Aneurysmal Diseases of the Aortoiliac Vascular Segment
1. Abdominal Aortic Aneurysm
The permanent dilatation of the vessel diameter to 1.5 times the norm is, by definition, referred to as an aneurysm. The average transverse diameter of the healthy infrarenal aorta is approximately 1.93 cm in men and approximately 1.67 cm in women [1]. From a diameter of 3.0 cm, an abdominal aortic aneurysm (AAA) is present according to international consensus.
Epidemiology and Aetiology
The most common location of aortic aneurysms, at 40–60 %, is the abdominal aortic segment, with the renal artery origins involved in 5 % of cases.
Population-based studies have shown a prevalence of AAA of 4 to 7.6 % in the group of men over 50 years of age and approximately 1.3 % in women of the same age [2, 3]. Men are thus affected considerably more often, at a ratio of 6:1. According to larger international registry studies, the perioperative overall mortality ranges between 1.6 % for intact AAA (iAAA) and 31.6 % for ruptured AAA (rAAA) [4]. With a lethality of up to 90 %, the prognosis of rAAA is particularly poor, so that effective strategies for elective treatment in the non-ruptured stage are required [5].
The most important risk factors for the development of AAA are smoking, a positive family history, age and atherosclerosis. The most significant risk factor, with an odds ratio of 5.07, is nicotine consumption [3].
Diagnostics
The AAA is frequently discovered as an incidental finding during routine examinations or as part of screening programmes and not infrequently remains clinically silent until rupture. In the presence of a 3 cm AAA, clinical examination provides indications of an AAA in only 29 % [6]. The gold standard for diagnosis and treatment planning of AAA is contrast-enhanced spiral computed tomography (sensitivity 93-100 %, specificity up to 96 %). Given the high radiation dose of a CT (27.4 mSV for three phases; plain abdominal radiograph for comparison: about 2 mSv), MRI represents an equivalent alternative, particularly in postoperative follow-up, with a sensitivity of 96 % and a specificity of up to 100 % [7, 8]. For initial and screening examinations of the abdominal aorta, colour-coded duplex sonography may be considered, which, depending on the examiner's experience, has a sensitivity and specificity of up to 100 % [9].
Treatment
In addition to conservative or medical treatment to optimise risk factors, open aortic replacement ("open aortic repair", OAR) and the endovascular procedure ("endovascular aortic repair", EVAR) are available for the invasive treatment of AAA. The approach should be selected individually and take into account the patient's individual circumstances (underlying conditions, life expectancy, patient preference).
The indication is fundamentally based on the existing rupture risk. For an AAA with a diameter of 4.4 cm this is less than 1 % per year and increases markedly from 5 cm. From an AAA diameter of more than 5 cm, the annual rupture risk is approximately 11 % [10, 11]. In elective treatment of an AAA, the individual rupture risk is set against a 30-day mortality of approximately 1.8 % for EVAR and 4.3 % for OAR [12]. However, the "early EVAR advantage" is offset in the long-term course, so that both procedures have an equivalent long-term outcome [13]. It follows that the elective surgical risk for AAA < 5 cm is higher than the annual rupture risk, which is why an indication for aneurysm exclusion applies only from 5 – 5.5 cm. Small aneurysms < 5 cm have an average annual growth rate of about 0.21 cm, which is why duplex sonographic follow-up examinations should be performed at 6- or 12-month intervals [11, 14]. Symptoms attributable to an AAA, as well as rapid size progression of more than 0.5 cm in 6 months, are associated with a significantly increased risk of rupture and therefore constitute an absolute indication for treatment.
For a long time, open surgical aortic replacement according to Creech represented the standard therapy for AAA [15], for which - partially coated - tube and Y-prostheses made of Dacron or PTFE are available. In three larger randomised studies, the 30-day mortality was reported as 3.0 % (OVER, USA), 4.3 % (EVAR-1, UK), and 4.6 % (DREAM, Netherlands). Mortality, revision rate, and lethality are significantly lower when the procedure is performed in specialised vascular surgery centres: perioperative mortality is about 2.2 % for vascular surgeons, 4.0 % for cardiac surgeons, and 5.5 % for general surgeons [16, 17]. Of particular importance for perioperative mortality are primarily cardiopulmonary complications, renal insufficiency, bleeding complications, and infections.
After the initial description of the procedure in 1988 by Nikolay Volodos [18], there was a continuous increase in EVAR procedures worldwide. In 2010, the EVAR share in the USA was 74 % [19] and in 2012 in Germany about 73 % [20]. Whether an EVAR can be performed depends, among other things, on the anatomical conditions and the morphology of the AAA as well as the access vessels. For complex anatomical conditions, so-called “custom-made” endografts with fenestrations and cutouts, e.g., for the visceral vessel origins, are now available. Their use should be reserved for specialised centres, as the mortality rate correlates significantly with the number of treated cases [17, 21].
2. Iliac Aneurysm
Iliac aneurysms can occur in the area of all pelvic vessels, but in most cases the common iliac artery is affected. There is very often an association with abdominal aortic aneurysms. About 16–20 % of patients with an abdominal aneurysm also have an iliac aneurysm [22, 23]. Isolated iliac aneurysms are significantly rarer (2 %) and then usually affect the common iliac artery [24].
Regarding the rupture risk of iliac aneurysms, the data in the literature are variable [25]. In their analysis, McCready et al. found that the average diameter of both symptomatic and ruptured iliac aneurysms was 7.8 cm [24]. Huang et al. observed a median diameter of ruptured iliac aneurysms of 6 cm and Lowry et al. of 7.5 cm. However, all publications observed that the range in the size of ruptured aneurysms is wide (3.5–18 cm), but that a rupture rarely occurs < 4 cm [26, 27].
In contrast to AAA, iliac aneurysms more frequently cause clinical symptoms. Symptoms are observed in over 60 % of patients [28]. Pain due to compression of adjacent structures is most commonly reported. In particular, the symptomatic internal iliac artery aneurysm often shows signs of a neurological compression syndrome with lumbosacral pain or sciatica (18 %), abdominal complaints (32 %), groin pain (12 %), hip and buttock complaints (8 %), and urogenital complaints up to congestion-related renal failure (28 %).
Analysis of the current literature suggests that the rupture risk for smaller aneurysms can be classified as extremely low. Only when the 3 cm limit is exceeded does the rupture risk appear to increase significantly [29].
Surgical therapy represented the "gold standard" in the treatment of iliac aneurysms in the past. It is performed depending on the location (aorta, common iliac artery, external and internal iliac artery) by interposition of a vascular prosthesis, preferably by aorto-(bi)iliac reconstruction ± revascularisation (or ligation) of the internal iliac artery via a retro- or transperitoneal approach. In the treatment of iliac aneurysms, endovascular techniques have gained increasing importance in recent decades. While in the past complex aortoiliac aneurysms presenting with rupture were treated almost exclusively by open repair, an endovascular approach is now increasingly chosen even in emergency management [27, 29].