Abdominal Aortic Aneurysm

Introduction

Abdominal aortic aneurysm (AAA) may present to the Emergency Department in several ways:

1. Incidentally discovered
2. Acute expansion
3. Contained leak
4. Shock due to rupture
   • Ruptured AAA carries a high mortality even with intervention
   • Retroperitoneal rupture may offer a transient stabilization window
5. Atypical presentation

Emergency Departments should implement a CODE AAA activation protocol for suspected rupture.

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Terminology

• Aneurysm: Permanent localized or diffuse dilation >1.5 times normal diameter involving all three vessel wall layers.
• Ectasia: Dilation < 50%
• Fusiform: Circumferential dilation, entire circumference of the vessel wall is dilated
• Saccular: Focal dilation, only part of the circumference is involved
• Pseudoaneurysm: Localized collection of flowing blood contained by adventitia only

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Epidemiology

• Rare before age 55
• Incidence rises with age:
  • ~4–7% in men over 65
  • ~1–2% in women over 65

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Pathophysiology

Causes

1. Degenerative changes (collagen/elastin)
2. Atherosclerosis
3. Infective (e.g., mycotic, syphilitic)
4. Trauma
5. Post-dissection
6. Inflammatory (e.g., PAN)
7. Genetic (e.g., Marfan’s, Ehlers-Danlos)
   • There is an approximately 15 % incidence of aneurysms among first degree relatives of patients with aortic aneurysms

Risk Factors

1. Age > 65
2. Male sex
3. Smoking
4. Family history
5. Atherosclerosis
6. Hypertension
7. Hyperlipidemia
8. History of other aneurysms

Risk Factors for Rupture

1. Diameter > 5.5 cm
2. Rapid growth (>0.5 cm in 6 months)
3. Smoking
4. Uncontrolled hypertension

Rupture Risk by Diameter

AAA Diameter (cm)	Annual Risk of Rupture
3.0 – 3.9	0%
4.0 – 4.9	~1%
5.0 – 5.9	1–10%
6.0 – 6.9	10–22%
> 7.0	30–50%

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Location

• 90–95% are infra-renal
• May extend into iliac arteries
• Thoraco-abdominal aneurysms may involve visceral/renal arteries

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Clinical Features

Typical Symptom Triggers

1. Rapid expansion
2. Compression of structures
3. Inflammation
4. Rupture

Clinical Presentations

1. Incidentally Detected:

• Detected via exam, ultrasound, or imaging
• In thin patients, arterial pulsation from the abdominal aorta may be detected in the epigastrium normally. This will be felt as a pulsation in the AP direction
• To help determine whether this actually represents an aneurysm (as opposed to normal arterial pulsations), it must be determined whether the pulsation is “expansile” (ie enlarges appreciably with systole).
• This is best done by aligning two fingers on either side of the pulsating mass and looking for appreciable separation of the fingers with each pulsation
• Generally AAA become clinically detectable at approximately 4 – 5 cm, however this will also depend largely on body habitus and the experience of the examiner

2. Acute Expansion:

• Sudden back/epigastric pain without rupture or leak

3. Contained Leak:

• Hemodynamically stable or unstable
• Back pain predominates

4. Frank Rupture:

• ‘Classic’ triad: Pain (acute, severe, located in back)
• Sudden severe back pain
• Hypovolemic shock
• Pulsatile abdominal mass (may be difficult to detect)

5. Atypical Presentations:

• Elderly with back pain (renal colic unusual; not always “arthritis”)
• Elderly with acute severe back pain with radiation to the legs (not always sciatica…)
• Chronic severe back pain (potential chronic contained rupture)
• Tenderness around aneurysm in known AAA
• GI bleed in post-graft AAA (consider aorto-enteric fistula)

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Investigations

Blood Tests

1. FBE
2. U&Es, Glucose
3. LFTs
4. Coagulation profile
5. VBGs/Lactate
6. Group and hold/cross match
   • Activate massive transfusion protocol if rupture suspected

Imaging

• Plain X-ray: May show mural calcification, not definitive
  • Mural calcification of the aneurysm wall may be seen on AXR.
  • On the AP the left margin of the aneurysm is usually distinguished. The right border is generally obscured by the vertebral column.
• ECG: To rule out cardiac causes; should not delay management
• Ultrasound: Best for unstable patients; detects aneurysm and free fluid
  • Excellent first up investigation that can be done in the ED, to confirm the presence of an AAA
• CT Angiogram: Gold standard for stable patients
  • Must include full thoracic inlet to iliac (and down to mid-thigh) aortic scan
  • Do not delay for renal function concerns in suspected rupture
  • Reliably shows the size and extent of an aneurysm and is able to demonstrate the leakage of blood from it

**In the case of possible rupture CT scan should not be delayed by concerns about a low eGFR – the risk of certain death outweighs any theoretical risk of nephrotoxicity. 

The investigation may be problematic in the unstable patient however if the clinical picture is clear enough, and backed up by ultrasound findings, then CT scan may not be necessary in the unstable patient as the vascular surgeon may have enough information to operate. Delays in obtaining an unnecessary CT scan to confirm the diagnosis in these situations can be detrimental to the patient.

If the patient is relatively unstable and the diagnosis is uncertain, on both clinical and ultrasound examination, then a diagnosis still needs to be established and the patient must have the CT scan to confirm it, regardless of “stability”, (unless an individual surgeon is prepared to do a laparotomy without diagnosis) – each case has to be assessed on its merits, rather than work to a formula.  

Preliminary emergent resuscitation must be undertaken to a point at least that ensures cerebral perfusion, rather than any set haemodynamic parameters.

All resuscitation equipment, monitoring equipment and appropriately trained staff should accompany the patient to the scanner, in anticipation of possible further deterioration.

• MRI: Reserved for elective cases; not suitable in emergencies

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Management

Incidental Findings

Aortic Diameter	Action
< 2.5 cm	None
2.5 – 2.9 cm	Repeat US in 5 years
3.0 – 3.9 cm	Repeat US in 12 months
4.0 – 4.4 cm	Repeat US in 6 months
4.5 – 4.9 cm	Repeat US in 3 months
≥ 5.0 cm	Urgent referral to vascular surgery

Criteria for Surgical Referral

1. AAA > 5.5 cm (men) / > 5.0 cm (women)
2. Rapid expansion > 1 cm/year
3. Symptomatic aneurysm
4. Patient fitness and preference

Symptomatic or Expanding AAA

• Emergency referral to vascular surgery

Contained Leak

• Urgent repair required

Shocked patient with frank AAA rupture

1. Resuscitation:

• Large bore IVs,
• Urgent blood cross match; activate local “massive transfusion” protocol
• Permissive hypotension: maintain consciousness, not BP targets

2. Monitoring:

• ECG continuous monitoring, pulse oximetry and non-invasive blood pressure monitoring

3. Analgesia:

• IV fentanyl preferred for stability

4. Surgical Repair Options:

Endovascular Repair (EVAR):

• Less invasive
• Preferred for unruptured cases and selected ruptured cases
• Requires lifelong surveillance (risk of endoleak)

Open Repair:

• Required when EVAR is not suitable
• Higher mortality in rupture cases (30–40%)

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Disposition

• All AAA cases should be referred to vascular surgery
• Activate CODE AAA for suspected rupture:
  • Vascular surgery
  • Radiology
  • ICU
  • Anaesthetics/OR
  • Blood bank

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References

Publications

• Robinson D, Mees B, Verhagen H, Chuen J. Aortic aneurysms – screening, surveillance and referral. Aust Fam Physician. 2013 Jun;42(6):364-9.

FOAMed

• Nickson C. Abdominal aortic aneurysm (AAA). LITFL
• Hartung MP. Abdominal CT: aortic aneurysm. LITFL
• Lobo V. POCUS: Abdominal aorta. LITFL
• Lam L. CT Case 084. LITFL
• Nazerian P. Aortic Dissection Detection Risk Score (ADD-RS). MDCalc