Peripheral and abdominal aortic aneurysms

Introduction

The normal diameter of the aorta varies with age, sex and bodyweight. It decreases in size as it leaves the thorax and enters the abdomen, tapering to its iliac bifurcation. However, the infra-renal aorta enlarges progressively with age. An aortic aneurysm is a permanent localised dilatation of all three layers of the vessel wall of at least a 50% increase in diameter compared to the expected normal diameter of the aorta. If the maximum normal diameter of the aorta is considered to be 2.1 cm, aneurysmal dilatation is said to occur when the diameter exceeds 3.0 cm.

The abdominal aorta is the most commonly affected artery and accounts for 90% of all aneurysms. Of these, 95% will originate below the level of the renal arteries. The aortic arch, thoracic aorta and thoracoabdominal aorta are involved in approximately 10% of aneurysms.

The morphology or shape of aneurysms may be classified as saccular or fusiform, although this description represents a continuous spectrum. Saccular aneurysms affect only a small portion of the aortic circumference while fusiform lesions involve the entire circumference of the vessel.

Epidemiology

The prevalence of abdominal aortic aneurysm (AAA) in men over 65 years is around 7–8%. ^, The condition is thought to be six times greater in men than in women. In determining the prevalence of AAA, the frequently asymptomatic nature of the disease is a major confounding factor. Data on prevalence stem from four sources: autopsy surveys, routine mortality and hospital in-patient statistics and population-screening surveys. It should be noted that all of these sources have their limitations and potential for bias; screening surveys offer, potentially, the most accurate estimate of prevalence.

The prevalence of screen-detected AAA in men in England is reported to be between 1.3–12.7%. This variation is accounted for by differing criteria for the definition of AAA and the age group screened. If the criterion of inner wall aortic diameter >29 mm is used as the definition for AAA, the prevalence in men aged 65 years within the UK screening programme in the year 2019–2020 was 0.9%. These figures are in keeping with data from other European and North American series. ^, Interestingly, data from autopsy-based surveys yield similar results. The prevalence of AAA at autopsy in the UK has been reported at 2.3% in men and 1.6% in women.

Ruptured AAA accounts for around 3000 deaths each year in England or around 1.7% of all deaths in men aged 65 years and older.

Cause-specific mortality data for England and Wales and Hospital Episodes Statistics for England have shown that AAA mortality and ruptured AAA admission have fallen in England and Wales by around one-third, while non-ruptured AAA admission has remained steady between 2001 and 2009. A fall in the global incidence and prevalence of AAA has been shown, particularly in high-income countries. National differences can be explained by variations in cardiovascular risk factors. Of these, the reduction in smoking prevalence correlates most closely with declines in AAA mortality.

Epidemiological data on peripheral aneurysm are less readily available. However, the association with AAA is well recognised and approximately 25% of patients with AAA have synchronous femoral or popliteal aneurysms. It is likely that peripheral aneurysm will have similar epidemiological trends to AAA.

Pathophysiology

The cause of aneurysms remains unclear. Historically, aneurysmal change was thought to be underpinned by atherosclerosis. However, because of histological and epidemiological differences, it is now recognised that atherosclerosis is a co-existent phenomenon and the majority of AAA (90%) are thought to represent a degenerative or non-specific process.

AAAs exhibit familial clustering. This raises the possibility of both genetic and environmental aetiological factors. Genes encoding for type III collagen, matrix metalloproteinases (MMPs) and protease inhibitors and plasminogen activator inhibitors have all been reported to play some role in AAA development or expansion. ^, However, no specific genes have been convincingly implicated to date and it is inferred that susceptibility to the development of AAA is an irreversible process with multiple genetic and environmental risk factors. Genetic influences are attributed to a few gene polymorphisms with large effects.

North American and European data suggest that there is a fourfold increase in risk of having an AAA for the brother of a patient having an AAA. ^, Familial AAAs are more common when the index case is female and rupture is said to occur at a younger age and more often than with sporadic aneurysms. ^,

Established independent risk factors for AAA include male gender, age, hypertension, hyperlipidaemia and smoking. ^{, ,} In particular, the relationship between tobacco use and AAA development is striking. Aneurysms are four times more prevalent amongst smokers than non-smokers and the comparative relative risks of chronic cigarette smokers developing an AAA are threefold greater than their risk of developing coronary artery disease. ^, For these reasons, it is thought that smoking is the foremost environmental risk factor for aneurysm development and growth. Interestingly, diabetes is associated with a reduced risk; this may relate to glycated crosslinks in aortic tissue or metformin therapy conferring a protective effect.

AAAs are characterised histologically by destruction of elastin and collagen in the tunica media and tunica adventitia, smooth muscle cell apoptosis with thinning of the medial wall, infiltration of lymphocytes and macrophages, and neovascularisation. Four pathological mechanisms are thought to play central roles in AAA development: proteolysis of connective tissue, inflammation, biomechanical stress and genetic influences.

Clinical features

About 75% of aortic aneurysms are asymptomatic and are discovered incidentally. A small proportion will present with symptoms related to pressure on adjacent structures (dysphagia, ureteric obstruction, caval obstruction).

A small subset of AAA cases may present with the triad of lower back pain, weight loss and raised erythrocyte sedimentation rate. This triad is characteristic of inflammatory AAA, which represents the most extreme end of the spectrum of chronic inflammatory change seen in degenerative aneurysms, and accounts for 10% of all AAAs. These cases are further characterised by a thickened aneurysm wall, retroperitoneal fibrosis that may cause obstructive uropathy and dense adhesions to adjacent viscera.

Most of the clinical symptoms caused by aortic aneurysm are related to aneurysm rupture or embolism of mural thrombus.

Aneurysm rupture is associated with an estimated overall mortality of 75–80% and a significant proportion of patients will not reach hospital. Of those that present, most will have a contained retroperitoneal haematoma causing tamponade and resulting in temporary haemodynamic stability. The characteristic triad of abdominal or back pain, hypovolaemic shock and a pulsatile abdominal mass is present in only a few patients and the symptoms may be vague, and an abdominal mass missed. Other symptoms and signs may include groin pain, syncope, paralysis, or flank mass. A ruptured aneurysm should be considered in any elderly patient with unexplained hypotension and abdominal symptoms. The diagnosis may be confused with renal colic, diverticulitis, pancreatitis, or disease affecting the lumbar spine. A small proportion of AAAs rupture into an adjacent structure causing a primary aortic fistula; rupture into the vena cava produces a large arteriovenous fistula. In this case, symptoms include tachycardia, congestive heart failure, leg swelling, abdominal thrill, abdominal bruit, renal failure and peripheral ischaemia. AAAs may rupture into the fourth part of the duodenum and presentation may be with a herald upper gastrointestinal bleed followed by massive haemorrhage.

Embolism : patients with embolisation of thrombus from an aortic aneurysm may also present with acute ischaemia of the lower limb because of occlusion of the femoral or popliteal artery. Small aortic aneurysms may also undergo acute occlusion because of thrombosis as the aortic lumen becomes progressively narrowed by the accumulation of mural thrombus; these patients may present with acute bilateral lower limb ischaemia.

Screening

Population screening for AAA is appealing for a number of reasons:

• 1.

  Most AAAs are asymptomatic.

• 2.

  The majority of patients who suffer rupture will die.

• 3.

  B-mode ultrasound is a highly sensitive and specific diagnostic tool for detecting AAA.

• 4.

  Elective aneurysm repair is an effective prophylaxis against rupture.

A meta-analysis of four randomised controlled trials of screening elderly men for AAA with 15-year follow-up, has shown a significant and substantial reduction in the risk of death from AAA and the need for emergency surgery with an associated increase in elective repair.

A detailed analysis of the clinical benefits, harms and cost-effectiveness of screening women for AAA concluded that it was unlikely to justify costs. Nevertheless, recent UK National Institute for Health and Care Excellence (NICE) guidelines do make a recommendation to consider screening in women aged >70 years with selected risk factors.

In England, the National Health Service (NHS) AAA screening programme began in 2009. It offers all men aged 65 years an invitation for ultrasound screening. The same screening strategy is used in Sweden and the rest of the United Kingdom. Interestingly, both programmes have demonstrated an incremental cost-efficiency ratio of £7000 per quality-adjusted life-year (QALY). Compared with existing screening programmes for breast and cervical cancer, AAA screening for men remains cost effective. Reassuringly, in over 3000 men referred for intervention from the screening programme in England, only 8% were turned down for medical reasons, and of those who underwent aneurysm repair, peri-operative mortality was 1.4%.

Diagnosis

Aortic aneurysms may not be detected by clinical examination alone. Calcification of the aneurysm wall may be apparent on plain abdominal or chest radiograph, but this method lacks sensitivity and is unsatisfactory for routine use.

B-mode ultrasound is the diagnostic investigation of choice for the detection of AAA, providing accurate assessment of the aneurysm diameter and some indication regarding site.

Computed tomography (CT) is the investigation of choice to delineate AAA morphology and relationship to the visceral and renal arteries ( Fig. 13.1 ). CT or magnetic resonance imaging (MRI) will detect thoracic aortic aneurysms.

Principles of management

The principles of management in asymptomatic AAA are based on an assessment of the risk of aneurysm rupture weighed against the morbidity and mortality associated with surgical repair.

The size and growth of AAA are strongly associated with the amount and duration of smoking; smoking cessation can moderate this risk. The association between hypertension and hypercholesterolaemia and AAA suggests that medical management of these conditions may have a benefit on aneurysm growth. However, several drugs have been investigated in randomised controlled and non-randomised trials – beta-blockers, angiotensin-converting enzyme inhibitors, angiotensin-receptor blockers and statins. None of these drugs has been shown to confer a benefit. Nevertheless, given the strong association between atherosclerosis and AAA, it is recommended that aneurysm patients receive best medical therapy (BMT) to reduce the incidence of cardiovascular events.

The natural history of untreated asymptomatic AAA is considered to be one of expansion and potential rupture. Measurement of the maximum anteroposterior diameter of the aneurysm predicts the risk of rupture. Randomised controlled data on AAA of <55 mm diameter has demonstrated a mean risk of rupture of 1% per year. However, there is little randomised evidence to inform the rate of rupture in large aneurysms. A multicentre study of 198 patients with AAA >5.5 cm for whom elective repair was not planned because of medical comorbidity or patient refusal, demonstrated a 1-year rupture risk for AAA of 5.5–5.9 cm of 9.4%, 10.2% for AAA of 6.0–6.9 cm and 32.5% for AAA of >7 cm. Interestingly, the risk of rupture in the smallest AAA diameter cohorts was significantly greater than that reported in randomised controlled trials. This finding has been reported across other studies and it is likely that patients with significant co-existent morbidity are at a higher risk of rupture than their healthier counterparts. The projected annual rates of rupture for AAA from a meta-analysis of 13 studies are shown in Table 13.1 . However, some small aneurysms will rupture, and some large aneurysms will not.

Patients with asymptomatic AAA of 30–54 mm should be kept under regular ultrasound surveillance to monitor growth. Current recommendations for surveillance intervals are yearly for 30–44 mm, every 3 months for 45–54 mm. Elective repair is only recommended in asymptomatic AAA of >55 mm diameter – this applies to both open and endovascular aneurysm repair (EVAR). The operative mortality associated with elective infra-renal AAA repair is between 1% and 6%. ^,

Symptomatic, intact AAA or rapid expansion (>1 cm/year) of an AAA >4 cm represents a relative indication for operative repair. It is thought that symptoms of pain attributable to an aneurysm are caused by acute expansion or imminent rupture and urgent repair is recommended.

Aneurysm rupture is usually an absolute indication for surgery because without repair, mortality is almost certain. Surgery in some patients may be futile because of comorbidity or poor pre-operative clinical condition (unconsciousness, cardiac arrest).

Repair of intact abdominal aortic aneurysm

There are two methods by which AAAs may be excluded – traditional open repair (OR) and EVAR. Open surgical repair of AAA is a durable and cost-effective procedure. EVAR is effective and safe in selected patients, with lower short- and medium-term morbidity and mortality rates than open surgery. Patients with AAAs have a markedly reduced life expectancy in comparison with age- and sex-matched controls. The 5-year survival of patients post open surgery varies from 62– 72% (compared with 83–90% in age- and sex-matched populations), with the majority of deaths caused by cardiovascular disease. ^, The introduction of endovascular solutions for AAA offers an option for patients in whom OR was unfavourable. Conversely, there are patients with aortic anatomical constraints that are ill suited for EVAR, and results of EVAR in adverse anatomies are worse in the medium and long term.