There are several anatomic patterns of AAAs. The descriptions are clinically relevant, as different techniques for repair are used for different types of aneurysms. The most common type is the infrarenal aneurysm, which is located distal to the renal artery origins with a neck of nondilated aorta between the aneurysm and the renal arteries. Infrarenal aneurysms may be repaired with commercially available endografts (See Chapter 16, Fig. 16.5) or by open surgical repair. The aorta may be clamped and sutured distal to the renal arteries with open surgical repair of an infrarenal aneurysm (Fig. 15.1).

Juxtarenal aneurysms have no intervening segment of normal aorta distal to the renal arteries. With this type of aneurysm, endovascular repair is possible with specialized techniques or fenestrated grafts. Commercial fenestrated grafts became available in the in the United States in 2012, with the initial FDA approval for the treatment of AAAs with short infrarenal aortic necks (4 to 15 mm). The proximal segments of these custom-fabricated endografts have holes (fenestrations) so that stents can be placed into the renal arteries, thus providing secure proximal fixation.¹⁸ Endovascular repair also makes use of specialized techniques, including the use of endostapling (where the proximal graft is anchored to a short aortic neck), snorkeling, or the use of endografts that do not rely solely on proximal stent fixation. Open surgical repair is more common for the treatment of juxtarenal aneurysms, but suprarenal or supraceliac aortic clamping is required, increasing the risk of visceral ischemia during the procedure.

Suprarenal aneurysms involve the aorta above the renal arteries, and the renal arteries arise from an aneurysmal segment. Suprarenal aneurysms require concomitant renal or visceral artery reconstruction when open surgical repair is performed. Use of fenestrated or branched endografts for suprarenal aneurysms is an option in some cases, though still experimental in the United States in 2014.

Thoracic aortic aneurysms (TAA) cannot be evaluated by ultrasound due to imaging limitations from the air in the chest (ultrasound is not transmitted through the lungs). Thoracoabdominal aortic aneurysms (TAAA) can involve both the thoracic and abdominal segments of the aorta.¹⁹ Repair of a TAAA requires exposure of the aorta above and below the diaphragm. TAAAs are functionally classified considering the nature of surgical repair required.²⁰ Suprarenal aortic aneurysms that extend proximally to include the celiac artery origin require aortic clamping and the proximal anastomosis be performed in the chest. This is a Type IV TAAA. Though the thoracic aorta cannot be visualized with ultrasound imaging, the possibility of a TAAA should be considered if the proximal extent of aneurysmal dilatation cannot be defined on an abdominal ultrasound examination. CT or MR angiography is needed for complete evaluation of any TAAA or suprarenal aneurysm.

Aneurysms can also be described by their morphology. The most common shape is fusiform—widest in the middle and tapering proximally and distally (Fig. 15.2). This descriptor is derived from the Latin fusus, meaning “spindle.” A more focal dilation is described as a saccular aneurysm. A saccular aneurysm resembles a small sac or outpouching. This term is derived from sacculus, the Latin word for small pouch or sac.

The most common cause of aortic and iliac artery aneurysms is a chronic degenerative change in the arterial wall leading to a focal arterial dilation. Though patients with aneurysms share risk factors with patients who have atherosclerosis,²¹ the term “atherosclerotic aneurysm” is no longer used, as the pathophysiology of AAA is distinct from atherosclerosis. The pathogenesis of an AAA is based on four broad cellular and molecular processes that include proteolytic degradation of the aortic wall, inflammation and immune responses, biomechanical wall stress, and molecular genetics.²² Multiple factors play a role in localized hemodynamic stress, and genetic predisposition attracts inflammatory cells (monocytes and lymphocytes) to the area of the infrarenal aorta.

Pseudoaneurysms, or “false” aneurysms, are the result of disruption of the arterial wall from injury or infection, or breakdown of an arterial anastomosis, such as the anastomosis of a prosthetic graft to a native artery (Fig. 15.3). Pseudoaneurysms tend to be saccular in appearance. Aneurysms occurring at, or adjacent to, the anastomosis of a graft (“juxta-anastomotic” or “para-anastomotic” aneurysms) are often pseudoaneurysms from disruption of the anastomosis, but they can also represent late aneurysmal degeneration of the remaining native vessel.²³,²⁴ Juxta-anastomotic aneurysms are rare enough that routine, regular surveillance is not recommended after uncomplicated open aortic procedures, at least not for several years after the initial operation.²⁵ Pseudoaneurysm formation at one or more anastomotic sites can be a late presentation of a prosthetic graft infection.²³,²⁶,²⁷,²⁸,²⁹,³⁰,³¹

Infection is an unusual cause for primary aneurysm formation, although the aorta, peripheral arteries, cerebral arteries, and visceral arteries (in descending order of frequency) can be affected by bacterial arteritis and false aneurysm formation. The most common causal pathogens are Staphylococcus and Streptococcus species. Imaging features of infected aneurysms include findings of a lobulated vascular mass, an indistinct irregular arterial wall, edema, or a perianeurysmal soft tissue mass. Perianeurysmal gas, aneurysm thrombosis, aneurysmal wall calcification, and disrupted arterial calcification have also been described with infected aneurysms, though these findings are less common.³²

Multicenter trials have found that there is no benefit for early surgical repair of small fusiform infrarenal AAAs (the most common type), compared to careful surveillance with serial ultrasound examinations.³³,³⁴ Small aneurysms can be safely observed.³⁵ Based on a review of published data, the estimated average annual rate of rupture or acute expansion is almost nil for an AAA less than 4.0 cm in diameter.³⁶ In a meta-analysis of 15,471 patients, small AAAs grew an average of 1.3 to 1.9 mm/year for 3.0 to 3.5 cm AAAs, 2.4 to 3.0 mm/year for 4.0 to 4.5 cm, and 3.6 mm/year for AAAs larger than 5.0 cm. Rupture rates of small aneurysms were found to be very low in this meta-analysis. The growth rates and rupture rates were found to be significantly higher in women than men.³⁶

The Society for Vascular Surgery (SVS) guidelines for management of small aneurysms suggest serial ultrasound imaging at intervals that depend on aortic diameter.² This strategy appears to be safe for men until the AAA reaches 5.5 cm in diameter, as the expected annual rupture rate is less than the risk of perioperative mortality with surgical repair observed in clinical trials.¹⁶,³⁷,³⁸,³⁹ Repair is generally recommended if the expansion rate is greater than 0.5 cm in 6 months or if symptoms develop.⁴⁰ Recommendations have been made to consider repair of AAAs at 5.0 cm in diameter for women, as rupture is more common in women with aneurysms, and female gender appears to be a risk factor for adverse outcomes.⁴¹,⁴²,⁴³

Because perioperative mortality with EVAR is less than with open repair, it has been suggested that EVAR could be considered at a smaller AAA size threshold. Two recently completed randomized controlled trials evaluated the efficacy of surveillance versus early EVAR for small AAAs. These trials found that mortality and rupture rates for AAAs less than 5.5 cm were low, and no clear advantage was shown with either an early or delayed EVAR strategy. Hence, the authors of both studies concluded that surveillance of small AAAs is safe if careful surveillance is possible.⁴⁴,⁴⁵,⁴⁶

Since AAAs are typically asymptomatic until rupture, screening for AAA is important. This allows AAA treatment in an elective setting. Physical examination is too insensitive to be relied upon for initial diagnosis, especially if the aneurysm is not large or if the patient is heavy,⁴⁷ but over 90% of aortic aneurysms are infrarenal in location where ultrasound imaging is practical.

AAA screening has a demonstrated benefit. A large United Kingdom trial, the Multicentre Aneurysm Screening Study (MASS), was a randomized, controlled trial of over 65,000 population-based men 65 to 74 years of age who smoked at least 100 cigarettes in their lifetimes. One-half of the men were invited for AAA screening, while the control group was followed for clinical events but not invited for screening. The results of this multiyear study demonstrated the ability of screening to significantly reduce aneurysm-related deaths as well as reduce all-cause mortality.⁴⁸,⁴⁹,⁵⁰ Recognizing the significance of this, Medicare has covered an aortic screening ultrasound (for new Medicare beneficiaries at risk) since 2007. Current recommendations call for a one-time ultrasound screening to be offered to men aged 65 to 75 years who have smoked 100 or more cigarettes in their lifetime or to men and women 50 years of age or older with a family history of AAA.¹⁴ In utilizing these criteria for screening a population at risk for AAA, a prevalence of 5.1% was detected in a contemporary series, which is similar to multiple other studies where the prevalence ranged from 3.9% to 7.2%.⁵¹

Aortic diameter measurements are used to diagnose an ectatic or aneurysmal aorta, stratify for risk of rupture, and guide the need for long-term follow-up. The normal diameter of the infrarenal aorta is generally less than 2 cm in transverse dimension. The aortic diameter is larger in men, and it increases
with body size and age.⁵²,⁵³ An aneurysm is diagnosed when the abdominal aortic diameter is 1.5 times the normal value or when the aorta is 3 cm or greater in diameter. The aorta is deemed “ectatic” if the aorta measures 2 to 3 cm in diameter (greater than normal aortic size but less than 1.5 times the diameter of the undiseased adjacent aorta). Ultrasound follow-up is recommended for patients with aortic diameters of 2.6 to 2.9 cm at 5-year intervals, 3.0 to 3.4 cm at 3-year intervals, and for those with aortas of 3.5 to 4.4 cm, annual follow-up is recommended. Patients with larger AAAs of 4.5 to 5.4 cm in diameter should be followed at 6-month intervals.²

Ultrasonography is recommended for both initial screening and the diagnosis of AAA but is relatively imprecise in measuring aneurysm size. The Intersocietal Accreditation Commission Standards and Guidelines for Vascular Testing state that aneurysms should be measured at the widest point of the aorta from outer wall to outer wall (Fig. 15.4). The outer wall to outer wall measurement was found to have less intraobserver variability and has been suggested as the method for measuring AAA size in the large MASS trial.⁵⁴ However, a more recent study has questioned the utility of outer to outer wall measurements and published data that interobserver reliability is greater with the inner wall to inner wall measurement.⁵⁵

Ultrasound imaging may be limited in patients with large body habitus, overlying bowel gas, or those who are unable to cooperate with the examination. In these instances, an abdominal CT scan is more appropriate for making the initial diagnosis of AAA. CT is more reproducible for diameter measurement than ultrasound, and CT is the primary modality for surgical planning. However, in almost all other cases, ultrasound is the preferred modality for initial AAA diagnosis and follow-up.

Given the utility of ultrasonography in screening for AAAs, point-of-care examinations are being used more commonly, especially in underserved and rural areas where primary care clinics are performing AAA screening. The increasing availability of compact, low-cost, ultrasound systems is making point-of-care studies more commonplace, leading experts to believe that ultrasonography will be a basic skill in routine medical practice in the near future (see Chapter 27).⁵⁶ The use of contrast-enhanced ultrasonography and 3D ultrasound have found some benefits in follow-up after EVAR, to detect endoleak or to find vascular branches (see Chapter 16), but these advanced applications are not particularly useful in the initial diagnosis of AAA.⁵⁷