INTRODUCTION — Abdominal aortic aneurysm (AAA) is a degenerative condition of the aortic wall that is relatively common and has the potential for lethal complications. Ultrasound screening studies have found that 4 to 8 percent of older men have an occult AAA; the prevalence is much lower in women [1-5]. Because the incidence of AAA rises sharply in individuals over 60 years of age, the future prevalence of AAA could increase substantially in association with the aging population. On the other hand, recent studies suggest that a reduction in the prevalence of smoking could have the opposite effect [6]. Clinical evaluation is aimed at identifying patients at risk for AAA, ideally making a diagnosis before the onset of any symptoms, and identifying the need for further evaluation in patients who have clinical signs or symptoms consistent with AAA.
The risk factors, clinical signs and clinical evaluation of AAA will be reviewed here. Screening for AAA is reviewed elsewhere. (See "Screening for abdominal aortic aneurysm".)
ANATOMIC ISSUES
Definition of AAA — An aneurysm is a segmental, full-thickness dilation of a blood vessel >150 percent of its normal diameter. Although “normal” diameter varies with age, gender, and body habitus, the average diameter of the human infrarenal aorta is about 2.0 cm; the upper limit of normal is typically <3.0 cm. Thus, for the majority of patients, an infrarenal aorta with a maximum diameter ≥3.0 cm is aneurysmal [7,8].
Anatomy of the abdominal aorta — The abdominal aorta is a retroperitoneal structure that begins at the hiatus of the diaphragm and extends to its bifurcation into the right and left common iliac arteries at the level of the fourth lumbar vertebra. The posterior abdominal peritoneum covers the abdominal aorta anteriorly and is reflected onto the posterolateral duodenum at its junction with the jejunum.
The aorta lies slightly left of the midline to accommodate the inferior vena cava which is adjacent to the aorta on its right (figure 1). The branches of the aorta include (superior to inferior) the left and right inferior phrenic arteries, left and right middle suprarenal arteries, the celiac axis, superior mesenteric artery, left and right renal arteries, left and right gonadal arteries, inferior mesenteric artery, left and right common iliac artery, middle sacral artery and the paired lumbar arteries (L1-L4).
The abdominal aorta is the most common anatomic site of arterial aneurysm. AAA most often affects the segment of aorta between the renal and inferior mesenteric arteries (figure 2); approximately 5 percent involve the renal (pararenal) or visceral arteries (suprarenal).
EPIDEMIOLOGY AND RISK FACTORS — The estimated prevalence of AAA in developed countries is 4 to 9 percent [1,9-12]. However, a screening study in Sweden found a prevalence of 1.7 percent, the lowest reported in a predominantly Caucasian population [6,13]. The decline may be attributable to a reduced prevalence of smoking. The prevalence of AAA is also age-dependent. Autopsy studies in Sweden found that the prevalence in men peaked at age 80, but continued to increase with age in women [14].
The annual incidence of AAA is difficult to measure. Screening studies in the United States and the United Kingdom have estimated the incidence of AAA in men over 50 to be 3.5 to 6.5 per 1000 person-years [15,16]. These studies also found that new AAAs develop in 2 to 2.6 percent of at-risk men 4 to 5.5 years after an initially normal study. Smoking continues to exert a profound influence on the development of AAA [17].
Although aneurysmal degeneration of the aorta is a multifactorial process, well-defined clinical risk factors are associated with the development of AAA. These risk factors include smoking, male gender, advancing age, Caucasian race, atherosclerosis, family history and the presence of other peripheral aneurysm. Female gender, non-Caucasian race and diabetes are associated with a decreased risk of AAA. These are briefly discussed below. (See "Natural history and management of abdominal aortic aneurysm".)
CLINICAL PRESENTATIONS — Patients with an AAA may be asymptomatic or symptomatic. The majority of patients are asymptomatic, but younger patients (≤50 years of age) are more likely to be symptomatic [36].
Asymptomatic — An asymptomatic (occult) AAA may be discovered during routine screening, during the course of an evaluation for acute or chronic abdominal pain or another unrelated medical complaint, or incidentally on imaging studies.
Screening — Ultrasound screening studies have shown that 4 to 8 percent of older men have an occult AAA [1-3]. For patients without symptoms or signs suggestive of AAA, the United States Preventive Services Task Force guidelines recommend one-time screening for AAA using ultrasonography for men between 65 to 75 years of age who have a history of smoking [37]. No recommendation for or against AAA screening was made for men age 65 to 75 years who have never smoked. Routine screening for AAA was not recommended for women [38]. (See "Screening for abdominal aortic aneurysm".)
For patients at risk for aneurysm, including men and women with a family history of AAA, and men age 65 to 75 who have smoked at least 100 cigarettes in their lifetime, the Screening Abdominal Aortic Aneurysm Very Efficiently (SAAAVE) Act in 2005 provides coverage for ultrasonography to evaluate for AAA as a Medicare benefit within six months of Medicare eligibility [39].
Incidental discovery on imaging — AAA is frequently detected as an incidental finding on ultrasonography, computed tomography, or magnetic resonance imaging performed for other purposes. In addition, this information is often not passed on to the referring physician [40-42]. The following findings are illustrative:
Pulsatile abdominal mass — Approximately 30 percent of asymptomatic AAAs are discovered when a pulsatile abdominal mass is palpated on routine physical examination. The ability to palpate and estimate the aortic diameter depends upon the patient’s body habitus, the size of the aneurysm, and the clinical experience of the practitioner. (See 'Abdominal palpation' below.)
Symptomatic — Aneurysms that produce symptoms are at increased risk for rupture. (See "Ruptured abdominal aortic aneurysm".)
Pain — Pain is the most frequent complaint in patients with an AAA. The pain is typically located in the abdomen but can also radiate to the back, flank, or groin. The pain is unaffected by position or movement. Pain from a symptomatic AAA can mimic many other acute conditions such as renal colic, diverticulitis, pancreatitis, inferior wall coronary ischemia, mesenteric ischemia, and biliary tract disease. (See "Differential diagnosis of abdominal pain in adults".)
Although AAAs can progress to rupture without any intervening symptoms, complaints of new or worsening pain, or findings of tenderness upon palpation of the aorta are suggestive of a rapidly expanding aneurysm or impending rupture. [43]
The initial presentation of AAA can be rupture. The manifestations of a ruptured AAA depend in part upon whether or not the rupture is contained. The pathognomonic triad of pain, pulsatile abdominal mass, and hypotension is present in only one-third of patients who present with ruptured AAA [44,45]. A contained posterior rupture resulting in a retroperitoneal hematoma can lead to ecchymosis in the flank (Grey-Turner sign) or groin. Such patients can be hemodynamically stable. In contrast, free intraperitoneal rupture is associated with the abrupt onset of pain, abdominal tenderness and distention, hemodynamic instability, and death unless rapidly treated. (See "Ruptured abdominal aortic aneurysm", section on 'Clinical presentation'.)
Nonspecific abdominal pain associated with systemic symptoms and a pulsatile abdominal mass may indicate the presence of an inflammatory or infectious AAA. (See 'Inflammatory aneurysms' below and 'Infectious aneurysms' below.)
Thromboembolism — AAA can present with distal thromboembolic events, which may be a sign of rupture of the aneurysm. The thromboemboli can be single or multiple. The manifestations include painful, blue digits and/or a painful, pulseless, cool leg. Rarely, AAA can present with acute thrombosis leading to bilateral lower extremity ischemia. The clinical manifestations of thromboembolic disease are discussed in detail elsewhere. (See "Embolism from aortic plaque: Thromboembolism" and "Acute arterial occlusion of the lower extremities (acute limb ischemia)", section on 'Clinical evaluation'.)
Cholesterol embolization from AAA is less common, and most often occurs after instrumentation (eg, arteriography, cardiac catheterization) or aneurysm repair (open or endovascular). (See "Embolism from atherosclerotic plaque: Atheroembolism (cholesterol crystal embolism)", section on 'Clinical manifestations'.)
Disseminated intravascular coagulation — Large or extensive AAAs may be associated with disseminated intravascular coagulation (DIC) causing hemorrhagic and thrombotic complications. The incidence of DIC is reported to be as high as 4 percent [46,47]. (See "Pathogenesis and etiology of disseminated intravascular coagulation".)
Systemic manifestations — Inflammatory or infected aneurysms can present with abdominal pain and systemic symptoms, which can confound or delay the diagnosis of AAA. Clinically distinguishing between an inflammatory and infected aneurysm is important since treatment differs.
Inflammatory aneurysms — Inflammation is a common component of most aortic aneurysms, but there is also a clinical entity called an inflammatory aneurysm [8,48]. About 5 percent of aortic aneurysms are classified as inflammatory aneurysms [49,50]. This type of aneurysm has distinct clinical and pathologic characteristics, although it may reflect an extreme manifestation of the processes present in all aortic aneurysms [48].
Observational data are inconsistent regarding distinctions between inflammatory and common aortic aneurysms [8,49-51]. In two reports, patients with inflammatory aneurysms were not different from those with noninflammatory aneurysms with regard to risk factors, treatment requirements, or prognosis, although they were more likely to have symptoms [49,51]. However, in a separate series, patients with inflammatory aneurysms were also more likely to have weight loss and a history of smoking, and had a higher operative mortality (7.9 versus 2.4 percent for noninflammatory aneurysms) [50]. During surgery, inflammatory AAA is associated with a thickened aneurysm wall, perianeurysmal fibrosis (shiny white at the time of surgery), and intense adherence of the aneurysm sac to adjacent intraabdominal structures [8]. These changes can sometimes be demonstrated on preoperative imaging studies (eg, computed tomography).
The triad of chronic abdominal pain, weight loss, and an elevated erythrocyte sedimentation rate in a patient with AAA is highly suggestive of an inflammatory aneurysm [50]. Patients with inflammatory aneurysms are often more symptomatic than patients with typical AAA, but the incidence of rupture may be lower. In a review that included 180 patients, rupture occurred in 4 percent of patients with inflammatory aneurysm compared with 20 percent of those with noninflammatory aneurysms [52].
Ureteral displacement or symptoms of ureteral obstruction can also occur due to the retroperitoneal inflammatory reaction.
Infectious aneurysms — Primary bacterial infection of the aortic wall is a rare cause of aortic aneurysm. Infected (previously termed mycotic) aneurysms can also arise secondary to infection of a pre-existing aneurysm [8]. If untreated, infected AAA is an insidious febrile illness associated with rapid expansion of the affected aortic segment, aneurysm rupture, and death [53]. The diagnosis and management of infected aneurysm is discussed in detail elsewhere. (See "Overview of infected (mycotic) arterial aneurysm".)
PHYSICAL EXAMINATION — Any patient with risk factors for or a clinical presentation suggestive of abdominal aortic aneurysm (AAA) should undergo a thorough abdominal examination. (See 'Epidemiology and risk factors' above.)
Abdominal palpation — Although abdominal examination may reliably diagnose a large AAA (>5.5 cm), a diagnosis is made by physical examination alone in fewer than 50 percent of those with AAA [16]. Detection of an AAA by physical examination is less likely when the aneurysm is small and when abdominal girth is large (waist >40 inches). In addition, an abdominal examination focused specifically on the diagnosis of AAA is more likely to identify an aneurysm than a nonfocused examination. Conversely, a diagnosis of AAA may be incorrectly made when examining a thin patient with a prominent but normal-sized aorta or in those with a nonvascular mass overlying an easily palpable transmitted aortic pulse. Patients with hypertension, a wide pulse pressure, or a tortuous aorta can also have prominent aortic pulsation that may be mistaken for an AAA. For these reasons, it is important to obtain an imaging study to confirm or exclude AAA if the physical examination is suspicious. (See 'Imaging' below.)
With the patient supine on the examination table and garments removed from the abdomen, the abdomen should be palpated to identify a widened pulse that suggests the presence of an aneurysm. AAA most often occurs in the segment of aorta between the renal and inferior mesenteric arteries. As a result, palpation between the xiphoid and umbilicus is particularly important. (See 'Anatomy of the abdominal aorta' above.)
The vascular examination should include auscultation of the abdomen since the presence of a bruit may indicate aortic or visceral arterial atherosclerotic disease, or, rarely, an aortocaval or aortoiliac fistula. Palpation of the abdomen to detect AAA is safe and has never been reported to precipitate aortic rupture. Most patients will experience mild tenderness with deep palpation of the aneurysm. This finding should not be used to brand the aneurysm as symptomatic in order to justify treatment.
The sensitivity of abdominal palpation increases significantly with AAA diameter.
Vascular examination — A complete peripheral arterial examination should be performed. Palpation of prominent femoral and/or popliteal pulses warrants further noninvasive evaluation for peripheral and abdominal aneurysm. Iliac aneurysms typically occur in association with AAA. In a study of 10,038 patients with iliac aneurysm, 89 percent had a coexistent AAA [56]. A 14 percent incidence of femoral and/or popliteal artery aneurysms was found in another study of patients with AAA [57]. (See "Popliteal artery aneurysm" and "Iliac artery aneurysm".)
DIAGNOSTIC EVALUATION — When suspected, the diagnosis of AAA should be confirmed with abdominal ultrasound or other imaging study. Abdominal ultrasonography is the most commonly used screening and confirmatory study for suspected AAA. It is noninvasive, inexpensive, and has a demonstrated sensitivity of 98 percent and specificity of 99 percent for the diagnosis of AAA [58]. It has also been shown to be an effective tool for identifying small aneurysms in patients at-risk [59], and to be cost-effective in reducing aneurysm-related mortality [60].
Laboratory studies — Laboratory studies are not routinely obtained as part of the evaluation of AAA. However, a white blood cell count, blood cultures, and erythrocyte sedimentation rate should be performed in patients with systemic symptoms (eg, fever, weight loss) to evaluate for an infectious cause of AAA or inflammatory aneurysm. (See 'Systemic manifestations' above.)
D-dimer levels may reflect increased deposition of thrombus within an aneurysm sac and thus predict aneurysm expansion [61,62]. One prospective study followed 1260 subjects (337 with AAA) recruited from a population screening study and 132 (41 with AAA) from a referral clinic [62]. In both groups, the dominant risk factor for AAA was D-dimer. The adjusted odds ratios were 12.1 (95% CI 7.1-20.5) and 24.7 (95% CI 13.7-44.6) for D-dimer cut-off values of >400 and >900 ng/mL, respectively. The average yearly aneurysm growth rate was positively and significantly associated with D-dimer. Although these data are interesting, we do not use D-dimer as a means to follow AAA progression.
Imaging — When an AAA is suspected based upon the physical examination, abdominal ultrasonography is the initial diagnostic test of choice and is also used to follow small aneurysms over time.
For the detection of AAA (aortic diameter >3.0 cm), abdominal ultrasonography has a sensitivity and specificity approaching 100 percent [63]. The routine sonographic evaluation involves measuring the anteroposterior (AP), longitudinal, and transverse dimensions of the aorta, and the examination should include imaging the iliac arteries (figure 3A-B). Patients are asked to fast prior to undergoing the examination to reduce the presence of overlying bowel gas that can obscure the aorta [1].
The main limitation of abdominal ultrasonography is that it is technician and equipment-dependent. If the ultrasound probe is not oriented perpendicular to the centerline, the AP diameter of the aorta will be overestimated [64]. A systematic review identified nine studies after 1990 that performed multiple AP measurements over a period of time, generally one to three weeks [65]. The method of AP measurements included anterior inner wall to posterior inner wall, anterior outer wall to posterior outer wall and midpoint anterior wall to midpoint posterior wall. Interobserver variability in the AP measurement in five of these studies was less than 5 mm; however, four studies reported greater variability (up to 10 mm). The use of a higher frequency transducer (5 MHz) reduces variability [64], and some have suggested that inner to inner wall measurements produce more consistent results [66]. At times, ultrasound may not give an accurate depiction of the iliac arteries, which may also be aneurysmal [65]. In approximately 1 to 2 percent of cases, the aorta cannot be imaged adequately because of technical difficulties (eg, bowel gas, aortic depth) [65].
Although ultrasonography is generally preferred, computed tomography (CT) can be used for the diagnosis of AAA and for serial monitoring of aneurysm size. CT has the advantage of evaluating the abdomen in more detail, which may be particularly important in patients with a nonspecific clinical problem such as abdominal pain. CT also defines the extent of the aneurysm better than ultrasound and is superior for imaging suprarenal aneurysms [67,68]. Limitations of CT compared with ultrasonography are greater cost, the requirement for contrast, and the cumulative risk of radiation with repeated scans [69].
As a result, CT for AAA should generally be limited to preoperative planning and postoperative follow-up of aortic graft repairs. When using CT to monitor AAA over time, the following need to be considered:
Other imaging modalities are less often used in the evaluation of AAA but may play a role in selected patients. Magnetic resonance angiography (MRA) may be more accurate than CT, but is expensive, time consuming, and may not be available [69,72]. Conventional arteriography alone does not reliably demonstrate the presence of an aneurysm, but it may play a role in planning surgical repair and is essential in performing endovascular repair.
Vascular laboratory studies — Further vascular evaluation may be indicated in patients with an abnormal peripheral arterial examination (eg, abnormal pulses or signs consistent with ischemia) or a clinical history suggestive of peripheral artery disease. (See "Noninvasive diagnosis of arterial disease".)
Although the prevalence of internal carotid artery stenosis is higher in subgroups of patients with AAA, the cost-effectiveness of routine screening in patients without a history of symptoms related to carotid artery stenosis remains unclear [73,74]. (See "Screening for asymptomatic carotid artery stenosis".)
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