Isolated Iliac Artery Aneurysms and their Management

Introduction

Iliac artery aneurysms (IAAs) commonly occur concurrently with more proximal arterial aneurysms. Isolated iliac aneurysms occur with an incidence of 0.4% to 1.9% in the general population. ^, Aneurysmal degeneration typically involves the common iliac artery (CIA, 70%–90%) and internal iliac artery (IIA, 10%–30%), or both of these segments contiguously; external iliac artery (EIA) involvement is extremely rare. ^, Bilateral common IAAs are present in at least 50% of affected patients.

Iliac arteries are defined as aneurysmal when their diameter is 1.5 times greater than the “normal” or expected diameter. On physical examination, isolated asymptomatic iliac aneurysms are difficult to identify, mainly due to their anatomical location deep within the pelvis. Most commonly, IAAs are identified during abdominal aortic aneurysm (AAA) screening studies or recognized as incidental findings on abdominal and pelvic computed tomography (CT) imaging obtained for other purposes. However, when they do become symptomatic and rupture, isolated IAAs are associated with high mortality due to late and missed diagnosis or difficulty with exposure and control during open operative repair (reported mortality 0%–33% for elective surgery; 0%–75% for emergency surgery). ^,

The operative approach to IAAs is guided by the anatomy, clinical stability, and the presence and severity of concomitant and proximal aortic disease. Like the experience with AAA repair, isolated IAA management has evolved toward an endovascular-first approach over the past two decades. The availability of dedicated iliac devices, including multiple branching stents, and the innovation of hybrid techniques have steadily improved the feasibility, ease, and durability of endovascular repair.

Pathogenesis

Mechanisms promoting IAA formation are multifactorial and involve both acquired and inherited risks (see Ch. 71 , Arterial Aneurysms: Etiology, Epidemiology, and Natural History). ^, Briefly, hypertension and cigarette smoking are the risk factors most prevalent in patients with iliac aneurysms. Other less common etiologies include previous trauma, iatrogenic injury, arteritis, connective tissue disorders, and infections. Historically, syphilis and tuberculosis were frequently associated with mycotic IAAs, but Salmonella is the most commonly identified pathogen today followed by Staphylococcus , a frequent culprit usually due to septic emboli. ^, In a Chinese study of 18 patients with infected aortic and iliac aneurysms, the majority (92%) were Gram-negative bacilli, including Salmonella species, Klebsiella pneumoniae, and Escherichia coli . Isolated case reports of Candida species have also been reported. It should be noted that in general for mycotic arterial aneurysms, Staphylococcus remains the most common pathologic species.

The proclivity for aneurysms to form in the common and internal iliac segments, sparing the external iliac artery, remains incompletely understood. Segments of the normal and aneurysmal adult aorta appear to have distinct DNA expression profiles, which may explain the site specificity and prevalence of various aneurysm phenotypes. ^, The EIA may be resistant to aneurysm formation because of the distinct embryonic lineage of its vascular smooth muscle cells as compared to the aneurysm-prone common and internal iliac arteries. ^, Immunobiological characteristics suggest phenotypical differences between the internal and external iliac arteries, emphasizing the concept that embryogenesis can influence the propensity of an artery to aneurysmal disease. ^{, ,}

Disturbed or asymmetric iliac artery flow may also promote aneurysmal degeneration. In a series of 329 World War II veterans with unilateral above-the-knee amputations, amputees were five times more likely to develop abdominal aortic and iliac aneurysms than risk factor-matched controls. Further, AAA morphology was reproducibly related to which leg had been amputated, indicating that asymmetrical flow patterns in the aortoiliac system predisposed to pathological remodeling and aneurysmal degeneration. Iliac aneurysms may also form proximal to pelvic or femoral arteriovenous fistulae.

Recently three distinct morphologies of common iliac artery aneurysms (CIAAs) have been identified: complex (involving a bifurcation), fusiform, and kinked (distal to a sharp bend or tortuosity in the CIAA). Utilizing computational fluid dynamics, abnormal blood flow in the CIAA was observed to promote proximal aortic remodeling with resulting lateral deflection of the abdominal aorta towards the CIAA side in most cases; these findings were confirmed in a validation cohort of 162 patients.

Natural History

There is a paucity of evidence regarding the true prevalence, natural history, and rupture potential of IAAs. Retrospective series have informed our understanding of this condition with regard to demographics, bilateral disease (found in 65% of patients in one series), and aneurysm expansion. ^, In these series, the rate of diameter enlargement for IAAs under surveillance varied based on the baseline diameter present at the time of diagnosis; for IAA ≤3 cm, expansion rates averaged 0.05 to 0.15 cm/year, whereas aneurysms greater than 3 cm enlarged 0.26 to 0.29 cm/year; no differences in growth rates were noted between isolated IAAs and those present in patients with concomitant AAA. ^,

IAAs are reportedly associated with a high propensity for rupture. They enlarge slowly over time, and are often first identified at the time of rupture. In early reports, as many as 42% of IAAs were ruptured at the time of initial presentation, an event associated with significant mortality (range, 0%–56%). ^{, ,} The mortality from emergent repair for rupture is approximately 28% (range, 0%–60%) as compared to a reported 5% mortality for elective IAA repair (range, 0%–50%). The mean size of ruptured IAAs varies in reports from 6 to 6.8 cm. Though rare, in a series of 11 patients with isolated external iliac artery aneurysms (EIAAs), four (36%) presented ruptured and one (9%) was symptomatic due to peripheral embolization. All ruptured EIAAs were greater than 4 cm in diameter. A literature review of internal iliac artery aneurysms (IIAAs) identified 94 cases with 40% ruptured at presentation, median aneurysm size of 7.7 cm (range, 2–13 cm), and 31% mortality. Thus, despite higher frequency and prevalence of imaging today, nearly half of IAAs in these series – common, external, and internal – still present ruptured.

In a report of 63 patients with ruptured IIAAs across 28 European vascular centers, the majority (nearly 94%) ruptured at diameters greater than 4 cm with only one IIAA rupture occurring at less than 3 cm, and four occurring at less than 4 cm (6.3% of all ruptures). Regarding anatomical distribution, isolated IIAAs occurred in 30% of cases and were more typically present concurrent with aortic aneurysms (42%), common iliac aneurysms (65%), or both (37%).

Summarizing the existing evidence, uncomplicated IAAs <3.5 cm in diameter rarely rupture, ^, supporting the generally accepted threshold of 3 to 3.5 cm for elective repair when asymptomatic (indications may vary between CIAAs and IIAAs). These recommendations vary based on anatomical location and complexity, rate of enlargement, presence of symptoms, and concomitant comorbid conditions. Symptomatic IAAs should be expeditiously repaired upon presentation at any diameter.

Presentation

The anatomic location and proximity to adjacent pelvic structures including bowel, bladder, the urinary collecting system, nerve roots, and pelvic veins can obscure the diagnosis in the absence of a perceptible mass. Vague lower abdominal pain often results from compression or impingement of these neighboring structures. Patients may also complain of intermittent claudication or lower extremity pain secondary to embolic arterial occlusion, lower extremity paresis, sciatic neuralgia, lumbosacral pain, ureteral obstruction, tenesmus, or even constipation. Though rare, lower extremity deep venous thrombosis has been reported in case reports as the presenting symptom of isolated IIAAs, due to the compression of the external iliac vein or the iliofemoral vein by the internal iliac artery. ^,

Sudden abdominal, groin, or flank pain associated with hemorrhagic shock frequently portends rupture. Although frequently contained in the retroperitoneum at the outset, IAA rupture and resulting hemorrhage eventually leads to irreversible shock if not controlled. Delays in diagnosis contribute significantly to reported mortality, with or without emergent intervention.

In rare instances, IAAs can elicit thromboembolic symptoms and peripheral arterial insufficiency. In a single-center retrospective review of 53 isolated IAAs over a 16-year period, intermittent claudication was identified in four cases due to iliac aneurysm thrombus causing stenosis, while ischemia was reported in three cases due to arterial embolism from the aneurysm. Distal embolization was the presenting symptom in 1 of 16 patients (9%) with symptomatic IAAs identified at a single institution over 6 years.