Abdominal Aortic Aneurysm (AAA)
NICE guideline [NG156] Abdominal aortic aneurysm: diagnosis and management. Published Mar 2020.
NICE CKS Abdominal aortic aneurysm screening. Last revised: May 2024.
Expanded on the diagnostic pathway decision, based on haemodynamic instability to improve clarity.
Date: 19/11/2025
Background Information
Definition
AAA is defined as a permanent localised dilatation of the abdominal aorta [Ref]
- Abdominal aortic diameter ≥3.0 cm, or
- Artery diameter ≥1.5 times the normal diameter
Classification
Anatomical classification of AAA: [Ref]
| Location | Description |
|---|---|
| Infrarenal (~85% cases) | Located below the renal arteries |
| Juxtarenal | Originates near but does not involve the renal arteries |
| Pararenal | Involves the origin of renal arteries |
| Suprarenal | Located above the renal arteries, including the origins of visceral arteries |
Aetiology
Major risk factors: [Ref]
- Male (6x more common than in women)
- >65 y/o
- Smoking
Other risk factors: [Ref]
- Hypertension
- Cardiovascular disease and atherosclerosis
- Peripheral aneurysm
Clinical Features
Unruptured AAA
Symptoms:
- Mostly asymptomatic – most cases discovered incidentally
- If symptomatic – usually abdominal / flank / back pain
- Features from distal embolisation
- Acute limb ischaemia
- Blue toe syndrome
Examination findings:
- Aortic bruit
- Pulsatile + expansile abdominal mass
Symptomatic AAA usually indicates aneurysm expansion or impending rupture.
Ruptured AAA
Typical clinical presentation:
- Sudden onset severe back and/or abdominal pain (+/- radiation to flank / buttocks / legs / groin)
- Presyncope (from hypotension)
- Pulsatile abdominal mass
- Grey Turner and/or Cullen sign
Be aware that features of a ruptured AAA may mimic those of renal colic.
Guidelines
Investigation and Diagnosis
This section refers to the assessment of a symptomatic AAA (not screening for AAA, which is covered in a separate section below), and the diagnostic pathway is determined by haemodynamic stability.
Haemodynamically Stable (Unruptured AAA / Mild Rupture)
1st line test: aortic ultrasound (trans-abdominal)
- Measure the inner-to-inner maximum anterior-posterior aortic diameter
- ≥3.0 cm is diagnostic of AAA
Gold standard: CT angiography
- Not routinely performed for just diagnosis
- To be performed before AAA repair (CT angiography provides a detailed anatomical assessment for surgical planning)
- Contrast extravasation from the abdominal aorta indicates a ruptured AAA
Haemodynamically Unstable (Major Rupture with Shock)
For unstable patients, the diagnosis is primarily clinical [Ref]
- Based on hypotension, acute abdominal/back pain, and a pulsatile mass
- Further imaging (i.e. ultrasound and CT angiography) should be abandoned → transfer immediately to operating theatre for intervention (see management section for details)
In haemodynamically unstable patients with ruptured AAA, ultrasound has limited value because it can confirm the presence of an aneurysm but cannot reliably detect active rupture or retroperitoneal bleeding. Its diagnostic accuracy for rupture is poor, and performing it risks delaying life-saving surgical intervention.
CT angiography, although the gold standard for diagnosing and characterising AAA, should not be performed in unstable patients because it requires transfer to the scanner, patient repositioning, intravenous contrast administration, and time for image acquisition. All of which can critically delay definitive surgical repair in a time-dependent emergency.
Management
Unruptured AAA
If an asymptomatic AAA is detected → refer to regional vascular service
- Within 2 weeks if AAA is ≥5.5 cm
- Within 12 weeks if AAA is 3.0-5.4 cm
Choice of definitive management (surgery vs conservative care):
- Urgent repair (within 2 weeks) is indicated if:
- Symptomatic (of any size), or
- Asymptomatic +
- ≥5.5 cm, or
- >4.0 cm + growing >1cm / year
- If repair is not indicated → offer conservative care
Surgical Repair
1st line: open surgical repair
Endovascular aneurysm repair (EVAR) is preferred if:
- Hostile abdomen (e.g. active intra-abdominal infection, adhesions, extensive prior abdominal surgery)
- Anaesthetic risks
Conservative Management
The main aim is to reduce the risk of progression: [Ref]
- Smoking cessation
- Blood pressure control
- Consider statin therapy in patients with AAA and evidence of aortic atherosclerosis
- Ongoing surveillance with ultrasound (see screening section below for timing)
Ruptured AAA
Initial management: hypotensive resuscitation (permissive hypotension)
- RCEM recommends aiming for SBP 90-120 mmHg
- Rationale: aggressive fluid resuscitation to normal blood pressure can dislodge fragile clot formation at the site of rupture, leading to increased bleeding
Absolute priority: immediate surgical repair / EVAR
- Do not delay surgery to stabilise the patient or to perform CT angiography
Screening
Routine screening for AAA:
- Population: ALL ≥65 y/o men
- Screening modality: one-off transabdominal ultrasound
Subsequent action depends on ultrasound findings:
| Aortic diameter | Interpretation | Re-scanning timeframe |
|---|---|---|
| <3.0cm | No aneurysm | Discharge, no further scanning |
| 3.0 – 4.4 cm | Small AAA | 12-monthly scan |
| 4.5 – 5.4cm | Medium AAA | 3-monthly scan |
| ≥5.5cm | Large AAA | Refer to vascular surgery |
References