The Clinical Challenges of Abdominal Aortic Aneurysm: Rapid, Systematic Detection and Outcome-Effective Management
The Clinical Challenges of Abdominal Aortic Aneurysm: Rapid, Systematic Detection and Outcome-Effective Management
Part I: Clinical Pathophysiology, Patient Presentation, and Diagnostic Pitfalls
Authors: Gary Hals, MD, PhD, Attending Physician, Department of Emergency Medicine, Palmetto Richland Memorial Hospital, Columbia, SC; Michael Pallaci, DO, Department of Emergency Medicine, Palmetto Richland Memorial Hospital, Columbia, SC.
Peer Reviewer: Dawn Demangone, MD, Assistant Professor of Medicine, Assistant Residency Director, Temple University Hospital, Philadelphia, PA.
The clinical scenario is familiar to the experienced emergency physician. An elderly man with a history of hypertension and cigarette smoking arrives to the emergency department (ED) by ambulance complaining of back pain; he is diaphoretic and hypotensive. The diagnosis of abdominal aortic aneurysm (AAA) rupture should be considered at the outset, with confirmation of the diagnosis and prompt surgical intervention being the principal management objectives.
A leaking or ruptured AAA is one of the most dramatic and life-threatening conditions encountered in the ED, and patients with this condition are presenting in ever-increasing numbers to EDs across the country.1 The increase in the incidence of AAA is due not only to a growing number of elderly, but also to an increase in the absolute frequency of this disease.1 The fact that many patients have no prior symptoms until their AAAs rupture also adds to the difficulty of management. Even more auspicious is that the natural history of AAA is progressive growth, which culminates in rupture. It should be stressed that while AAA rupture is by far the most frequent complication, infection of aneurysm, aortoenteric fistula, and atheroembolism are additional clinical manifestations of AAA that can complicate both the diagnosis and prognosis of this disease.
Given the natural history of this condition and a mortality rate ranging between 50% and 80%, emergency physicians must be aware of both classic and more insidious presentations of AAA.2,3 Unfortunately, even when patients present with "classic" signs and symptoms, misdiagnosis is still common and is reported to be as high as 30-60% in some studies.4,5 Explanation for the unacceptably high misdiagnosis rate have centered on the possible confusion between signs of AAA and more common conditions. In this regard, two common presentations for a leaking AAA include: 1) back pain with hematuria; and 2) left lower quadrant (LLQ) pain with GI bleeding. Inasmuch as renal colic and acute diverticulitis are much more common causes of these symptom complexes, respectively, it is easy to understand why the diagnosis of symptomatic AAA can be difficult even for the experienced ED clinician. Delaying the correct diagnosis of patients with leaking AAA has been reported to double the mortality rate of these patients.6
With these issues in clear focus, this article will address the diagnostic and clinical challenges faced by the ED physician caring for patients with AAA. The authors also address current thinking about the pathogenesis of AAA, typical and atypical presentations, ED evaluation and stabilization, and late complications following AAA repair.
— The Editor
Introduction
Abdominal aortic aneurysms (AAAs) account for approximately 15,000 deaths each year in the United States and rank as the 13th leading cause of death.7,8 From an incidence perspective, AAA is found in 2% of the elderly population (age > 65 years), with a higher incidence in males vs. females (9:1). Rupture of an AAA usually is a lethal event, carrying an overall mortality rate of 80-90%;9,10 a significant percentage of these patients die before arrival to the hospital. Among those who reach the operating room, the mortality rate is still 50%.11,12 Fortunately, prompt diagnosis and surgical repair before rupture can reduce the mortality rate; the operative survival rate of patients undergoing elective repair is reported to be about 95%.12
Definitions. An aneurysm is defined as an irreversible, localized dilatation of an artery to at least 1.5 times of its normal diameter.13 Nevertheless, there has been some controversy surrounding the exact definition of what constitutes an AAA and its precise anatomical characteristics. Although up to six different definitions have been proposed, the most accepted description is that an aortic aneurysm is an aorta that is dilated 1.5 times that of the adjacent intact aorta.14 Since the normal diameter of the abdominal aorta is around 2 cm, with some variation in men vs. women (aorta in men > 55 years averages 2.1 cm, and 1.8 cm in women), an aorta measuring 3 cm commonly is used as a defining criterion for an AAA.15
A true aneurysm is characterized by involvement of all three layers of the vessel (intima, media, and adventitia), as opposed to a pseudoaneurysm, which involves only the adventitia and/or surrounding tissue. (See Figure 1.) In other words, a pseudoaneurysm is actually a disruption in the intima and media of the aorta, and only the adventitia/surrounding tissue retains blood within the aortic cavity.
Most AAAs are true aneurysms, whereas pseudoaneurysms of the aorta are primarily seen as late complications of repair. An AAA frequently is confused with an aortic dissection, which can occur in (or involve) an AAA, but which more typically begins in the thoracic aorta. An AAA and acute aortic dissection are separate processes precipitated by different etiologies, clinical findings, diagnostic tests, and treatment strategies. An inflammatory aneurysm is characterized by extensive perianeurysmal and retroperitoneal fibrosis, and dense adhesions of adjacent abdominal organs.16
Clinical Anatomy. The aorta is a retroperitoneal structure that lies immediately anterior to the lumbar spine. It enters the abdomen after passing through the aortic hiatus of the diaphragm at T12 along with the thoracic duct and azygous vein. The aorta yields five main vascular branches between the diaphragm and its bifurcation. Almost immediately below the diaphragm it bifurcates into the celiac trunk and superior mesenteric artery (SMA). The renal arteries branch off the aorta about 1 cm inferior to the SMA at approximately the L1-L2 interspace. Note that the L1-L2 level is an important landmark, as greater than 90% of AAAs arise inferior to the renal arteries, and extend from this location to involve the iliac arteries.17 (See Figure 2a.)
The next branch is the smaller inferior mesenteric artery (IMA), and the last are the two iliac arteries. The aorta splits into the right and left common iliac arteries about 2-3 cm inferior to the IMA at the level of the umbilicus. The vascular branches arising distal to the renal arteries are often involved in the aneurysm and may require grafting at surgery. There are two main types of aortic aneurysms classified according to their shape. Most AAAs are fusiform with tapering at both ends, while the other type is saccular, with the aneurysm joined to the aorta by a smaller entrance. (See Figure 2b.)
Clinical Pathogenesis and Risk Factors
Over the course of a lifetime, the aorta is exposed to cumulative physical stress caused by arterial blood pressure. Why then do some individuals develop an AAA, whereas others do not? The precise mechanisms leading to the development of an AAA remain unclear, although it is likely to be the result of a combination of events. In general, the factors leading to aortic aneurysm include those that weaken and increase stress on the aortic wall. Those factors most often identified in this pathophysiological process include: atherosclerosis, hypertension, breakdown of elastin and collagen, inflammation, genetic factors, tobacco smoking, and age. For those interested in more detailed analysis of AAA pathogenesis, an excellent review of the literature is available.18
Atherosclerosis. Atherosclerosis was once thought to be the leading cause of AAA, but atherosclerosis generally leads to occlusive arterial disease. Despite this basic difference, there appears to be an etiologic relationship between the two disorders. These conditions share multiple risk factors and there is a clear association between AAA and peripheral artery disease. However, several recent studies have shown that the two diseases have clear epidemiological differences. For example, elevated serum lipid levels have not been found in patient populations with AAA using case-control studies.19 Furthermore, the makeup of patients with AAA shows a higher proportion of males.20,21
Some researchers have suggested that atherosclerosis may be a response to aneurysmal dilatation rather than its cause.22,23 Others think, based on animal studies, that regression of atherosclerotic plaques can weaken the wall of the aorta and lead to aneurysm formation.24 The human abdominal aorta has few vasa vasorum to provide nutrients for the media, and it relies on diffusion from surrounding areas. Plaques in the aorta may also impair nutrient diffusion to the media, which can lead to further weakening. Investigations show that atheromatous plaques form preferentially in areas of low shear stress, and the lateral aortic wall just superior to the bifurcation is a region of low shear stress. It seems unlikely that coincidence is the reason that this is a common site for aortic aneurysms to form.24 Obviously a relationship exists between AAAs and atherosclerosis. However, the details of this association are still being elucidated.
Elastin and Collagen. Another area of recent research has focused on defining the role elastin and collagen, the two principle biochemical components of the aortic wall, play in aneurysm formation. The two proteins are arranged with smooth muscle cells in concentric lamellae in the aortic media. Elastin, as the name suggests, is a very flexible protein. It can expand to double its original length and yet still recoil, without damage, to original size. The properties of elastin provide the aorta with the ability to stretch and recoil to original caliber as blood pressure fluctuates with the cardiac cycle. This cyclic stretch and rebound are responsible for the continuous augmentation large arteries provide to cardiac output during diastole.25
Collagen, on the other hand, has opposite properties. It is a strong protein that is very resistant to stretching; tests show it has a tensile strength 20 times that of elastin.26 In fact, if it is stretched to just slightly beyond its original length, structural damage occurs.27 Collagen in the aorta is coiled when the aorta is at rest. Therefore, as the aorta expands, elastin acts first as the main load-bearing element and stretches easily. As the elastin reaches its maximal length, the collagen is uncoiled so that it now provides support for the aorta. With collagen being very strong and resistant to stretch, the aorta now assumes these properties. Collagen has been likened to a "safety net" for the aorta.18
There is mounting evidence that destruction of elastin is a key element in the formation of aortic aneurysms. The abdominal aorta normally contains much less elastin than the thoracic aorta, and aneurysms are more common in the abdomen.28 The amount of elastin found in the wall of an AAA is considerably reduced (35% to 8%) when compared to a normal abdominal aorta.29 Moreover, elastin is not synthesized in the adult aorta, and has an impressive half-life of 70 years.30 This time span correlates with the age of disease onset in many patients, and suggests that aneurysmal formation may be dependent on the amount of total elastin present. (Additional evidence demonstrates that aneurysms can be produced in vivo in animal models by giving intra-arterial elastase, a naturally occurring enzyme that destroys elastin.)31
Finally, elevated elastase levels can be found in human AAAs, an activity that persists after aneurysm repair, suggesting that this is likely to be a primary event and not a response to aneurysm formation.32 However, loss of elastin alone will produce only a slight dilatation of the aorta.33 As collagen is produced throughout life, the next step is thought to be replacement of the lost elastin by collagen fibers. Indeed, collagen comprises a larger proportion in diseased aortas.34 Since collagen cannot stretch without damage, significant fragmentation of collagen occurs with time, leading to increasing diameter of the aorta. Although elastin and collagen are important components in the pathogenesis of aneurysm formation, other microfibrillar proteins may also play a role. This is most clearly established in the case of Marfan’s syndrome, in which a genetic mutation leads to changes in fibrillin, which can lead to aneurysms and dissection.35
Genetic Factors. It is well established that genetics play a role in the development of aneurysms, and that a positive family history is a risk factor for AAA. (See Table 1 for risk factors.) Approximately 15-20% of first-degree relatives of patients with AAA will also have the disease, and a surprising 20-25% of brothers of patients with AAAs will develop an aneurysm.36,37 Reminiscent of sickle cell disease, gene mutations causing only a single amino acid substitution on the collagen molecule have been identified as predictive of AAA in one family studied.38 In contrast to sickle cell, however, this mutation is actually rare in the general population of AAA patients. Other researchers are looking for genetic causes of increased proteolytic enzyme activity, such as elastase and collagenase, since these enzymes are known to be present in increased concentration in the media of AAA.39 Despite intensive investigation, there is no clear consensus about the role of genetics in the average patient with AAA.
| Table 1. Risk Factors for AAAs |
| • Age > 60 years |
| • Male sex |
| • White race |
| • Family history of AAA |
| • Smoking history |
| • History of hypertension |
| • History of coronary artery disease |
Tobacco Use. The link between smoking and AAA prevalence has been established for more than 20 years.40 Furthermore, in one study, AAA rupture was four times more common in smokers and 14 times more common in smokers who hand-rolled their cigarettes.41 The exact mechanism by which this risk is incurred remains unclear. Even as the tar content of cigarettes has decreased by 50% over past two decades, age-adjusted AAA rupture has increased.41 Hence, attention has focused on other toxic components of smoke.
Chronic obstructive pulmonary disease (COPD) recently has been shown to be a risk factor for rupture of aneurysms less than 5 cm;42,43 consequently, alpha-1 antitrypsin has been suggested as a link between the two diseases. Alpha-1 antitrypsin is a major inhibitor of proteolytic enzymes, including elastase. Since deficiency of this enzyme is associated with emphysema and rapid destruction of elastin in the lungs, it has been suggested that a similar process could occur in the aorta.42 At this time, however, clear support for this hypothesis has not been established.
Aortic Inflammation. Inflammatory aneurysms are identified at surgery as those that have thickened walls and dense adhesions to surrounding structures. Interestingly, approximately 4-15% of patients will be found to have an inflammatory AAA at time of surgery.44,45 These patients also typically have the clinical triad of weight loss, abdominal pain, and elevated erythrocyte sedimentation rate.44
At one time, this type of aneurysm was thought to be a separate and distinct disease process from non-inflammatory aneurysms. However, recent research has found that some degree of inflammatory infiltration is present in most AAAs, and the current hypothesis is that an "inflammatory aneurysm" is an extreme example within a continuous spectrum of AAA disease.46-48 Furthermore, it has been suggested that increased infiltration of the aneurysm by lymphocytes and histiocytes is associated with clinical findings of increased tenderness and increased diameter of the aneurysm.18 Therefore, the inflammatory process could be an important factor in the equation of aneurysmal dilatation. The fact that inflammatory infiltration does correlate with aneurysm expansion in a rat model supports this hypothesis.49
Age. AAAs are uncommon in patients younger than 50 years of age, although they are encountered with increasing frequency as a patient’s age increases; overall, 2-4% of patients older than age 50 will have an AAA.50 The average age at time of diagnosis is between 65 and 70 years.51
Interestingly, even the "normal" aorta dilates and develops a thicker wall with age.52 As discussed in the section on elastin and collagen, loss of elastin and replacement of collagen in the wall of the aorta leads to damage of collagen fibers as they are forced to support wall stresses in the aorta at lower distending pressures. As elastin is not replaced as we age, this process occurs in all individuals. In this regard, the aorta of a 20-year-old patient is nearly three times more elastic than that of a 70-year-old patient.52,53
Hypertension. Hypertension has long been known to be a risk factor for AAA. Patients with hypertension and AAA are also at an increased risk for rupture.54,55 As with other risk factors associated with AAA, it remains unclear whether hypertension is a direct cause of AAA or exacerbates a pre-existing flaw in the aortic wall. It seems clear, however, from the observed effects that normal changes in blood pressure have on AAA, that increased blood pressure plays a role in the pathogenesis of this condition.54
As blood travels down the aorta from root to bifurcation, the pulse pressure actually increases.56 Several factors contribute to this. The aorta tapers in diameter as it descends from thorax to bifurcation, a feature that increases the pressure wave amplitude.56 In addition, branches of the aorta reflect the pressure wave back into the aorta further increasing the pressure. Finally, the abdominal aorta contains less elastin than the thoracic aorta, making it stiffer and less compliant. As a result, the pressure wave reaches its peak just before the bifurcation, perhaps explaining the remarkable localization of AAAs to the distal abdominal aorta. The act of expansion itself further increases stress on the aortic wall. Laplace’s law states that wall tension is proportional to pressure and radius and inversely proportional to wall thickness. This relationship explains how dilation of the aorta leads to a feedback cycle where dilation increases wall stress, which leads to further dilation, until the aorta eventually ruptures.
Natural History of AAA: Expansion and Rupture
Left untreated, most AAAs inexorably enlarge over time until they eventually rupture. Patients die from internal hemorrhage.57 While other outcomes and complications of AAAs are possible (i.e., embolism or dissection), aortic rupture remains the most common and most dangerous complication for the majority of patients with evolving AAA.
The most significant risk factor for rupture is size of the aneurysm, but interestingly, length or expansion rate have not been shown to correlate with risk of rupture.57-59 While the average growth rate is quoted at between 2 mm and 5mm per year, individual aneurysms will grow at different rates.22 One study found that 20% of aneurysms grew more than 4 mm per year, while the remaining 80% grew at a reduced rate, with 15% having no detected growth.60 What this means is that some aneurysms remain stable for years whereas others grow rapidly without pause; it is impossible to predict with accuracy which aneurysms will follow which time course.
Perhaps, in part due to the forces of Laplace’s law, larger aneurysms (> 5 cm) typically grow faster and carry a higher risk for rupture than do smaller ones.61 Interestingly, many of the studies have been performed exclusively on male patients. A recent review noted that women with AAAs represent 34% of deaths from AAA rupture in 1988, but only around 20% of all surgical cases.62 The investigators suggested that since women naturally have smaller aortic diameters, a 5 cm aneurysm in a woman represents more advanced disease than a similar size AAA in a male patient. Further study is needed to more accurately define this correlation.
In addition, there is disagreement in the literature about the risk of rupture of intermediate size aneurysms (4-6 cm). Autopsy data from 1977 show a rate of rupture of 60% for those at 10 cm or greater, 23% for those between 4 cm and 5 cm, and 9.5% for those less than 4 cm.63 However, aneurysm size at autopsy may not be as accurate as it is in a living patient.60 Furthermore, the data are not consistent and actually vary widely from study to study. Risks of rupture of 1-3% for aneurysms 4-5 cm have been published as recently as 1998.64 Population-based studies from many areas of the world quote numbers as low as a surprising 0% risk of rupture for aneurysms less than 5 cm.65-67 Finally, the United Kingdom small aneurysm trial data on 1090 patients with aneurysms less than 5.5 cm in diameter had an overall risk of rupture of only 1% per year for those less than 4.0 cm.68 Of note is that this is a much larger study than any of the previously mentioned reviews.
Not surprisingly, this controversy has fueled considerable debate about which patients with which size aneurysm truly are at risk of rupture. A recent investigation found that in a population of 161 patients with AAA rupture, the mortality was the same (70% for < 5 cm and 66% for > 5 cm), regardless of size.42 They found that 10% of aneurysms in their series ruptured at less than 5.0 cm, with 1.2% rupturing at less than 4 cm. COPD and diabetes were associated with a higher rupture risk of smaller aneurysms.
Another study also found COPD and diastolic hypertension to be associated with a higher risk of rupture.54 This review reported that the highest risk of rupture was associated with aneurysm size greater than 5.0 cm, presence of COPD, and diastolic blood pressure higher than 105 mmHg. The risk of rupture in these patients was 100%, while it was only 2% in patents with aneurysmal size less than 3 cm, no COPD, and diastolic blood pressure lower than 75 mmHg for the same time period (mean, 37 months).54 A study of 514 patients with aneurysms found advanced age, severe cardiac disease, previous stroke, and cigarette smoking were associated with more rapid growth of smaller aneurysms.69
Using a different approach, one author polled members of the Society for Vascular Surgeons on their estimate of risk of rupture in a patient with an AAA varying from 6.5 cm to larger than 7.5 cm in diameter. Remarkably, with 267 surgeons responding, the median estimate of rupture risk was more than twice as high as published estimates, with some guessing the risk even four times higher than published data.70 As the author points out, other physicians cannot expect consistent recommendations for their patients until better data become available. These data are expected in the next year at the end of a large VA-based study currently in progress.
The variability of data concerning risk of AAA rupture creates a fierce debate about recommendations for elective repair of AAA. While it is true that ED physicians typically are not making decisions concerning when to perform elective repair of AAA, the fact remains that ED physicians may be asked to offer consultation to patients who present to the ED with asymptomatic AAA. In addition, the frequent use of abdominal computed tomography (CT) and ultrasound (US) in the ED has led to an increase in the incidental finding of small AAAs while a patient is being evaluated for other pathology.71 In fact, small aneurysms now account for 50% of clinically recognized aneurysms.59 Taken together, these facts imply that the ED physician should at least be aware of the current standard of care regarding elective repair of AAA.(See Table 2.)
| Table 2. Surgical Indications for AAA44 |
| • All symptomatic aneurysms |
| • All saccular aneurysms |
| • Poor risk patient: size > 6 cm |
| • Good risk patient: size > 5 cm |
| • Young, good risk patient: size 4-5 cm |
Little debate exists regarding repair of aneurysms larger than 5-6 cm; if the patient is without prohibitive co-morbid conditions, then elective repair is recommended to avoid the high risk of rupture. Mortality from rupture is greater than 90%, while it is less than 5% for elective repair, including data on less-than-perfect operative candidates.59 Likewise, most surgeons would recommend frequent observation of patients with aneurysms of less than 4 cm rather than immediate repair.
Currently, three trials are under way in Canada, England, and the United States to answer the question of elective repair of aneurysms between 4 cm and 5 cm in diameter. Initial data from the British study of 1090 patients ages 60-76 years with aneurysms of 4-5 cm in diameter suggest that the risk of repair is higher than expected. The operative mortality of 5.8% in this study is higher than the 3% rate in previously published studies.68,72 In addition, there was no survival advantage for early intervention; the six-year mortality was the same (64%) for operation vs. following with serial US until aneurysm size was larger than 5.5 cm.72 This trial, then, suggests that elective repair of smaller aneurysms (< 5 cm) is not warranted as a general recommendation. The authors of an accompanying editorial point out that with a lower elective operative mortality, resulting from more judicious patient selection, prophylactic repair of smaller aneurysms might be more compelling.73 (See Table 2.)
Finally, a new endovascular stent-grafting technique for treatment of AAA is being evaluated for eventual replacement of the traditional surgical approach.74 This procedure allows the surgeon to place a stent inside the aneurysm under fluoroscopic guidance from a remote femoral artery puncture, thereby removing the aneurysm from circulation and preventing rupture. Still in an experimental stage, this technique is being used more frequently outside the United States. Complications range from migration or thrombosis of the stent, to embolization, continued dilation of the aneurysm, and leakage of blood into the aneurysm from either end of the stent. Controlled clinical trials are now under way in the United States, and the technique is expected to meet with approval.74
Clinical Presentation
Although the clinician may tend to lump the presentation of AAA into those patients who come to the ED with an incidental finding vs. those who have a catastrophic presentation, myriad signs of symptomatic AAA have been described.
Unruptured. The majority of AAAs are detected when they produce symptoms in their "host;" unfortunately, rupture of an AAA is often responsible for the first symptoms the patient experiences. While a significant number of patients do present for evaluation of vague symptoms that ultimately can be linked to the presence of an AAA, the exact incidence of symptoms in patients without AAA rupture remains unknown.75 Such symptoms may include vague abdominal or back pain, the cause of which may not be clear. These symptoms may be related to erosion into surrounding structures (i.e., vertebral bodies) as the aneurysm expands; this is thought to be a rare occurrence. Some patients complain of an abdominal fullness or notice pulsations.
A high index of suspicion is required in patients with known AAA who complain of pain or tenderness. It should be stressed that most intact aneurysms are not tender, and that new tenderness is highly suggestive of rapid expansion or early rupture. Less frequently, patients with AAA will present with embolic complications or a fistula between the aorta and GI tract or venous structures (see section on "Atypical Presentations"). Compression of the adjacent duodenum can lead to gradual narrowing. Accompanying symptoms include weight loss and vomiting, a constellation that has been termed the SMA syndrome.76 Compression of the ureters can cause true renal colic, and compression of nerves and nerve roots can lead to radicular symptoms.
Physical findings of AAA primarily consist of palpating a pulsatile abdominal mass. Other physical findings are of little diagnostic value as they are too nonspecific or insensitive. Bruits are detected in only 5-10% of patients with AAA, and they may result from other sources (i.e., renal or mesenteric artery stenosis).77 Femoral pulses are typically normal in patients with AAA, unless hypotension from rupture is present. A recent meta-analysis evaluating the diagnostic accuracy of palpation found that results depended on the size of the aneurysm. For aneurysms 3.0-3.9 cm, accuracy was only 29%; for those with aneurysms 5.0 cm or greater, it increased only to 76%.78 Obesity in the patient was an obvious factor complicating palpation for AAA presence.
The presence of aortic pulsations alone is not predictive of AAA, as the aorta in thin elderly people can be felt easily. Moreover, a torturous aorta in a thin person can mislead the examiner into believing the aortic diameter is bigger than it really is. The accurate way to exam a patient for the presence of an AAA is to palpate on each side of the aorta to determine its width; a value larger than 2.5 cm should prompt further work-up. Concerns that one will cause a rupture if an AAA is present by simple palpation are unfounded. The authors of the meta-analysis found a positive predictive value of only 43% in screening patients for AAA by abdominal exam without other adjunctive modalities. Hence, the bottom line appears to be to palpate patients in whom the diagnosis is in question without fear of inducing rupture, and if an AAA is not detected, a more accurate confirmatory test (i.e., US or CT) is certainly indicated.
Ruptured AAA. Rupture of an AAA usually is a catastrophic event, and patients often present in extremis. The triad of abdominal pain, pulsatile abdominal mass, and hypotension are reported as the "classic" presentation for ruptured AAA. Unfortunately, as with so many diseases, the classic presentation is not the most common one. Indeed, only 30-50% of patients with AAA rupture will present with this triad.79 The majority of AAA ruptures cause retroperitoneal bleeding as opposed to bleeding into the abdominal cavity itself. The so-called free rupture is more likely to cause rapid exsanguination and death before the patient can reach the ED.73 Retroperitoneal bleeding is seen in 76-90% of cases, and as a rule the bleeding is to the left.80 Therefore, back pain is a more common presenting complaint, and should trigger the ED physician to consider and rule out AAA as the diagnosis.
The pain produced by retroperitoneal bleeding is often the initial trigger for the patient’s presentation to the ED. Specifically, most patients complain of back, flank, or abdominal pain that is sudden in onset, severe, and constant in nature. It may radiate into the inguinal region, scrotum, or thigh, or even into the chest if the retroperitoneal hematoma spreads superiorly. Severe, sudden pain in a patient with AAA does not always indicate rupture or that bleeding has occurred. Rapidly expanding aneurysms can produce a similar clinical presentation.81 However, because rapidly expanding aneurysms are at a high risk for impending rupture, all patients with severe pain and a documented AAA should be considered to have a ruptured AAA until proven otherwise. The sudden and intense nature of the pain can cause nausea, vomiting, and/or vasovagal syncope; transient hypotension from blood loss can produce brief syncope in these patients as well.
Initially, most patients can be stabilized in the ED if significant blood loss is not ongoing; in fact, persistent hypotension is usually a late and ominous finding. The duration of symptoms in patients with proven AAA rupture is variable. Some patients with small, contained bleeding in the retroperitoneum may have waited days or even weeks to present for care.82 Therefore, prolonged duration of pain does not rule out the diagnosis of rupture, and a seemingly stable patient with AAA and a subacute history of back pain still requires evaluation for possible rupture.
Because larger aneurysms are frequently associated with rupture, physical findings in these patients often include a palpable abdominal mass.83 For example, a retroperitoneal hematoma can sometimes be detected as a non-pulsing mass in the left lower quadrant. However, if the aneurysm is small and the patient is obese, or if abdominal guarding or distension are present, one is much less likely to palpate a mass. In addition, reduced blood pressure can dampen aortic pulsations produced by the aneurysm. Ecchymosis can develop from significant bleeding, and can be seen in the abdominal wall, flank, scrotum, penis, inguinal region, perineum, or perianal area.84 A hematoma can even present as a mass in the scrotum, simulating an incarcerated hernia.85 Finally, although not common, the hematoma can compress the femoral nerve, resulting in femoral neuropathy. This produces hip and thigh pain, quadricep muscle weakness, reduced sensation over the anteromedial thigh, and weakened patellar reflex.75
Misdiagnosis: Avoiding the Pitfalls
Unfortunately, misdiagnosis of symptomatic AAA is common. In fact, some series report misdiagnosis rates as high as 30-60%.5,86 In particular, patients with a ruptured AAA are often "missed" if they present in atypical fashion, while the patient with "classical" symptoms may be misdiagnosed as renal colic or diverticulitis.
Several features of symptomatic AAA act in concert to increase risk of ED misdiagnosis. For example, the majority of these patients (> 80%) have no idea that an aneurysm is present at time of rupture, giving the ED physician no historical clues to suggest the diagnosis.87 Furthermore, AAA rupture is much less common in elderly patients than other diseases that can mimic this condition. That is, when an elderly patient presents with LLQ pain and guaiac positive stools, the likelihood of diverticulitis is much greater than AAA rupture. Complicating the clinical diagnosis is that the "classical triad" of back/abdominal pain, hypotension, and pulsatile mass is actually the exception rather than the rule. One study found only 9% of patients with AAA rupture presented with this triad.86
A number of studies have reported the final diagnoses in patients thought to have ruptured AAA but who were found to have other conditions discovered during surgery.88 One group of researchers found a misdiagnosis rate of 10% over a 10-year time period at their institution.89 Figure 3 summarizes the final diagnoses in these patients. It should be stressed that almost 75% of these patients still would have required emergency surgery based on their final diagnosis. In addition, 10 of the 16 patients in this series did indeed have AAAs found at surgery; CT scan was done in three of these patients and the results were "misleading" and not helpful in ruling out rupture of AAA. These data provide additional support for the prudent practice of taking unstable patients with a suspected diagnosis of ruptured AAA directly to the OR with limited diagnostic studies.
Many of the conditions that can be confused with AAA rupture are listed in Figure 4.86 Renal colic is the most frequent misdiagnosis. The fact that AAA can be misdiagnosed as ureteral colic can be explained on the basis of anatomical and clinical pathophysiology. Compression of a ureter by an expanding aneurysm or hematoma can produce renal colic through ureteral obstruction.86 Consequently, the physician may recognize the renal colic, but fail to ascribe it to an underlying process, such as expanding or ruptured AAA. Pursuit of the underlying etiology is imperative, since most patients with renal colic usually are sent home, where AAA rupture is likely to prove fatal. In the following section, features of AAA linked to misdiagnosis in patients with both typical and atypical presentations will be emphasized.
Classical Presentation. The so-called "classic patient" with AAA rupture can be difficult to recognize. A number of findings and clinical features can complicate the diagnosis. First, the presenting symptoms of rupture are not specific for this entity. Acute abdominal pain has myriad causes in elderly patients, most of which are more common than AAA rupture. Likewise, back pain is often present for other reasons in ED patients, the majority of which are not life threatening. As a result, patients may be inappropriately triaged from the start, putting them at even greater risk.
Elderly patients with back pain and hematuria must be evaluated for AAA. If AAA is present, they should be considered to have acute rupture until proven otherwise. The same can be said for patients with LLQ pain and lower GI bleeding. In addition, any elderly patient with abdominal pain and a mass, especially in the LLQ, should be considered to have a symptomatic AAA unless CT or US shows a normal aorta. In addition, any elderly patient presenting with shock and presumed sepsis needs to be evaluated for presence of an aneurysm. These patients often have mental status changes that preclude them from providing a history of pain which might lead the physician to the correct diagnosis.
Finally, acute myocardial infarction (AMI) can occur as a result of AAA rupture. Patients with AAA often have other arterial disease, and the associated hypotension and cardiovascular stress can precipitate an AMI. Fortunately, most of these patients can give a history of abdominal or back pain in addition to chest pain. Complaints of abdominal or back pain should not be ignored in these patients, especially if they precede the onset of chest discomfort. As will be reiterated later, the best advice is to "think" aneurysm and look for it early to avoid misdiagnosis.
Atypical Presentations. There are several atypical presentations that AAA, with or without rupture, can produce. These atypical presentations include the following: chronic contained rupture, aneurysm dissection, inflammatory aneurysm, aortovenous (AV) or aortoenteric (AE) fistula, and embolic complications. AE fistulas primarily are found as a late complication of AAA repair, and will be discussed in a later section. These complications of AAAs are by no means common, but they do happen and only will be "picked up" by a physician who is looking for them.
Although most patients with AAA rupture present acutely, some have been known to wait for weeks or months prior to detection.90 In these cases, the rupture occurs in the retroperitoneal space, where it may be contained without further leakage.
Characteristically, these patients have continuous, chronic back pain and a number of complications caused by the retroperitoneal hematoma, such as femoral neuropathy.91 Even though they may have survived with a "stable" rupture for long periods, they still are at risk for progression to hemorrhage at any time.92
Dissection of the abdominal aorta can occur, but this is the rare exception; fewer than 4% of aortic dissections begin in the abdominal aorta.93 These can occur without the presence of AAA. Fortunately, the symptom complex produced by abdominal aortic dissection is similar to AAA rupture, and the distinction may be recognized during the workup for AAA rupture or detected at surgery.
Inflammatory AAAs are defined by a thickened aneurysmal wall, extensive perianeurysmal and retroperitoneal fibrosis, and dense adhesions of adjacent abdominal organs.16 Clinically, these patients may present with abdominal or back pain, but also have signs of chronic, smoldering infection such as weight loss, low-grade fevers, and elevated erythrocyte sedimentation rate.94 Although initially thought to be the result of a completely separate pathological process, most clinical investigators recognize that all AAAs exhibit some degree of inflammatory change; as a result, inflammatory AAAs are thought to be an extreme example of the continuum of AAA disease.44,95 They are identified in 4-12% of patients with AAAs.96
A recent study of 274 patients with AAAs found no ruptures in those patients with inflammatory aneurysms.94 Despite this report, it is known that inflammatory AAAs can rupture, but it is not known if they are less prone to this complication or if chronic contained rupture leads to inflammatory changes in the beginning. Advanced cases on CT and US show a characteristic "mantle core sign" where the lumen of the aorta is compressed into a dumbbell shape by the inflammatory reaction. Ureteral compression (sometimes leading to acute renal failure) is seen in about 10-25% of patients in association with these aneurysms, and often results when the aneurysm extends into the iliac arteries.97,98 Treatment is the same as for other symptomatic AAAs—expeditious removal—although it may be technically more demanding because of the dense adhesions surrounding the aneurysm.
AV fistulas also can be seen in patients with AAA, but are found in only 1% of elective AAA repairs and in 4% of patients with AAA rupture.99 AV fistulas occur when the AAA erodes into an adjacent vein. Most commonly, the inferior vena cava or left renal vein is involved.14 Although an AV fistula can open simultaneously with AAA rupture, the majority of patients with AV fistula have no other sites of aneurysm leakage. The fistula results in shunting of blood from the arterial to venous system and increased venous return. Increased venous pressure can lead to distension of veins in the legs or abdominal wall. It can also produce hematuria through distension and leakage of veins in the bladder wall, or by increased venous pressure in the kidneys. Renal insufficiency can develop from the combination of increased pressure in renal veins, high-output heart failure, and decreased renal blood flow caused by the shunt.
Patients with AV fistula can present with symptoms of high-output failure, including: dilated heart, tachycardia, wide pulse pressure, dyspnea, pulmonary edema, and eventual hemodynamic decompensation. The shunt also can produce a decrease in blood flow in structures distal to the shunt. Patients can present with cool lower extremities and diminished pulses. Since aneurysms that produce AV fistulas usually are large, nearly 90% are palpable in the abdomen.75 An abdominal bruit can be heard in 75% of patients, and 25% also will have a palpable thrill.100,101 To summarize, patients with large aneurysms (> 10 cm) and signs of high-output cardiac failure, hematuria, renal insufficiency, signs of venous distension or any combination of the above, need to be evaluated for AV fistula. This can be done with an arteriogram or evaluated by the surgeon at time of AAA repair.
Any patient presenting to the ED with signs of peripheral emboli should be evaluated for the presence of an AAA. Peripheral embolism is another unusual but documented complication of AAA. Overall, embolism to the lower extremities is the presenting symptom in less than 5% of patients with AAA.14 Rarely, the entire aneurysm will thrombose, simultaneously producing ischemic symptoms in both extremities; more commonly, microemboli are passed "downstream" and generate ischemic symptoms in the distal lower extremities. Larger emboli can cause one leg to become ischemic, but the more common situation is ischemia in one or two toes caused by microemboli (the "blue toe syndrome").2 Emboli to other intra-abdominal vessels can produce intestinal ischemia, renal ischemia and failure, and neurological deficits (via the spinal artery of Adamkiewicz).
Medico-Legal Aspects of Misdiagnosis. Given the high rate of misdiagnosis of patients with symptomatic AAA, one would expect failure or delay of diagnosis to be a common cause of malpractice suits. While not frequent in overall terms compared to other more common causes of lawsuits (i.e., missed MI), AAA claims are represented in most studies of suits against emergency physicians. In one study of malpractice claims from 1980 to 1987 in Massachusetts, failure to diagnose AAA accounted for 2% of claims (6 of the total 262), and 3% of the dollars paid ($384,000 of $11,800,000).102 A more recent article also classified AAAs as "high risk" for cases paid against emergency physicians.103 Other members of the high risk-class included: chest pain, wounds, fractures, pediatric fever/meningitis, central nervous system bleeding, and epiglottitis.103
One would expect the data to back up these claims; in other words, patients with misdiagnosis should have a higher mortality rate. One group reported a 50% increase in mortality (35% to 75%, respectively) when studying patients with misdiagnosis from 1975 to 1979.6 Interestingly, more recent studies have not found the same results. Delay to diagnosis of AAA rupture was not found to be associated with an increased mortality in these patients.5,86 Several facts are likely to be responsible for this. These include the high baseline mortality (up to 80%) associated with AAA rupture, even when it is rapidly diagnosed.3 Patients with or without delay of diagnosis in one series had similar mortality rates of 44% and 58%, respectively.86
Especially important is that misdiagnosis may be more likely in the subgroup of hemodynamically stable patients. They survive long enough to make it to the ED and appear less critical than those with hypotension on presentation. Perhaps, they survive their protracted course because they have more subtle symptoms and a better prognosis from the onset than those with unrestricted rupture. This suspicion was confirmed in a recent study on the effect of length of ED stay on mortality of patients with ruptured AAA.104 These investigators reported that prolonged presurgical time was associated with more hemodynamically stable patients, and these patients did have a lower mortality rate. Even so, every effort should be made to avoid delays to diagnosis and treatment.
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Physician CME Questions
81. Which of the following is not considered a risk factor for development of AAA?
A. Hypertension
B. Tobacco use
C. Female sex
D. Age older than than 65 years.
E. Positive family history
82. Which of the following is true?
A. Some AAAs will stop growing at a certain size and therefore have no risk of rupture.
B. AAAs below 4 cm are not at risk for rupture.
C. An abdominal mass can be palpated in most patients with a ruptured AAA.
D. Less than 50% of patients present with the classic triad of abdominal/back pain, pulsatile mass, and hypotension.
83. Regarding clinical presentation of patients, which is true?
A. Palpating aortic pulsations is diagnostic of AAA presence.
B. Palpation of an AAA alone has been shown to produce aneurysm rupture.
C. Rapidly expanding AAAs can produce symptoms similar to AAA rupture.
D. None of the above
84. Misdiagnosis of AAA is reported to be as high as:
A. 70-90%.
B. 30-60%.
C. 10-30%.
D. none of the above.
85. Two common presentations for a leaking AAA include:
A. back pain with sciatica or hepatojugular reflux.
B. back pain with hematuria or LLQ pain with GI bleeding.
C. back pain with neurogenic bladder or RLQ pain with hematuria.
D. all of the above.
E. none of the above.
86. The most significant risk factor for rupture of an AAA is:
A. degree of inflammations.
B. degree of hypertensions.
C. location of the aneurysm.
D. the size of the aneurysm.
E. none of the above.
87. Bruits are detected in about what percentage of patients with AAA?
A. 100%
B. 90-100%
C. 80-90%
D. 70-80%
E. none of the above
88. Pain produced by bleeding into what anatomic compartment is a common initial trigger for the patient’s presentation?
A. GU tract
B. Spleen
C. Esophagus
D. Retroperitoneum
E. None of the above
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