cause cardiac tam­ponade or renal impairment if the dissection ex­tends to the renal arteries.

Most dissections arise either in the ascending aorta within several centimeters of the aortic valve or in the descending thoracic aorta just be­yond the origin of the left subclavian artery in the region of the ligamentum arteriosum. Dissections have been classified into three types: type I begins in the proximal ascending aorta and extends to the descending aorta, type II is confined to the ascending aorta, and type III begins in the de­scending aorta and extends distally. Patients with proximal dissections tend to be younger and have a higher incidence of Marfan’s syndrome and cys­tic medial necrosis; distal dissections more com­monly involve older patients with hypertension.

Dissecting aortic aneurysms are usually asso­ciated with chest pain radiating into the abdomen or the back. The pain is usually severe, sudden, maximal in intensity at its onset, and described as sharp or tearing. An occasional patient presents with aortic insufficiency and heart failure without the history of an acute painful episode. Physical examination may reveal hypertension/Pulses are often but not always asymmetrical. Abnormal pul­sations may be felt in the region of the sterno­clavicular joints. The electrocardiogram is usually nonspecific but may show evidence of complieating pericarditis or myocardial infarction. The chest x-ray often but not always reveals widening of the mediastinum.

CT scan with contrast injection is very accurate in diagnosing both ascending and descending dis­secting aortic aneurysms. Echocardiography can sometimes demonstrate ascending aortic involve­ment. Angiography is necessary to identify the site of the intimal tear, to determine the full extent of aortic involvement, to evaluate the competency of the aortic valve, and to determine the relation­ship of the dissection to the aortic branches.

The prognosis is usually poor. Surgical therapy should be undertaken when a dissection involves the ascending aorta or arch, or the distal aorta when there is evidence of leaking or retrograde extension, impairment of cerebral or other vital circulations, inability to control pain or blood pressure, and when hypertension is absent and thought not to be the etiology of the dissection. Patients with distal dissections tend to be older, have more generalized atherosclerosis, and have more cardiopulmonary disease. Thus, surgery generally is indicated in type I and type II, while medical therapy may be the treatment of choice for uncomplicated distal dissection (type III). Medical therapy must be administered to surgical patients both during the preoperative stabiliza­tion period and chronically postoperatively to prevent the risk of progression or repeat dissec­tion. Medical management involves control of hy­pertension and lessening the force of ventricular contraction. Potent antihypertensive agents such as intravenous sodium nitroprusside or trime-thaphan may be used. Trimethaphan has the ad­vantage of also decreasing the force of myocardial ejection but exhibits both unpleasant side effects and tachyphylaxis. Therefore, sodium nitroprus­side, combined with beta blockade to lessen the force of contraction, is usually the medical treat­ment of choice.