Abdominal Aortic Aneurysm
An abdominal aortic aneurysm (AAA) is a focal abdominal-aortic dilatation that is often silent until it becomes symptomatic or ruptures; management is determined by symptoms, diameter, growth, anatomy, operative risk and patient preference.
Definition
An abdominal aortic aneurysm is a permanent localised dilatation of the abdominal aorta to an inner-to-inner maximum anterior-posterior diameter of 3.0 cm or more on ultrasound. It is a structural disease of the aortic wall and may be asymptomatic, symptomatic, or ruptured.
Epidemiology
AAA is more common with increasing age, male sex, smoking, hypertension, chronic obstructive pulmonary disease, other atherosclerotic disease and family history. In England, the NHS AAA Screening Programme invites men in the year they turn 65; NICE also recommends considering ultrasound for women aged 70 or over with relevant risk factors when AAA has not already been excluded.
Pathophysiology
Aortic-wall degeneration and remodelling weaken the vessel, allowing progressive dilatation. Increasing diameter raises wall stress and can accelerate enlargement, while mural thrombus and atherosclerotic change may coexist. Rupture causes retroperitoneal or intraperitoneal haemorrhage; distal embolisation, aortoenteric or aortocaval fistula and pressure effects are less common complications.
First principles
An aneurysm is a structural failure, not a blockage
An AAA is a permanent localised dilatation of the abdominal aorta. NICE and the NHS AAA Screening Programme use an inner-to-inner maximum anterior-posterior diameter of 3.0 cm or more on ultrasound to define an AAA. The important clinical problem is progressive wall weakness and the possibility of rupture, not loss of arterial inflow.1,2
Size changes the balance between surveillance and repair
As diameter increases, wall stress and rupture risk generally rise. This is why small and medium asymptomatic AAAs are monitored, while symptoms, rapid growth or an ultrasound diameter of 5.5 cm or more trigger vascular assessment for possible repair. Diameter is not the only decision variable: morphology, anatomy, life expectancy, fitness, comorbidity and the person's preferences matter.1,2
Rupture is a time-critical haemorrhage
A ruptured AAA may initially be contained in the retroperitoneum, but ongoing bleeding can cause rapid cardiovascular collapse. New abdominal or back pain, collapse or loss of consciousness should prompt consideration of rupture, particularly in someone over 60, who smokes or previously smoked, has hypertension or has a known AAA. Atypical pain does not make the diagnosis safe to dismiss, and rupture is more likely in women than men.1,3
Repair must be planned around anatomy and risk
Open repair and endovascular aneurysm repair (EVAR) have different perioperative and long-term trade-offs. NICE recommends open repair for eligible unruptured AAAs meeting repair criteria, with EVAR or conservative management considered when open repair is contraindicated or other individual factors make it preferable. This is a specialist shared decision after anatomical CT angiography and assessment of operative risk.1
Presentation
Most AAAs are asymptomatic and are found on screening or imaging for another reason. Symptomatic disease causes new abdominal, back or loin pain; rupture classically causes sudden severe pain with collapse, hypotension or a pulsatile abdominal mass, but the full triad may be absent.1,4,3
Cardinal features
- Usually asymptomatic and detected by screening or incidental imaging
- Pulsatile, expansile abdominal mass may be present
- New abdominal, back or loin pain can indicate symptomatic expansion or leak
- Sudden severe pain, syncope or cardiovascular collapse suggests rupture
- Rupture is more likely in women than men at a given diameter
- Distal embolisation or acute limb ischaemia can occur but is less common
Red flags
- Sudden severe abdominal, back or loin pain with hypotension, syncope or collapse: treat as suspected ruptured AAA
- Known AAA with new pain or haemodynamic deterioration
- A tender or rapidly enlarging aneurysm on imaging
- A pulsatile abdominal mass with shock or unexplained anaemia
- New abdominal or back pain in a person over 60, who smokes or previously smoked, or has hypertension: do not rule out AAA because another diagnosis seems plausible
- Acute limb ischaemia or distal embolisation in a person with aortic disease
Investigations
Immediate bedside aortic ultrasound for suspected symptomatic or ruptured AAA
Offer immediate bedside aortic ultrasound when symptomatic or ruptured AAA is being considered. Discuss immediately with a regional vascular service if an AAA is seen, or if ultrasound is unavailable or non-diagnostic and suspicion remains.
Expected finding: A dilated abdominal aorta, with free fluid or retroperitoneal haemorrhage not required to justify immediate specialist discussion in the appropriate clinical context.
1Aortic ultrasound measurement
Use ultrasound for suspected asymptomatic AAA and surveillance. Record the inner-to-inner maximum anterior-posterior aortic diameter in accordance with the NHS AAA Screening Programme so serial measurements are comparable.
Expected finding: AAA is 3.0 cm or more; 3.0 to 4.4 cm is small, 4.5 to 5.4 cm is medium, and 5.5 cm or more requires referral to a regional vascular service.
1,2Thin-slice contrast-enhanced arterial-phase CT angiography
Offer CTA when evaluating a person for elective repair to define the aneurysm, branch vessels, iliac access and suitability for open or endovascular repair. Consider CTA in suspected rupture when the person is being evaluated for repair and is stable enough for imaging; it must not delay urgent vascular discussion or transfer.
Expected finding: Aneurysm extent and morphology, relationship to renal and iliac vessels, and signs of leak, rupture or a competing diagnosis.
1Baseline assessment and postoperative surveillance
Assess haemodynamic status, haemoglobin, renal function, coagulation and crossmatch needs in suspected rupture while resuscitation and transfer proceed. After EVAR, tailor surveillance to the perceived risk of complications; investigate suspected endoleak with contrast-enhanced CTA or contrast-enhanced ultrasound when CTA is contraindicated.
Expected finding: Anaemia, shock or acute kidney injury may indicate severity in rupture; after EVAR, sac expansion or an endoleak requires vascular review.
1Management
| Step | Detail | Source |
|---|---|---|
| Suspected rupture: resuscitate, discuss and transfer immediately | Treat suspected ruptured or symptomatic AAA as a vascular emergency. Call the regional vascular service immediately, obtain immediate bedside aortic ultrasound if available, and do not delay specialist discussion or transfer for a non-diagnostic scan or routine CT in an unstable patient. Once accepted for emergency assessment, ensure the person leaves the referring unit within 30 minutes of the decision to transfer. Consider a restrictive volume-resuscitation approach (permissive hypotension) during transfer, with local major-haemorrhage and anaesthetic protocols guiding blood products and airway management.1,5 | NICE NG156 recommendations 1.1.7 to 1.3.6 |
| Diagnose and refer according to diameter and symptoms | Use ultrasound for suspected asymptomatic AAA and for surveillance. Refer a newly diagnosed AAA of 5.5 cm or more to a regional vascular service for review within 2 weeks; refer a 3.0 to 5.4 cm AAA within 12 weeks. Symptomatic or suspected ruptured AAA follows the emergency pathway regardless of diameter. In England's screening pathway, small AAAs are rescanned annually and medium AAAs every 3 months; an aneurysm that has grown by more than 1 cm in 12 months should be referred to a vascular surgeon.1,2,4 | NICE NG156 and NHS AAA Screening Programme care pathway |
| Reduce cardiovascular and aneurysm risk during surveillance | Offer referral to a stop-smoking service and manage hypertension in line with NICE guidance. Address the person's wider cardiovascular risk, including lifestyle, diabetes and lipid modification where indicated, using current NICE cardiovascular-prevention guidance and medicines guidance rather than prescribing an AAA-specific drug regimen. Surveillance does not remove the need for urgent review if new pain or collapse develops.1,6 | NICE NG156 and NICE NG238 |
| Offer elective repair when the benefit outweighs the risk | Consider repair for an unruptured AAA if it is symptomatic, is asymptomatic and larger than 4.0 cm with growth of more than 1 cm in 1 year, or is asymptomatic and 5.5 cm or larger. Discuss repair versus conservative management using size and morphology, life expectancy, fitness, comorbidity, operative risk, anatomy, the need for future procedures and surveillance, and the person's preferences. Offer open repair unless contraindicated; consider EVAR or conservative management when open repair is unsuitable or individual factors make EVAR preferable.1 | NICE NG156 recommendations 1.5.1 to 1.5.5 |
| Follow up after EVAR and manage endoleaks through vascular services | EVAR requires imaging surveillance because graft complications and further intervention can occur. Tailor surveillance to perceived risk. If an endoleak is suspected, use contrast-enhanced CTA or contrast-enhanced ultrasound when CTA is contraindicated; do not exclude an endoleak using a negative colour-duplex ultrasound alone. Consider intervention for type 1 and type 3 endoleaks, and for type 2 endoleaks when the aneurysm sac is expanding. Follow the treating vascular team's pathway after open repair or EVAR.1 | NICE NG156 recommendations 1.7.1 to 1.8.3 |
| Safety-net all people with an AAA | Give clear urgent-return advice: sudden or worsening abdominal, back or loin pain, collapse, fainting, breathlessness, marked weakness or a cold/painful limb requires emergency help. Tell the person to mention the known AAA to ambulance and emergency staff. A new symptom is not explained safely by a previous surveillance result; it requires urgent reassessment for leak, rupture or another acute vascular diagnosis.1,3 | NICE NG156 and NHS AAA information |
Illustrations
Differentials
Renal colic
Loin-to-groin pain and urinary findings; in an older or high-risk person, exclude AAA before accepting renal colic as the explanation.
Acute aortic syndrome
Abrupt chest or back pain with dissection or intramural-haematoma features on appropriate aortic imaging.
Acute mesenteric ischaemia
Pain out of proportion to examination, often with vascular risk, atrial fibrillation, metabolic acidosis or raised lactate.
Perforated viscus or intra-abdominal sepsis
Peritonism, inflammatory features or free air/local inflammation on imaging rather than aortic pathology.
Acute pancreatitis
Severe epigastric pain radiating to the back with compatible pancreatic enzyme or imaging findings.
Musculoskeletal back pain
Mechanical or positional pain without haemodynamic compromise, a pulsatile expansile mass or vascular imaging abnormality.
Complications
- Rupture with retroperitoneal or intraperitoneal haemorrhage
- Distal embolisation or acute aortic thrombosis causing limb ischaemia
- Aortoenteric fistula, particularly after previous aortic repair
- Aortocaval fistula with high-output cardiac failure
- Ureteric obstruction or vertebral erosion from local pressure
- Endoleak, sac expansion or graft complication after EVAR
Prognosis
Small asymptomatic AAAs are usually managed with surveillance, risk reduction and specialist review rather than immediate repair. The balance changes with symptoms, rapid growth or large diameter. Rupture has a very high mortality, so early recognition, immediate vascular discussion and rapid transfer are central to management.
Guidelines
- Abdominal aortic aneurysm: diagnosis and management (NG156) (NICE, 2020)
- NHS Abdominal Aortic Aneurysm Screening Programme care pathway (NHS England, 2025)
References
- NICE NG156: Abdominal aortic aneurysm: diagnosis and management (NG156 recommendations)Published 19 Mar 2020
- NHS AAA Screening Programme: care pathway (England screening pathway updated 27 October 2025)Published 27 Oct 2025 | Updated 27 Oct 2025
- NHS: Abdominal aortic aneurysm (NHS symptoms and treatment information)
- NHS: Abdominal aortic aneurysm screening (NHS screening information and surveillance intervals)Updated 11 Nov 2024
- NICE NG156: Emergency transfer and permissive hypotension recommendations (NG156 recommendations 1.3.1 to 1.3.6)Published 19 Mar 2020
- NICE NG238: Cardiovascular disease: risk assessment and reduction, including lipid modification (NG238 recommendations)Published 14 Dec 2023
Evidence checked: 2026-08-03
This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.