Background
Abdominal aortic aneurysms (AAAs) are segmental dilatations of the aortic wall that cause the vessel to be larger than 1.5 times its normal diameter or that cause the distal aorta to exceed 3 cm. These can continue to expand and rupture spontaneously, exsanguinate, and cause death.
AAA rupture is an important cause of unheralded deaths in people older than 55 years, claiming more than 15,000 lives annually in the United States alone.
Pathophysiology
A marked decrease in aortic elastin, an increase in collagen production and degradation, inflammatory changes, and imbalances of matrix metalloproteinases and their inhibitors have been noted in pathologic studies. Atherosclerosis may be only a facilitating factor.
Genetic predisposition plays some role, especially in disorders such as Marfan disease and type IV Ehlers-Danlos syndrome. Familial clustering of cases also has been documented.
Clinical risk factors that predispose individuals to these degenerative changes in the arterial wall include smoking, advanced age, male sex, chronic obstructive pulmonary disease (COPD), hypertension, and family history.Females, African-Americans, and persons with diabetes appear to have a lower prevalence of AAA.
Larger aneurysms tend to enlarge at a higher rate, as noted by Bernstein.
Table 1. Growth Rate of AAAs
| Initial Size of AAA, cm | Mean Growth Rate, cm/y | 95% Confidence Interval of the Mean Growth Rate |
|---|---|---|
| 3.0–3.9 | 0.39 | 0.20, 0.57 |
| 4.0–4.9 | 0.36 | 0.21, 0.50 |
| 5.0–5.9 | 0.43 | 0.27, 0.60 |
| 6.0–6.9 | 0.64 | 0.16, 1.1 |
The natural history of an individual case is difficult to predict, and AAAs can have intervals of stability and slow and rapid expansion. The general recommendation is to consider elective aneurysmorrhaphy for aneurysms with a diameter of 5 cm or greater or for small aneurysms that have an average growth rate of more than 0.5 cm/y. Approximately 20% of aneurysms expand faster than 0.4 mm/y; the rest do so at slower rates.
AAA rupture
In addition to the initial aneurysm diameter, independent predictors of rupture include current smoking, lower forced expiratory volume (FEV-1), and higher mean blood pressure, all risk factors possibly amenable to modification. Findings from various series have suggested that diabetes mellitus, elevated levels of serum markers such as amino-terminal type III procollagen propeptide (PIIINP), a rapid rate of AAA expansion, and unfavorable morphology and aortic wall compliance are also predictive of rupture. Of note, the prevalence of diabetes in the population with AAAs is relatively low.
Attempts have been made to index the aneurysmal diameter to individual body size or anatomic points of reference (eg, body surface area, supraceliac aortic diameter, aneurysm length), but no feature has been found to be highly predictive of rupture. Rupture has been observed in some AAAs smaller than 5 cm in diameter but wider than the transverse dimension of the third lumbar vertebral body.
Findings from a few studies have suggested that most AAAs rupture into the left retroperitoneum. The retroperitoneum contains the leak by means of mechanisms that cause clotting or tamponade. This rupture can also cause abdominal, back, or flank pain; this symptom is related to impingement of the hematoma on adjacent structures.
Aneurysms that continue to leak or those that rupture into the peritoneal cavity can result in hemodynamic collapse and, often, death.
Frequency
United States
The estimated incidence of ruptured AAA in a population study from Sweden was 0.06 case per 1000 people.This rate increased with age; the age-adjusted incidences were less than 0.01, 0.37, or 1.36 per 1000 people younger than age 60 years, those aged 70-79 years, and those older than 90 years, respectively.
The best-known predictor of rupture rate is the maximal aneurysm diameter. Simplified estimates for the annual rupture rates based on size are as follows:
Table 2. Annual Rupture Rates Based on Size
| Maximum Aneurysmal Diameter, cm | 5-Year Rupture Rate, % |
|---|---|
| <4.0 | 2 |
| 4.0–4.9 | 3–12 |
| 5.0–5.9 | 25 |
| 6.0–6.9 | 35 |
| >7.0 | 75 |
The annual rupture rate in the UK Small Aneurysm Trial (N = 2257, with about half in randomized arm and half in the registry) was 2.2% per year for the first 3 years of follow-up. The initial aneurysm diameter was 3-6 cm, and the mean was approximately 4.4 cm.
AAA rupture after stent-graft placement has been observed with specific devices, but device evolution and operator experience in both patient selection and device placement make it difficult to provide any stable estimates of the incidence at this time.
Mortality/Morbidity
The mortality rate for ruptured AAA is substantial. As many as 2 of 3 patients with ruptured AAA die before arriving at the hospital.Of those who reach the hospital, as many as one fifth of those who die do so before the operation, and the overall mortality rate still averages approximately 49%.
Multivariate analyses by Harris and coworkers revealed adverse predictors for early mortality, as shown in Table 3 below.
Table 3. Predictors of Early Mortality
| Risk Factor | Percentage of Patients With Factor, % | Associated Mortality Rate, % | Mortality Rate in Patients Without Factor, % |
|---|---|---|---|
| Cardiac arrest | 19 | 81 | 26 |
| Loss of consciousness | 26 | 72 | 24 |
| Intraoperative BP <> | 28 | 62 | 24 |
Note: For all comparisons, P <.05 unless otherwise noted. BP indicates blood pressure.
*P value <.10.
Deaths that occurred after the second postoperative day were predicted by variables shown in Table 4 below.
Table 4. Variables Predictive of Mortality
| Risk Factor | Percentage of Patients With Factor, % | Associated Mortality Rate, % | Mortality Rate in Patients Without Factor, % |
|---|---|---|---|
| Renal failure | 30 | 75 | 8 |
| Respiratory failure | 28 | 69 | 19 |
In the consecutive series of 180 patients with ruptured AAA, the following factors were independently related to the mortality rate: age; systolic BP less than 80 mm Hg; and a history of hypertension, angina, or myocardial infarction (MI). In patients who survived the surgery, the causes of death were as follows:
- Renal failure or multisystem failure (32%)
- Cardiac failure (29%)
- Respiratory failure, including pneumonia (17%)
- Coagulopathy (12%)
- GI hemorrhage (3%)
- Perforated duodenal ulcer (1.5%)
- Renal hemorrhage (1.5%)
- Hemorrhage from graft anastomosis (1.5%)
- Stroke (1.5%)
- Aspiration (1.5%)
Major postoperative complications observed in a series of 174 patients at the Mayo Clinic are shown in Table 5 below.
Table 5. Major Postoperative Complications
| Event | Percentage of Patients Affected, % | Case Fatality Rate, % |
|---|---|---|
| Respiratory failure | 48 | 34 |
| Tracheostomy | 14 | 44 |
| Renal failure | 29 | 76 |
| Sepsis | 24 | 45 |
| MI/CHF* | 24 | 66 |
| Bleeding | 17 | 90 |
| Stroke | 6 | 50 |
| Ischemic colitis | 5 | 67 |
| Lower extremity ischemia | 3 | 17 |
| Paraplegia/paraparesis | 2 | 50 |
*CHF indicates congestive heart failure.
Other major postoperative morbidities reported in the literature include the following:
- Delayed hemorrhage from iatrogenic injuries
- GI or genitourinary tract injuries
- Pancreatitis
- Duodenal obstruction
- Prosthetic graft infection
One in 5 patients undergo a repeat operation. One half undergo repeat laparotomy, and the other half undergo a tracheostomy or vascular procedure.
Late in-hospital deaths occur at a mean of 25 days ± 23.
The long-term outlook for patients is grim. Table 6 below shows survival statistics of the 147 patients with ruptured AAA (defined as blood outside the aortic wall) in the prospective Canadian Aneurysm Study.Younger age and total intraoperative urine output were the only factors that were independently predictive of survival.
Table 6. Cumulative Survival of Ruptured AAA Patients, as Determined at Kaplan-Meier Analysis
| Time Since First Diagnosis | Survival Rate of Cohort, % | Patients Surviving > 1 mo After Surgery, % |
|---|---|---|
| 1 month | 49 | 96 |
| 1 year | 41 | 87 |
| 2 years | 37 | 77 |
| 3 years | 36 | 73 |
| 4 years | 29 | 60 |
| 5 years | 26 | 53 |
Roughly 40% of early and late ( <5>
The availability of vascular surgeons, as opposed to general surgeons, was independently associated with a decreased 30-day mortality rate.
Sex
- In 2001, the National Center for Health Statistics reported that annual deaths attributable to AAA are almost 2-fold higher for males compared with females.
- An autopsy study revealed that the peak incidence of AAA was a decade later for women compared with men. Specifically, the peak incidence for women was at 90 years (4.5%) and 80 years in men (5.9%).
- Females were noted to have 3 times more ruptures than males in the UK Small Aneurysm Trial.In addition, females who have ruptured AAA may have a higher mortality rate than that of their male counterparts.
Age
- The incidence of AAA increases significantly after age 55 years in men and after age 70 years in women.
- The mean age of men with ruptured AAA is 70.6 years (range, 44-89 y) versus 77.3 years (range, 67-86 y) for women.
- Consequently, a number of competing comorbidities occur in this elderly cohort. These diminish the long-term survival rates even after successful AAA repair. On average, two thirds of postoperative patients died within the next 5 years, mostly due to causes related to cardiovascular disease.
Anatomy
Approximately one third of AAAs originate close to or at the level of the renal arteries, and suprarenal involvement has been reported in as many as 10% of patients.
In an old series by Rosch, 1 in 3 patients had accessory renal arteries, of which 7% arose from the AAA, and 1 in 5 patients had celiac or superior mesenteric artery stenosis. As many as 30% of patients with AAA can have renal artery stenosis. These are relevant findings because colon ischemia, as well as renal failure, can occur after aneurysmectomy.
A Japanese series of 97 cases of ruptured AAA revealed the following distribution of sites of rupture:
- Right lateral wall - 28%
- Pelvic arteries - 22%
- Posterior wall - 19%
- Left lateral wall - 17%
- Anterior wall - 10%
- Suprarenal - 4%
In a series of 226 AAAs in Italy, bleeding occurred into the following regions41:
- Retroperitoneal - 85.3%
- Peritoneal - 7.1%
- Inferior vena cava (IVC) or iliac vein - 5.8%
- Enteric - 1.8%
Although ruptures into the retroperitoneum typically originate from the left posterior aspect of the AAA, ruptures into the intestine tend to occur from the right anterior aspect.
Clinical Details
As many as three fourths of AAAs are initially asymptomatic. Most aneurysms are incidentally discovered during routine physical examination, during a diagnostic imaging study, or during surgery for other abdominal pathology.
Hypotension, pulsatile abdominal mass, and flank or back pain constitute the classic triad for ruptured AAA. However, this triad may be incomplete in as many as 50% of patients. Cardiac arrest can be the clinical presentation in a fourth of patients. In 1985, Donaldson et al described the presentation of their series of 81 patients that survived to undergo surgery for ruptured AAA.
Symptoms
- Abdominal pain - 58%
- Back pain - 70%
- Syncope - 30%
- Vomiting - 22%
Findings
- Mass - 91%
- Tenderness - 78%
- Systolic blood pressure less than 80 mm Hg - 42%
- White blood cell count greater than 10,000/µL - 79%
- Hematocrit less than 38-42%
- AAA apparent on abdominal plain film - 74%
Atypical presentations
- Pain radiating to the groin
- Back pain from AAA erosion into the vertebral body, with rare false aneurysm formation into the left psoas muscle
- Acute femoral neuropathy with or without thigh ecchymosis due to femoral nerve compression as blood dissects inferiorly between the iliacus and psoas muscles and the overlying fascial pockets
- Partial upper GI obstruction from AAA compression of the third portion of the duodenum
- Lower extremity ischemia and visceral thromboembolism caused by embolization of AAA mural thrombi
- Acute bilateral limb ischemia from aortic thrombosis
- GI bleeding secondary to aortoenteric fistula, usually involving the third part of the duodenum
- High output CHF, widened pulse pressure with machinery-like murmur, hematuria, rectal bleeding, priapism, or lower extremity swelling related to a fistula from the aorta to IVC or renal, lumbar, or common iliac vein
Preferred Examination
Intravenous access with 2 large-bore catheters should be established if a ruptured AAA is suspected. Blood should be drawn for stat determination of the CBC and kidney profile and for blood typing and screening. The vascular surgery team should be involved from the outset.
Ideally, in a hemodynamically stable patient, nonenhanced and enhanced helical or spiral CT of the thorax, abdomen, and pelvis should be expeditiously performed. This examination provides key information about the extent of aneurysmal disease, and it can be used to confirm and localize the site of rupture.
In the patient with an unstable presentation, an emergency operation is indicated. Time may permit only rapid bedside ultrasonography (US) and Doppler study of abdominal aorta and iliac arteries to confirm the presence of aneurysms.
The maximal aneurysm diameter is adequately assessed by using B-mode ultrasonography, CT scanning, and MRI. Aortography reveals only the lumen of the AAA because laminated clot obscures the outer limit of the aneurysm wall. Therefore, it often causes underestimation of the true aortic diameter.
Limitations of Techniques
Although CT and MRI provide detailed information about the blood vessels and their surrounding structures, these examinations require time; therefore, they may be unsuitable for use in patients in unstable condition. When contrast material is used in conjunction with CT to delineate blood-filled structures, it poses a risk of acute renal failure, particularly in hypovolemic elderly patients who may already have baseline nephrosclerosis or diabetic nephropathy.
Sonography is a quick and convenient modality, but it is much less sensitive and specific for the diagnosis of aneurysmal rupture. The absence of sonographic evidence of rupture does not rule out this entity if clinical suspicion is high.
