Iliac Artery Aneurysms
Steven M. Santilli
Iliac artery aneurysms are commonly defined as a localized dilatation of the iliac artery larger than 1.5 cm in diameter. Awareness of iliac artery aneurysms is increasing, paralleling the rise in abdominal aortic aneurysm (AAA) detection and repair. Though commonly found in association with AAAs, iliac artery aneurysms are occasionally isolated to the iliac artery segment. Early reports suggested a lethal natural history for iliac artery aneurysms that are 3 cm in diameter or larger, leading to recommendations for repair of iliac artery aneurysms at 3 cm. Recent evidence has been presented to suggest that iliac artery aneurysms expand slowly, are usually asymptomatic, and rarely rupture at smaller sizes. With rising awareness and detection as well as minimally invasive means to repair iliac artery aneurysms, a sound knowledge of iliac artery aneurysms is required to best serve patients with this condition.
Diagnostic Considerations
There are currently no uniformly accepted screening protocols for the detection of iliac artery aneurysms; diagnosis usually occurs during evaluation for another clinical condition. Confirmatory diagnostic tests include physical examination, plain x-ray, ultrasound, computed tomography (CT), magnetic resonance imaging (MRI), or conventional angiography.
Physical examination is relatively reliable in the detection of AAAs in patients with a favorable habitus. However, due to the location of the iliac arteries deep within the pelvis, detection of these aneurysms is rare on physical examination unless their diameter exceeds 4 cm. Because the vast majority of iliac artery aneurysms are less than 4 cm in diameter, physical exam is not a reliable diagnostic test.
Plain radiographs can detect a calcific rim in some patients with AAAs, but there are no data to suggest that they are useful for the diagnosis of iliac artery aneurysms.
Ultrasound is a reliable technique to diagnose iliac artery aneurysms. The average variance between ultrasound determination of the diameter of an iliac artery aneurysm from CT scanning was 0.3 mm. Due to ultrasound’s ubiquitous availability, relative low cost, and accuracy, duplex scanning is the screening test of choice for the diagnosis of iliac artery aneurysms.
CT scanning is considered the gold standard in the diagnosis and measurement of iliac artery aneurysms. The images allow the accurate measurement of size and location to plan repair. With the addition of CT angiography, the need for later planning arteriography prior to repair is eliminated. However, the use of CT angiography as a screening test is discouraged due to its relatively high cost and the exposure of the patient to radiation.
MRI as a technology is continuing to evolve and improve. In the diagnosis of iliac artery aneurysms, most centers limit the use of MRI to patients with either a contrast allergy or those with, or at risk for, dyeinduced renal failure. The advantages include no exposure to radiation or nephrotoxic contrast agents, but MRI is relatively expensive. As technology continues to advance, using MRI for diagnosing iliac artery aneurysms will increase and will potentially replace CT scanning.
Conventional arteriography has no role in the diagnosis of iliac artery aneurysms. In many patients, thrombus lines the aneurysm sac, making an accurate diagnosis of diameter impossible. Arteriography is used as a planning test prior to iliac artery aneurysm repair, though CT angiography has reduced its use.
Pathogenesis
The etiology of iliac artery aneurysms is multifactorial. Causes of arterial aneurysms are:
Connective tissue disorders
Mechanical
Arteritis
Infectious
Pregnancy-related
Pseudoaneurysms
Degenerative
Graft failure
Most iliac artery aneurysms are degenerative, but pseudoaneurysms of the iliac arteries are an increasingly common cause of iliac artery aneurysms as endovascular procedures increase in frequency.
The etiology of degenerative iliac artery aneurysms is currently unknown, but it is believed that these aneurysms arise from a mechanism similar to that of AAAs. Histologically, the infrarenal aorta and iliac arteries have no medial vasa vasorum, which could contribute to the development of pathological conditions, including aneurysms. Most atherosclerotic risk factors, except diabetes, are associated with the development of aneurysms, yet evidence suggests that aneurysmal disease is felt to be fundamentally different from occlusive disease. While there appears to be a genetic susceptibility to the development of aneurysms, no specific mutation of a major arterial connective tissue protein has been identified.
Table 22-1 The Number of Patients and Iliac Artery Aneurysms Per Size Category | ||||||||||||||||||||||||||||||||||||
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Current work is focusing on the role of protelytic enzymes and their inhibitors in the formation of arterial aneurysms. In particular, matrix metalloproteinases are being investigated to determine their precise role in the pathogenesis of arterial aneurysms.
Indications and Contraindications for Repair
In general, traumatic iliac artery pseudoaneurysms and infectious aneurysms are considered for repair due to their compromised anatomic wall and probable tendency to expand or rupture. The decision to repair degenerative iliac artery aneurysms is based on the known natural history data described below.
Knowledge of the natural history of iliac artery aneurysms aids in determining the indications for repair. Current natural history data are demonstrated in Tables 22-1 and 22-2. Iliac artery aneurysms less than 3 cm in diameter expand at a slow rate, while those larger than 3 cm expand at a faster rate. Iliac artery aneurysms are unlikely to cause symptoms unless they expand to greater than 4 cm, and rupture is rare until they expand to more than 5 cm. With current natural history data in mind, recommendations for treating iliac artery aneurysms are:
Table 22-2 Expansion Rates Per Size Category | ||||||||||||||||||||||||||
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