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| Abdominal Aortic Aneurysm by Vanessa Cordell |
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Abstract
Abdominal aortic aneurysms can occur due to smoking, hypertension, trauma, inflammatory disease of the aorta and syphillis. Most abdominal aortic aneurysms are found when the patient is assessed for other complications. Patients are usually asymptomatic. Ultrasound is frequently used in the initial assessment of the AAA. Sonographic appearance shows the dilated vessel and can show the absence of tapering of the aorta. Unless the aneurysm has ruptured, the mechanism of treatment depends on its size. There are complications with AAA, but few are fatal. |
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Keywords
abdominal aortic aneurysms, AAA, aneurysm |
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Introduction
Abdominal Aortic Aneurysms are defined as localized dilatations at least 1.5 times greater in diameter than an adjacent normal segment. They may be saccular or fusiform and are adjacent to a normal-sized arterial segment. Knowing the normal anatomy plays an important role in diagnosing and treating a patient with and abdominal aortic aneurysm. Causes and characteristics vary depending on the patient. Patients with AAA are usually asymptomatic. Because AAA repair is so demanding there are several complications. |
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Abdominal Aortic Aneurysms usually occur just below the renal artery origin to the bifurcation of the iliacs. To find these with ultrasound, you must know the anatomy and measurements of the aorta and it’s branches. The abdominal aorta generally is visualized from the diaphragm to at least the bifurcation and frequently beyond. The normal abdominal aorta has smooth margins and a well-defined wall, and tapers gradually from cephalad to caudad as it gives off its visceral branches. The maximum normal diameter of the aorta averages 2 cm in adults and varies little with respect to age, gender, race, and body size. The aorta is located slightly to the left of the midline and lies adjacent to the spine throughout its course. |
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Aortic aneurysms result primarily from arteriosclerosis which weakens the wall of the aorta enough that the pressure inside forces it to balloon outward. A blood clot often develops in the aneurysm and may spread along its entire wall. High blood pressure and cigarette smoking increase the risk of an aneurysm. Trauma, inflammatory disease of the aorta, hereditary connective tissue disorders, and syphilis can all predispose a person to aneurysms. AAAs are generally fusiform with gradual dilatation of the vascular lumen. Frequently, these are eccentric so that one aspect of the wall is more severely affected. The aneurysmal aorta has a thin and fibrotic tunica media with only a few elastin layers remaining. Generally aneurysms increase in diameter at the rate of approximately 2 mm per year. Most abdominal aortic aneurysms are relatively symmetrical; tortuosity may make them appear asymmetric. |
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| An AAA will usually appear as a pulsating abdominal mass, but this is not absolute. Symptoms are present in only 25-30% of all cases. Pain is present in only 20-30% of patients with abdominal aortic aneurysms and this type of pain varies from mild pain to lumbar discomfort, with an array of additional symptoms in the mid-abdomen. The patient may also be hypotensive. The pain may be constant or intermittent in nature. Most patients have no symptoms, and the aneurysm is discovered incidentally by radiographic examination for other complaints, or following detection of prominent abdominal pulsations. Constant mid-lumbar, lumbar or pelvic pain, which may be severe and have a piercing quality, suggests an expanding abdominal aneurysm and impending rupture. Occasionally, large AAAs cause symptoms from local compression (i.e. nausea or vomiting from duodenal compression, hydronephrosis from ureteral compression or venous thrombosis from iliocaval venous compression). |
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| Some patients report attacks of abdominal pain or back pain. Patients might have tenderness in the vicinity of an uncomplicated aneurysm. Abdominal pain may simulate peptic ulcer disease, pancreatitis, biliary or renal colic, acute appendicitis, intestinal obstruction, peritonitis or a herniated interventional disc. These symptoms are often termed “impending rupture,” since the appearance of pain may suggest early rupture within minutes, days, weeks, or months. The classic presentation of ruptured AAA includes abdominal/back pain, hypotension, and a pulsatile abdominal mass. All 3 findings are found in only 20% of patients with proven AAA rupture. |
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| Pain is useful but a late clue. Many patients with aneurysms have no symptoms and are diagnosed by chance during routine physical examinations or when by imaging modalities used for other symptoms. A doctor may feel a pulsating mass in the midline of the abdomen. Rapidly growing aneurysms that are about to rupture frequently hurt or feel tender when pressed during an abdominal examination. In obese people, even large aneurysms may not be detected. |
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Several laboratory procedures help diagnose aneurysms. An abdominal x-ray may show an aneurysm that has calcium deposits in the walls. An ultrasound scan clearly reveals the size of an aneurysm. A computed tomography scan of the abdomen, particularly if performed after injection of contrast media, is even more accurate in determining the size and shape of an aneurysm. |
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Ultrasound scanning is the least expensive and most frequently used examination particularly for initial confirmation of a suspicion of AAA, during physical examination and long term follow-up of small AAAs. Ultrasound findings of an abdominal aortic aneurysm include increased aortic diameter, focal dilatation, lack of normal tapering distally, presence of thrombus, and occasionally aortic dissection. Size measurements are more accurate in the anteroposterior than the lateral dimension. Bowel gas can cause difficulty in obtaining the required images, so you may ask the patient to take 2 simethicone tablets and fast for 8 hours prior to the exam. Ultrasound cannot accurately determine the presence of retroperitoneal hemorrhage and often cannot accurately determine upper extent of an AAA. The sonographic appearance of an abdominal aortic aneurysm no longer has the smooth well-defined margins as present in a normal aorta. The walls of an abdominal aortic aneurysm are dilated causing layers of thrombus to line the vessel. |
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When performing this exam the patient is positioned supine, scanning sagittal, midline just inferior to the xiphoid process. In longitudinal evaluate the entire aorta from the diaphragm to the bifurcation determining the location and longitudinal extent of the aortic aneurysm. Measure the aneurysm anteroposterior, outer to outer in longitudinal plane. In transverse document the maximum diameter of the aorta at the diaphragm, SMA, and distally near the aortic bifurcation. Measuring the aneurysm anteroposterior, transverse and outer and outer. Documenting all abnormalities along the way.
CT scanning is more expensive than ultrasound and involves radiation exposure. It is more accurate and provides a more precise definition of the extent of the AAA.
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| Unstable patients with a distented abdomen, a known history of AAA, or a pulsatile mass should be expeditiously explored. Occasionally, a cause other ruptured AAA is found, but nearly always requires surgical treatment anyway. |
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For aortic aneurysms there is an absence of tapering of the aorta below the mesenteric and renal vessels. Aortic aneurysms have various configurations. Some are bulbous with a sharp junction or neck, between the normal and aneurysmal portions; and others are fusiform with a gradual transition between the normal and aneurysmal portions. Tortuous aortas usually deviate to the left of the spine, but some may deviate anteriorly, creating a prominent kink at the aneurysm neck. |
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Thrombus in an aneurysm is usually not organized, although some evidence indicates it may reduce wall stress and risk of rupture. Because of the presence of thrombus, the outer dimensions of an aneurysm are often much greater than the dimensions of the lumen. |
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| Unless an aneurysm is rupturing, treatment depends on its size. Most surgeons do not consider operating on patients with an aneurysm less than 4 to 5 cm in maximum diameter. At 5 cm, the incidence of spontaneous rupture begins to climb alarmingly and is mandated for elective repair. Surgery to repair the aneurysm consists of inserting a synthetic graft from the proximal aneurysm to the bifurcation of the iliacs. The death rate from this type of surgery is about 2%. |
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Rupture or threatened rupture of an abdominal aneurysm requires emergency surgery. The risk of death during an operation for a ruptured aneurysm is about 50%. When an aneurysm ruptures, the kidneys are at risk of injury from disrupted blood supply or from shock related to blood loss. If kidney failure develops after the operation, the chances of survival are very poor. An untreated ruptured aneurysm is always fatal. |
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| Elective repair or replacement of an abdominal aortic aneurysm in a reasonable surgical candidate carries a mortality rate of less than 2% in experienced hands. Urgent surgery to repair a leaking aneurysm may result in a mortality rate as high as 60%. |
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| In the patient who has multiple coexistent medical problems may make the risk of surgery unacceptable. Ultrasound may be used to monitor the size of the aneurysm. If scans every 3 months confirm stability, surgery may be deferred. If an abrupt increase in diameter of the aneurysm is documented the risk of not operating may force a semielective repair. |
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| Aortic surgery is technically demanding and despite a low incidence operative complications can be severe and fatal. There are several types of complications due to aortic surgery. Venous bleeding which usually results from injury to the iliac or left renal veins during initial exposure. Sigmoid colon ischemia following AAA repair is a rare but devastating complication, more likely to occur following ruptured AAA repair. Ischemia may also result from embolization or ligation of the IMA and internal iliac arteries. Despite careful preoperative screening and intraoperative management, cardiac complications, particularly MI, constitute the most serious threat to patients following AAA repair. This type of complication remains the leading cause of death after elective AAA surgery. Pulmonary complications are common but seldom fatal following AAA repair. Renal failure, once a frequent complication of AAA repair, is now unusual and dialysis requirement is rare except after complicated, ruptured AAA repair. Later complications after successful AAA repair are infrequent. Some later complications can include pseudoaneurysm, graft infection, thrombosis, and other aneurysms. |
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Conclusion
Abdominal aortic aneurysms are the 13th leading cause of death, accounting for more than 15,000 deaths per year. Ultrasound is an adjunctive for determining and helping in the diagnosis of an abdominal aortic aneurysm. Patients will not go to surgery based on the ultrasound alone. Ultrasound is a good tool for following up on patients with abdominal aortic aneurysms, but is not the basis of diagnosis. |
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References:
§ Calligaro MD, Keith D., Matthew J. Dougherty MD, and Larry H. Hollier, MD. Diagnosis and Treatment of Aortic and Peripheral Arterial Aneurysms. Philadelphia: Harcourt Brace & Company, 1999.
§ Cronenwatt, MD, Jack L., Lawrence N. Sampson, MD. Current Diagnosis & Treatment in Vascular Surgery. Connecticut: Appleton & Lange, 1995.
§ McGahan, MD, John P., Barry B. Goldberg, MD. Diagnostic Ultrasound A Logical Approach. Philadelphia: Lippincott-Raven, 1997.
§ Mittelstaedt, MD, Carol A., Lawrence M. Vincent, MD. Abdominal Ultrasound. New York, Edinburgh, London, Melbourne: Churchill Livingstone, 1987.
§ Zweibel, MD, William J. Introduction to Vascular Ultrasonography. Philadelphia: Harcourt Health Sciences Company, 2000.
§ Powell, Janet T. Surgical and Endovascular Treatment of Aortic Aneurysms. Armonk: Futura Publishing Company, 2000.
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Created by Vanessa Cordell on 9/17/02
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Copyright CVTC 2003. All Rights Reserved.