Epidemiology
In autopsy studies, the incidence of abdominal aortic aneurysms (AAA) ranges from 1.8% to 6.6 % and is increasing. Males are more likely to have an AAA than are females by a 3:1 to as much as an 8:1 margin (1,2,3). The majority of AAA occur in the sixth and seventh decades of life, because aging causes changes in the elastic tissue, collagen, smooth muscle and ground substance which make the aorta less distensible and less able to absorb the forces from left ventricular contraction (4,5).
AAA and Sudden Death or Driver Incapacitation
Rupture or dissection of the aorta accounts for 1.3% to 3.8% of sudden death in drivers (6,7,8). The 10-year prospective study (1/1/1978 to 12/31/1987) by Christian re- focused attention on AAA as a significant cause of sudden death in drivers. This study identified seven who died of a ruptured AAA out of a total of 56 drivers (12.5%) who died from cardiovascular disease (CVD) within two hours of arrival at an emergency room. All seven cases occurred in men, three while driving. None of the seven had prior
knowledge of the condition (9).
Anatomy of Abdominal Aortic Aneurysms
Aortic aneurysms can be classified by morphology, etiology or location. In general, the location determines the cause, clinical presentation, and treatment. An aneurysm can be located in the thoracic, the thoracoabdominal or abdominal area. The abdominal aorta begins at the diaphragm and descends to the fourth lumbar vertebra, where it divides into the common iliac arteries. The abdominal aortic aneurysms are almost always located below the renal arteries (3).
Risk Factors and Associated Conditions
Approximately 90% of aneurysms are secondary to atherosclerosis (4,10). AAA are eight times as frequent among those smoking one or two packs of cigarettes per day than among non-smokers (11). Hypertension is present in 60% of patients with aortic
aneurysms and 80% of persons with aortic dissection. Because the risk of rupture is proportional to the stress on the artery wall, hypertension probably plays a role in aneurysm formation; however, it is not clear whether hypertension accelerates the
aneurysm’s growth rate (12). More recently, there has been increasing investigation into the role genetics plays in the development of AAA (2).
139 Diagnosis
Most aneurysms are asymptomatic and discovered either on routine examination or on an abdominal x-ray done for another reason. On x-ray, the calcified wall of the aneurysm is seen in approximately 50% of cases (13). Auscultation of an abdominal bruit may indicate the presence of an aneurysm. Detection of an AAA on physical examination depends on the aneurysm’s size and the person’s obesity. Larger aneurysms are more easily detected. Approximately 90% of AAA greater than 6 cm are identified on clinical examination. Aneurysms larger than 4 cm may be palpable in non-obese persons. The overall detection rate on examination is 31% when compared to detection by ultrasound, which has an almost 100% sensitivity and specificity (2,3,13,14,15).
Complications
An AAA can be an acute life-threatening risk, a condition requiring ongoing follow-up, or a disease associated with other CVD. Rupture is the most feared complication of an aortic aneurysm (16), and is the tenth leading cause of death among men older than 55 years of age, with a mortality rate of 78% to 94%. (1,14).
While it is difficult to predict when an aneurysm is going to rupture, the risk increases geometrically as the aneurysm increases in size. AAA less than 4 cm rarely rupture. AAA smaller than 5 cm have a 1% to 3% per year rate of rupture, AAA of 5 to 6 cm have a 5% to 10% per year rate of rupture, and AAA greater than 7 cm have approximately a 20% per year risk of rupture (1,17).
The average growth rate has been reported to be 0.21cm to 0.40 cm per year; but greater increases in size occur in at least one-quarter of patients (3). Factors that correlate with the rate of growth of an aneurysm include the largest aneurismal cross-sectional area, tobacco use, and tortuosity (18).
Bickerstaff reported that the mean period from detection of an AAA to rupture was 48.7 months (5). In another study, when the criteria used for operative intervention was an aneurysm > 5 cm or an increase in size > 0.5 cm in six months, less than 50% of patients entering the study with an AAA less than 5 cm needed surgery within a mean of three years (19). A more aggressive approach for surgical repair is to operate when the surgical mortality is less than the per year risk of rupture. With improved surgical outcomes, and without contraindication for surgery, aneurysms greater than 4 cm are electively repaired to prevent dissection and rupture (1).
Less common complications include distal embolization, sudden and complete
thrombosis, infection, chronic coagulopathy, aortic intestinal fistula, and development of AV fistula between the aorta and vena cava (4).
140 Commercial Driver Certification
Though the decision to operate on a person with an aneurysm is often controversial and is beyond the scope of the medical examination, certain guidelines for CMV driver
certification are useful. An AAA larger than 5.0 cm should disqualify the driver because of the high risk of rupture. A vascular specialist should review an AAA larger than 4 cm before a commercial driver is certified to drive. The decision by the driver’s health care provider not to surgically repair an aneurysm does not mean that the driver can be certified to drive safely. However, a recommendation to operate on an aneurysm disqualifies the driver until the aneurysm has been repaired and a satisfactory recovery period has passed.
Detection of an AAA should prompt a search for the presence of other CVD. AAA are found in approximately 5% of patients with coronary disease, 10% of those with cerebrovascular disease, and 20% of patients with peripheral vascular disease (14,15).
Thoracic Aortic Aneurysms
The thoracic aorta runs from the base of the left ventricle to the diaphragm. The thoracic aorta can be divided into the ascending, transverse (arch), and descending segments (3). While relatively rare, thoracic aneurysms are increasing in frequency (20). Aortic size has been considered the major factor in determining risk for dissection or rupture. The rate of rupture or dissection has ranged from 8.8% for a 3.5 cm aneurysm to 27.9% for an aneurysm greater than 6 cm. The growth rate of thoracic aneurysms has been reported to average 0.10 cm per year. Those that required sur gery had more rapid growth rates (21). Following repair, at least a 3- month period before driving a commercial vehicle is recommended.
Aneurysms of Other Vessels
Much of the information on aortic aneurysms is applicable to aneurysms in other arteries. Although less common than aortic aneurysms, aneurysms can develop in the visceral arteries: splenic, subclavian, celiac, gastroduodenal, and renal; and can develop in
peripheral arteries: including the femoral, iliac, and popliteal arteries; and venous vessels. Rupture of any of these aneurysms can lead to sudden incapacitation and death (4,22-26). Certification of the driver should require review by a vascular specialist. Surgical repair requires an appropriate waiting period before driving again. The location of the
aneurysm and surgical repair required affect the length of the waiting period before certification. In general, a 3- month waiting period is recommended.
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