Abdominal Aortic Aneurysm (Screening and Investigation)

• An abdominal aortic aneurysm (AAA) is defined as pathological dilatation of the aorta.

• Although the diameter of the aorta varies with age, sex and body mass index, the upper limit of normal diameter is generally defined as >1.5 times normal. For an average older male the normal diameter of the infra-renal aorta would be 15-24mm; >3cm would be defined as aneurysmal 1.

• AAAs are described according to the location of the proximal margin with respect to the renal arteries – infrarenal (i.e. inferior to the level of the renal arteries – the great majority), juxtarenal, suprarenal, or pararenal. 

• Their morphology is also part of the description - they may be fusiform (most common), saccular, or mixed. Both location and morphology have implications with respect to potential treatment.

Prevalence

• The risk of AAAs increases markedly over the age of 60 years. They are 4-6 times more common in men than women and develop later in women. They occur more frequently in white than black people.

• The prevalence of AAAs in older men is reported to range from 1-13% (2). A 2011 review reported screening studies of men aged 65-80 y.o. identified AAAs in up to 8% 3.

• In a 2014 review the mortality rate attributable to AAAs was reported to be about 15,000 per year in the United States (US) and 6000–8000 per year in the United Kingdom and Ireland 2.

• In view of this relatively high prevalence and the high morbidity and mortality associated with AAA rupture, many countries have introduced screening programs, using ultrasonography (US), of asymptomatic men in the target older male age group.

• However, the prevalence of AAA appears to have fallen considerably in most countries. For example, only 0.74% of men aged 65 years had an AAA in the UK screening program 4.

Aetiology/Risk factors

• Usually, no specific cause of an AAA is found.

• There is a very strong association with smoking (past or current), male sex, and age. Other associations include: hypertension; positive family history, with increased prevalence among first-degree relatives of patients.

• While patients frequently have evidence of atherosclerosis affecting the aorta or other arteries, a causal association with atherosclerosis is not clear cut. For example, diabetes appears to be a slowing factor in AAA growth, as do some medications, perhaps especially metformin 5.

• More specific causes of AAAs do occur - infections (acute or chronic); connective tissue and syndromic disorders (eg. Marfan and Ehlers-Danlos syndrome). These specific causes will not be considered here although the same imaging principles generally apply.

Clinical features

• Most AAAs are asymptomatic and are detected in screening programs or incidentally when the patient is clinically examined or imaged for unrelated reasons.

• Aneurysms producing symptoms, such as abdominal pain or back pain, or that are tender on palpation, are at an increased risk of rupture and should be assessed urgently.

• About 5% of AAAs are classified as “inflammatory”. These present with abdominal or back pain, tenderness to palpation and raised blood inflammatory markers.

• The complication of rupture of an AAA can be the first presentation or occur in a patient with a known AAA. It has a high morbidity and mortality and is a clinical emergency.

• Rarely an AAA may result in thrombosis and/or distal embolism.

• Most AAAs are asymptomatic and are detected in screening programs or incidentally when the patient is clinically examined or imaged for unrelated reasons.

• In view of the high morbidity and mortality of AAA rupture, some countries have introduced screening programs (with ultrasonography) of asymptomatic men in the target older age group (typically > 65 y.o.), as well as other high-risk groups.

• In most countries, due to a lower prevalence of AAAs, population screening of women has not been deemed cost-effective.

• However, there is evidence of a reduction of prevalence of AAAs in the last couple of decades, in part due to reduced smoking and modern medication that reduces risk factors. This has resulted in a reduction of the detection rate in screening programs targeting > 65 y.o. men 6.

• This, in turn, has led to the most recent European guidelines reconsidering the population targeted for screening, from all males > 65 y.o. to only those in “high-risk” groups6 .

• All authorities recommend that population screening uses ultrasound (US).

There are several international expert consensus guidelines:

• The Society for Vascular Surgery, 2018 7

• US Preventive Services Task Force recommendations, 2019 8, 9

• NICE (UK), 2020 10 / Public Health England 11

• European Society for Vascular Surgery (ESVS), 2024 6

• The European Society of Cardiology (ESC), 2015 12

• Japanese Circulation Society (JCS), 2013 13

The various recommendations are detailed in the Table Recommendations vary below, but there are commonalities:

• Target populations:

• Males, age > 65, especially smokers

• High risk groups: first-degree relatives of patients with AAAs; smokers; patients with peripheral aneurysms

• Females: evidence for screening is weaker than for men with no consensus agreement. Screening is either not recommended or only for high-risk women (eg. those with a positive family history)

• Surveillance/Intervention:

• Intervention when the AAA diameter is > 5.5cm; there is evidence for a lower threshold (5cm) for intervention in women.

• Surveillance when the AAA diameter is 3-5.4cm; there is some variation but in general the recommended surveillance interval is:

• 3-yearly for AAAs 3-3.9cm

• yearly for AAAs 4-4.9cm

• 3-6 monthly for AAAs 5-5.4cm

• Two guidelines recommend repeat screening in 5 or 10 years for “sub-aneurysmal” aortas (2.5-2.9cm)

US is the imaging modality of choice for screening of asymptomatic individuals for AAA. Screening for AAAs. It is relatively cheap, widely available, safe and has very high sensitivity and specificity (approaching 100%) 14.

Some of the technical considerations of US performance as reviewed by Fadel et al. 15 and Ristow et al.16 include:

• It is widely accepted that the diameter of the aorta should be measured in the AP plane perpendicular to the long axis of the aorta. Axial resolution is superior to lateral resolution in US and the AP diameter is more reproducible and correlates better with CT-derived measurement.

• The transverse measurement may be over-estimated when there is aortic tortuosity.

• However, some aneurysms have larger transverse than AP diameters and transverse diameters should be documented in these cases 15.

• It should be noted that measurement during diastole may be 2mm lower than during systole. For this reason, some authorities advocate ECG gating and off-line caliper placement for measurement to reduce variability 6.

• There are three methods of caliper placement for measurement: 

• Outer wall to outer wall (OTO),

• Inner wall to inner wall (ITI), and 

• Leading edge to leading edge (LELE)

• The OTO and ITI are the most commonly used. ITI wall measurements are about 3-6 mm smaller than OTO wall measurements, and LELE measurements are intermediate 17.

• A recent systematic review and meta-analysis reported that OTO and ITI wall measurements had better inter-observer reproducibility but this was not significantly different 18 and probably of little clinical implication 6. However, other publications have shown considerable variability, for example 17, and have shown LELE measurements to be the most reproducible.

• The small increase in measured diameter using the OTO wall measurement method, compared to other methods, may be of significance when the borderline between normal and abnormal aortic diameters is considered, as well as the threshold for treatment i.e. causing the threshold for intervention to be reached earlier. One study 17 that used LELE as the standard reference reported that the change in prevalence of AAA in a screening program would be from -22% (ITI) to +36% (OTO), dependent on which US measurement was used. This may be of significance on the thresholds for monitoring and for intervention. 

• Thus, while the use of OTO wall measurement would seem to precipitate earlier intervention, ITI wall measurement provides the most relevant measure of threshold for repair with fewer unnecessary operations on small AAAs, and has been proven to be safe in the UK screening program. However, for patients with sub-aneurysmal aortas on ITI wall measurement, strict follow-up is required to monitor any increase above the sub-aneurysm/aneurysm threshold 19.

Unfortunately, there is no single universally accepted method for aortic measurement, for example: 

• The OTO wall measurement method is recommended by The American Institute of Ultrasound in Medicine and the European Society of Cardiology 15.

• The UK screening program uses the maximum AP ITI diameter 10.

• The ACR Appropriateness Criteria 20 indicate that measurements should include the LELE AP diameter (and add that measurement should be taken from the proximal, mid, and distal infrarenal aorta and of the common iliac arteries).

• The LELE method is used in the Swedish National Screening program 17.

Therefore, it is important to use one method consistently within each clinical/screening program and to recognise the specific impact of that method on the epidemiology and decision making and to ensure reproducibility.

The acceptable standard for measurement repeatability is that the limits of agreement should be <5 mm (meaning that the difference between measurements is < 5 mm for 95% of measurements) 6, 21.

Consistency of methodology and reporting standards within the program should be ensured by reporting:

• The plane of measurement

• The position of calipers ie. OTO, ITI or LLE

• Whether measurements are taken during systole or diastole

• Morphology and extent of an aneurysm

• Multiple measurements from different levels of the visualised aorta and common iliac arteries

While it is necessary to try to delineate the full extent of the aneurysm and involvement of the common iliac arteries at US examination, this may not always be possible. It should be stressed that if a AAA is detected, CT/CT angiography is indicated prior to any intervention to fully delineate the extent of the AAA, involvement of visceral and iliac branches and potential access vessels, and to visualise the whole length of the aorta including the thoracic aorta to exclude contiguous or separate thoracic aneurysms.

3-D US, because it is largely independent of transducer angulation, may allow greater comparability of measurements and thus has shown promise for improved accuracy and reproducibility in aortic diameter measurements by allowing for measurements to be made in the plane orthogonal to the centre-line of the abdominal aorta 22, 23.

Contrast-enhanced US (CEUS) is indicated predominantly in the investigation and monitoring of endoleaks following AAA repair 24. However, CEUS may also show vascularisation of intraluminal thrombus within some AAAs 25. There is a need for further research to find out whether the degree of vascularisation of the thrombus may have a significant impact on the risk of rupture of aneurysms.

It should be stressed that for pre-intervention planning for AAA treatment via either endovascular or open surgical repair, US is insufficiently precise and does not provide imaging information on access vessels and abdominal aortic branches 22. Therefore, CT angiography is required for planning intervention. Of note, CTA may give a larger aneurysm diameter than US (usually a difference of 1-3mm), most likely due to the technical issues involved in US measurement 6 as described above.

The recommendations of the various international guidelines, regarding screening populations and management of findings, are tabulated below (data from 26, 6-8, 10-12 :

Table notes:-

M=male; F=female; NR=not recommended

*NICE 10 risk factors:

• Chronic Obstructive Pulmonary Disease

• Coronary, cerebral or peripheral vascular disease

• Family history of AAA

• Hyperlipidaemia

• Hypertension

• Current or past smoking history

# ESVS (2024) 6 risk factors:

What can be considered a high-risk group varies based on local conditions, such as disease prevalence, life expectancy, and healthcare structure.

ESVS guidelines recommend screening “high-risk” populations and provide the following as risk factors for AAA. (Number of plus signs indicates the suitability for screening) 6 :

• Male aged > 65years (+) but particularly a former or current smoker (++)

• First degree relative with AAA (+++)

• Peripheral aneurysms (+++)

• Organ transplanted (++)

** SVS: or older than 75 years with tobacco history and not previously screened

##Data from the RESCAN study 27 indicate that the rupture rate of a 4.2cm AAA in a woman was about the same as that of a 5.5cm AAA in a man. Although a 4.5cm AAA may be an appropriate threshold for intervention in a woman, the operative mortality is higher in women. Good evidence is lacking, but the ESVS guidelines 6 suggest it is prudent to use a measurement of about 5cm in women to proceed to consideration for repair.

Growth rates and surveillance intervals

The RESCAN study 27, 28 assessed data collected from 18 different studies from Europe, Canada, the USA, and Australia. More than 15,000 individuals with a small AAA and a mean of 4 years of follow up were included. From the data, the authors estimated the mean growth rate of AAAs to be 2.2 mm per year, increasing by 0.4 mm/year in smokers. Of interest, the growth rate was 0.5 mm/year in diabetics. The risk of rupture was four times higher in women than men.

The RESCAN collaborators suggested 2-3 year surveillance intervals for AAAs measuring 30-39 mm, yearly intervals for 40-49 mm, and six-monthly surveillance for patients with AAAs between 50-54 mm.

The RESCAN study was published in 2012/13. A recent systemic review and meta-analysis 29 was carried out to examine contemporary growth rates in view of recent epidemiological changes (especially reduction in smoking rates). The authors found that the RESCAN recommendations were still valid.

The factors that increase the likelihood of rupture are as follows:

• Aneurysm diameter. This is the strongest predictor of rupture. 

• The risk increases markedly for aneurysms >5.5 cm in diameter. 

• All published international guidelines recommend consideration of intervention for fusiform aneurysms > 5.5 cm.

• The risk of rupture rises exponentially with the aneurysm’s maximal diameter and is higher in women than in men at similar diameters; women present with ruptured AAAs on average 10 mm smaller than men 12.

• A lower threshold (> 5 cm) for intervention is recommended for women by three of the international guidelines 6, 7, 13. Data from the RESCAN study 27 indicates that the rupture rate of a 4.2 cm AAA in a woman was about the same as that of a 5.5 cm AAA in a man. Although a 4.5 cm AAA may be an appropriate threshold for intervention in a woman, operative mortality is higher in women than in men. Good evidence is lacking, but the ESVS 6 guidelines suggest it is prudent to use a measurement of about 5 cm in women to proceed to repair. 6

• The 5-year cumulative rupture rate for incidentally diagnosed AAAs >5 cm is 25-40%; this compares with 1-7% for AAAs of 4-5 cm diameter.

• Estimates of annual rupture risk according to AAA diameter are as follows 30:

• Less than 4.0 cm: 0%

• 4.0 cm to 4.9 cm: 0.5% to 5%

• 5.0 cm to 5.9 cm: 3% to 15% 

• 6.0 cm to 6.9 cm: 10% to 20% 

• 7.0 cm to 7.9 cm: 20% to 40% 

• 8.0 cm or greater: 30% to 50%

• Expansion rate.

• This may also be important in rupture rate. 

• Growth tends to be greater in smokers and less rapid in patients with diabetes, especially diabetic patients on metformin 5, and peripheral vascular disease.

• Mechanical wall stress is likely to influence expansion rate but further investigation of the factors involved is required (see below) 26.

• The larger the AAA, the higher the growth rate 12.

• The NICE 10 guidelines recommend intervention in asymptomatic AAAs larger than 4.0 cm that have grown by more than 1 cm in 1 year 10.

• A recent study 31 reported the development of a statistical model (based on observed data sets of consecutive CTA measurements) to describe the natural growth rate of maximum AAA diameter. Further research is required. 

• Other aneurysm features. There is some evidence that anatomical features, in addition to diameter, may influence the risk of rupture.

• One published study has associated the degree of tortuosity of the aorta with aortic aneurysm diameter (and involvement of the iliac arteries)32. However, the relationship is not clear cut 33 and seems to also be dependent on cross-sectional diameter asymmetry. This remains to be resolved 26, 34.

• Mechanical wall stress is also a predictor of AAA rupture and may be dependent on several factors including local pressure, aortic wall stiffness, as well as the anatomy of the aorta 32 35.Peak wall rupture index (the ratio between aortic wall stress and strength estimated assuming a constant wall-thickness) has been shown to predict the risk of AAA events and significantly improve risk stratification compared to aortic diameter alone 36.

• Unfortunately, these factors are not easily measured and the relationship of mechanical wall stress to risk of rupture is complex.

• The presence of intraluminal thrombus (despite decreasing mechanical wall stress 37) appears to predict an increased rate of AAA growth and early rupture 37, 38, 39.

• A recent study compared ruptured and unruptured large (>8 cm) AAAs. Large intact AAAs had lower total aneurysm volumes and shorter left common iliac arteries, and higher intraluminal thrombus/aneurysm volume rates, compared to the ruptured AAAs 40.

Threshold for intervention

• The risk of rupture increases markedly for aneurysms >5.5 cm in diameter.

• The 5-year cumulative rupture rate for incidentally diagnosed AAAs >5 cm is 25-40%; this compares with 1-7% for AAAs of 4-5 cm diameter.

• All published international guidelines recommend consideration of intervention for fusiform aneurysms >5.5 cm.

• A lower threshold (>5 cm) for intervention is recommended for women by three of the international guidelines 6, 7, 13. Data from the RESCAN study indicates that the rupture rate of a 4.2 cm AAA in a female was about the same as that of a 5.5 cm AAA in a male. Although a 4.5 cm AAA may be an appropriate threshold for intervention in a woman, operative mortality is higher in women. Good evidence is lacking, but the ESVS guidelines suggest it is prudent to use a measurement of about 5 cm in women to proceed to repair 6.

• The NICE guidelines recommend intervention in asymptomatic AAAs larger than 4.0 cm that have grown by more than 1 cm in 1 year 10.

Management of AAAs in individual patients should be discussed in the context of multidisciplinary meetings of expert specialists, including interventional radiologists, vascular surgeons and physicians.

The NICE guidelines 10 indicate that, in the decision to proceed to repair of an unruptured AAA in an individual patient, apart from aneurysm size and morphology, the balance of risks versus benefits must be considered in discussion with the patient. Relevant factors include the patient’s age, life expectancy, co-morbidities, inherent risks of the procedure, the uncertainties surrounding the natural history of the condition, and the need for ongoing post-procedural surveillance, etc., as well as patient preference.

1. Elective Repair (EVAR)

The following pertains predominantly to the commoner asymptomatic infrarenal aortic aneurysms:

• Several RCTs have favoured EVAR in the management of unruptured AAAs 6, 26. A 2014 Cochrane review 41 found that the operative or in-hospital 30-day mortality rate with EVAR was lower than OSR in patients fit for surgery. A more recent meta-analysis 42 of the available RCTs confirmed early mortality to be lower in the EVAR group. 

• However, the advantage of EVAR was lost over the long term due to overall mortality being similar in both groups 42. The short-term survival advantage of EVAR over OSR does not translate into long-term survival benefit 6, 41, 43. There may be long-term survival benefit of OSR over EVAR 44.

• A meta-analysis published in 2023 45 confirmed that in “younger” patients (defined as 60-70 years old) EVAR has a lower peri-operative mortality and morbidity and a shorter hospital and intensive care unit stay than OSR, but the overall and aneurysm-related mortality in the short- to medium- term are not significantly different between EVAR and OSR.

• With regard to aneurysm-related mortality, there was no difference in mortality up to 3 years post-procedure, but after this there were more aneurysm related-deaths in the EVAR group 42.

• There is also a greater re-intervention rate for EVAR, and an increased risk of aneurysm rupture compared to OSR 43, 44, 46, 47. However, most are catheter-based interventions.

•  In high-risk patients, even though the aneurysm-related mortality of EVAR is lower compared with no intervention, EVAR may confer no overall survival benefit 48.

• The lower early survival benefit may favour EVAR over OSR in elderly, frail patients. However, in patients with limited life expectancy, elective AAA repair is not recommended 6.

• A 2019 publication comparing the NICE and the ESVS 2019 guidelines reported these guidelines differ in their recommendations with regard to the preferred mode of repair of unruptured AAAs, explained, at least in part, by their differing perspectives, methodologies, and quality assurance 49.

• The NICE guidelines (published 2020) 10 favour OSR unless there are contraindications. These recommendations are based on earlier RCTs and UK-specific economic modelling, and take into account the higher re-intervention rate after EVAR, EVAR management’s higher cost, and its lack of mortality benefit. The NICE guidelines state:

• For unruptured AAAs, OSR should be considered unless contraindicated by an individual patient’s surgical risk due to abdominal co-pathology, anaesthetic risk or medical co-morbidities 

• EVAR should be considered in patients with abdominal co-pathology, such as a ‘hostile’ abdomen, horseshoe kidney, a stoma, or other considerations that contraindicate open surgery

• EVAR or conservative management should be considered in patients who have anaesthetic risks and/or medical comorbidities that would contraindicate open surgical repair

1. Emergency Repair (OSR) in ruptured AAA

• For patients with a ruptured abdominal aortic aneurysm and suitable anatomy, endovascular repair is recommended as the first-line treatment option 6, 50, 55(6, 50-55).

• This has become feasible due to the development of rapid access protocols to enable emergency CT and the use of intra-operative aortic balloon occlusion.

• In the UK IMPROVE Trial, EVAR (in nominated aortic centres) showed improved survival and quality of life compared to OSR, with equivalent re-intervention rates and reduced cost 50.

• The SVS 7 and ESVS 6 guidelines both recommend EVAR as the first-line treatment for AAA rupture, if feasible.

• The NICE guidelines state that EVAR provides more benefit than OSR for most patients, especially men >70 y.o. and women of any age, but OSR is likely to provide a better benefit-to-harm ratio in men under 70 y.o. However, the guidelines do not recommend complex EVAR with a ruptured AAA if OSR is suitable, unless part of an RCT 10, 56.

IN SUMMARY

Regarding elective repair of AA:

• For elective repair of AAAs, the published international guidelines provide mixed recommendations with regard to EVAR versus OSR. The NICE guidelines 10 favour OSR unless the patient is high-risk, ESVS 6 recommendations also vary with the patient characteristics (see below), and SVS 7 does not make a specific recommendation. 

• In clinical practice, EVAR has gained an increasing role (vs OSR).

• The published evidence indicates that EVAR provides a significant short-term survival benefit over OSR, but an increased rate of complications occurs in the long-term.

• However, it is important to note that due to the continuously evolving technology of EVAR devices, the data regarding re-intervention and rupture rates post-EVAR in published RCTs must be interpreted with caution. Newer technology may result in lower rates 6.

• Therefore, debate continues regarding the superiority of EVAR or OSR. The most recent ESVS guidelines (2024) 6 recommend that: 

• For most patients with suitable anatomy and reasonable life-expectancy, EVAR should be considered the preferred treatment for elective repair

• For most patients with long life-expectancy, OSR should be considered preferable

• Both of these recommendations have similar levels of recommendation

• EVAR is preferred to OSR in high-risk elderly and frail patients 10, 57.

• Conservative management should be considered in patients with limited life-expectancy 6.

• EVAR is preferred in patients with contra-indications to OSR 10.

• EVAR should be considered in patients with abdominal co-pathology, such as a ‘hostile’ abdomen, horseshoe kidney, a stoma, or other considerations that contraindicate open surgery 10.

• EVAR or conservative management should be considered in patients who have anaesthetic risks and/or medical comorbidities that would contraindicate open surgical repair 10.

• Whether the initial survival advantage afforded by EVAR is sufficient to justify the long-term risk of rupture, reintervention, and long-term mortality should be determined on a case-by-case basis by the multidisciplinary team overseeing the clinical care of the patient 43.

• On current evidence, in situations where EVAR and OSR are of equal access and technical feasibility, decision-making with regard to elective AAA repair needs to balance the short-term benefits of EVAR versus the long-term benefits of OSR 58.

Regarding Emergency Repair in ruptured AAA:

• Recent studies have shown the feasibility of EVAR in emergency repair of ruptured AAAs.

• Consensus of recent existing guidelines is toward EVAR versus OSR.

• The NICE guidelines 10 favour EVAR in men >70 y.o. and in all women, when ‘standard’ EVAR is required but do not recommend EVAR when ‘complex EVAR would be needed.

Saccular aneurysms are spherical, localised aneurysms involving only a portion of the aortic circumference. It is important to exclude infection 6. Non-infected saccular aneurysm management has not been fully determined. The natural history and rupture risk is largely unknown, but it should be noted that there is a possible increased rupture risk compared to the usual fusiform aneurysms, and therefore earlier treatment with a lower threshold diameter for repair than for fusiform aneurysms may be considered 6, 7, 59.

• Most patients with AAA are asymptomatic until or unless they rupture, their AAA being discovered incidentally on imaging or physical examination for unrelated causes or on screening.

• However, some patients will present complaining of symptoms such as low back pain, abdominal pain, flank pain or pulsatile groin pain.

• The nature of the pain in symptomatic AAAs is typically steady, lasting for hours to days at a time, and has a gnawing quality. Unlike musculoskeletal back pain, in the case of aneurysm pain there is no definite relation to movement. Expansion and impending rupture are heralded by the development of new or worsening severe pain, characteristically located in the back or lower abdomen, sometimes with radiation into the groin, buttocks, or legs 1, 60.

• Patients with a known or suspected AAA who develop symptoms, such as abdominal pain or back pain, or that are tender on palpation, are at an increased risk of rupture and should be managed urgently.

• In a patient who develops acute pain and is haemodynamically stable, urgent CT should be performed to determine whether rupture has occurred. If so, this constitutes a surgical emergency. If the AAA is unruptured or there is a contained rupture, urgent intervention is indicated once conditions for intervention are optimised 7.

• CT signs of a contained rupture include a large aneurysm sac, increase of aneurysm size, a thrombus and high-attenuation crescent sign, focal discontinuity in circumferential wall calcification, and the ‘draped aortic’ sign 12.

Symptomatic AAAs are thought to have a higher risk of rupture than asymptomatic AAAs 6. Therefore, in patients with symptomatic but unruptured AAAs repair should be considered regardless of the AAA diameter 10, 60. Emergency repair with its disadvantages of higher peri-operative complications must be balanced with the risk of delayed therapy. However, some brief delay allows adequate assessment of the patient and optimisation of the patient’s condition and of the operating and anaesthetic staff. Until the optimal conditions are achieved, the patient must be carefully monitored 6.

Inflammatory AAAs61constitute about 5-10% of all AAAs:

• They belong to a group of chronic peri-aortitis diseases, which are also defined as idiopathic peri-aneurysmal retroperitoneal fibrosis. They are associated with autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, and giant cell arteritis.

• Unlike degenerative AAAs, which are usually asymptomatic until they rupture, inflammatory AAAs are often symptomatic (28-100%) and cause chronic abdominal and/or back pain.

• However, the risk of rupture is probably lower than for degenerative AAAs. 

• The triad of chronic abdominal or back pain, weight loss, and moderately elevated systemic inflammatory markers, though highly suggestive of inflammatory AAAs, is only present in 15%.

• Physical examination may demonstrate a tender pulsatile mass.

• Inflammatory markers (ESR, CRP) may be elevated.

• The findings on CT/CTA are pathognomonic - enhancing peri-aortic inflamed tissue (‘mantle sign’), with a thickened wall secondary to chronic inflammation and dense peri-aneurysmal fibrosis sparing the posterior wall in most cases.

Rupture means acute bleeding outside the aortic wall into the retroperitoneum and/or into the peritoneal cavity. A rupture may be contained if the haematomais (temporarily) sealed in the retroperitoneum.

A symptomatic (but unruptured) AAA is associated with abdominal and/or back pain without breach of the abdominal wall, which is tender to palpation, or one that presents with another complication, such as embolisation 6.

• Acute abdominal pain and shock are characteristic in the case of a ruptured AAA, sometimes preceded by a less intense abdominal pain for a contained rupture 12.

• However, symptoms and physical examination are unreliable 62 with variable reported sensitivity 6. The presence of abdominal or back pain and syncope may only be about 62%, 54% and 28% sensitive, respectively, in a ruptured AAA. A pulsatile abdominal mass may only be 47% sensitive for the diagnosis of ruptured AAA 63.

• The classical triad of abdominal and/or back pain, hypotension and a pulsatile abdominal mass are present in about 50% of patients with a ruptured AAA 6.

• In those presenting without shock, misdiagnosis is frequent (myocardial infarction and ureteric colic are the most frequent misdiagnoses 64), but it is essential to distinguish a rupture from an unruptured symptomatic AAA.

• In patients with a known AAA who present with acute abdominal and/or back pain, rupture should be suspected 7.

• In view of its high morbidity and mortality, it is justified to have a low threshold for investigation for a possible ruptured AAA in ED 63.

Imaging of suspected ruptured AAA

• Emergency ultrasound (preferably at point-of-care) is highly sensitive in detecting the presence of AAA but cannot exclude or reliably diagnose a leaking/ruptured aneurysm 6, 65.

• However, the presence of a AAA in a patient with relevant symptoms, especially if they are unstable, should strongly suggest a ruptured AAA and precipitate emergency management and transfer to a specialist unit and, if feasible, a CT/CTA.

• CT has high accuracy in the diagnosis of a ruptured AAA 62, 63.

• Immediate CT/CTA of the whole aorta and iliac vessels is indicated for patients with a suspected ruptured AAA, to confirm the diagnosis and allow planning for emergency repair 62, 66.

• Although many patients with a ruptured AAA will not reach a hospital alive, those who do reach hospital are usually stable enough to undergo CTA 50, 67, 68, which is necessary if EVAR is contemplated.

• About one third of these are initially misdiagnosed 69.

• Those patients who are too unstable to undergo CTA may undergo emergency transfer to the operating theatre for on-table angiography followed by a decision for EVAR or OSR Endovascular Repair (EVAR) versus Open Surgical Repair (OSR). Alternatively, an intra-operative angiogram +/- aortic occlusion balloon may be performed 6, 70.

• Efficient and successful care of patients with suspected AAA rupture depends on the implementation of a protocol for the safe and rapid transfer of patients with suspected rupture or symptomatic unruptured AAA to a specialist centre 10 and the implementation of a structured management protocol for the in-hospital phase of care 71 and is enacted by a co-ordinated, experienced multidisciplinary hospital team. In addition, an adequate inventory of equipment, stents, etc must be maintained and available for OSR and EVAR.

EVAR vs. OSR in a ruptured AAA

• For patients with a ruptured AAA and suitable anatomy, endovascular repair (EVAR) is recommended as the first-line treatment option 6, 50-55.

• This has become feasible due to the development of rapid access protocols in specialist centres to enable emergency CT and the use of intra-operative aortic balloon occlusion, if required.

• In the UK IMPROVE Trial, EVAR (in nominated aortic centres) showed improved survival and quality of life compared to OSR, with equivalent re-intervention rates and reduced cost 50.

• The SVS 7 and ESVS 6 guidelines both recommend EVAR as the first-line treatment for AAA rupture, if feasible.

• The NICE 10 guidelines state that EVAR provides more benefit than OSR for most patients, especially men >70 y.o. and women of any age, but OSR is likely to provide a better benefit-to-harm ratio in men under 70 y.o.. However, the guidelines do not recommend complex EVAR with a ruptured AAA if OSR is suitable, unless part of a RCT 10, 56.

• US examination is the imaging modality of choice for screening of asymptomatic individuals Ultrasound examination (US) for screening for AAA.

• US can detect the presence of an AAA in nearly all patients, with sensitivity and specificity approaching 100% 22, when performed -

• in the context of screening 14, and 

• at the point-of-care in the Emergency Department in symptomatic patients including when performed by non-radiologists 72, 73.

• In a small proportion of patients, US is unsuccessful due to body habitus, overlying bowel gas, or other factors.

• For pre-intervention AAA repair planning via either endovascular repair (EVAR) or open surgical repair (OSR), US may not fully delineate the extent of the AAA, nor provide imaging information on visceral aortic branches, iliac vessels or access vessels 22. CT/CT angiography is required to achieve these aims, including visualisation of the whole aorta including the thoracic aorta to exclude contiguous or separate thoracic aneurysms.

• Measurements should include the AP diameter in the proximal, mid, and distal infrarenal aorta and of the common iliac arteries 20 and must be performed in a plane perpendicular to the aortic longitudinal axis, which will vary in the presence of aortic tortuosity 6.

• Compared to CT, US underestimates AAA diameters by an average of 1 to 3 mm.

• Intraluminal thrombus is frequently seen in larger AAAs. It is generally chronic (laminated) and stable, and not associated with the risks of thrombus propagation or embolisation seen in other arterial locations. The presence of thrombus within the lumen of a AAA may predict increased rate of growth and early rupture 37, 38.

• 3-D US has shown promise for improved accuracy and reproducibility in aortic diameter measurements by allowing for measurements to be made in the plane orthogonal to the centre-line of the abdominal aorta 22.

• The role of US in suspected AAA rupture is discussed in Suspected Rupture

CTA is of great importance in therapeutic decision-making, in planning intervention, and in determining the method of repair 6:

• Although multi-phase imaging is often performed, single arterial phase thin-slice imaging is sufficient for EVAR planning 10.

• It enables assessment of the longitudinal extent of the aneurysm and involvement of aortic visceral and iliac branches and access vessels.

• Due to superior spatial resolution and rapid image acquisition, CTA with 3-D volumetric reconstruction and vessel analysis is now the accepted gold standard for evaluation prior to EVAR.

• The use of iodinated contrast medium, thin-section acquisition, timed satisfactorily to coincide with peak intraluminal enhancement, and 3-D volumetric post-processing techniques allow the aorta and branches to be viewed in multiple planes, including obliques. Luminal diameter, cross-sectional area and aneurysm sac volume can be measured 74.

• Measurements should include the AP diameter in the proximal, mid, and distal infrarenal aorta and of the common iliac arteries 20 and must be performed in a plane perpendicular to the aortic longitudinal axis, which will vary in the presence of aortic tortuosity 6.

• Dedicated post-processing software allows the volumetric data set to be reformatted in multiple planes with 3-D rendering.

• 3-D reconstruction software reduces the impact of vessel tortuosity on diameter and length measurements and reduces inter-observer variability 74, 75.

• Software provides automatic centreline positioning for semi-automatic diameter measurement.

• Inter-observer variability of measurement of relevant dimensions required for intervention planning has not been good. This is particularly important around the treatment threshold for repair and for monitoring of AAA growth towards that threshold 6. However, post-processing software may improve discordance rates and decision-making 76, 77.

• CTA should include the entire length of the aorta and iliac vessels and include the thoracic aorta when indicated.

The ESVS guidelines 6 recommend -

• CTA for treatment planning once the anteroposterior diameter threshold for elective abdominal aortic aneurysm repair has been met on ultrasound, and for the diagnosis of rupture.

• Aortic diameter measurement with CTA is recommended using dedicated post-processing software analysis with consistent calliper placement in an orthogonal plane perpendicular to the long axis of the aorta.

CT/CTA is highly accurate in the diagnosis of AAA rupture 62, 63. The role of CT/CTA in this scenario is discussed inSuspected Rupture

MRA, with its excellent soft tissue contrast, is the procedure of choice for treatment planning when CT is contraindicated due to iodine allergy or concerns regarding ionizing radiation.

Contrast-enhanced MRA (CE-MRA) is preferred due to its high signal-to-noise ratio. In addition to being an alternative to CTA, MR offers the advantages of good soft tissue characterisation and 4-D flow imaging. CE-MRA is preferred to non-contrast protocols 16, 74 - it is less susceptible to flow and susceptibility artifacts and has a high signal-to-noise ratio for evaluating small vessels and fine structural details. CE-MRA should be added to conventional T1- and T2-weighted spin-echo sequences. CE-MRA has been found to be as effective as CTA for assessing suitability for EVAR 78.

The disadvantages of MRI include: lower spatial resolution than CT; contra-indicated when some implanted devices (eg. pacemakers) are in situ; patient claustrophobia; lack of imaging of vascular calcification; long acquisition times compared to CT, important especially when investigating patients with likely AAA rupture; suboptimal imaging of small vessels; and, the risk of Nephrogenic Systemic Fibrosis (NSF) with some gadolinium-based contrast agents.

For pre-EVAR planning, non-contrast MR with T1 weighted spin-echo and flow-based methods (such as Time-of-Flight or phase contrast) avoids the risk of NSF and will demonstrate aneurysm morphology and anatomy, but limited spatial resolution and low SNR may lead to suboptimal images for assessing small vessels and small side branches 74, 79. In addition, flow-based sequences are susceptible to flow artifacts that may overestimate the degree of stenosis or falsely demonstrate an occlusion 74, 80.

Non-contrast MR may also be used for accurate monitoring of AAA diameter, when indicated.

Intra-luminal thrombus (ILT) within AAAs can be identified and characterised by MRI 16. Although ILT is a possible factor in AAA progression and risk of rupture 37, 38, its exact role is not fully understood. However, there is evidence that components of ILT (fibrous matrix, lipid-rich necrotic core, calcium, or haemorrhage) may be characterised by T1 and T2-weighted MRI 16, 81. Fresh thrombus, haemorrhagic, or partly organised thrombus can be differentiated from old, organised thrombus. Suppression of the blood signal using black-blood imaging can be helpful in assessing thrombus burden and age by providing a sharp delineation of the surrounding vessel wall 16. Fresh thrombus has been shown to be associated with more rapid aortic growth than old thrombus 82, 83.