Abdominal Aortic Aneurysm (AAA)

OVERVIEW

• Abdominal Aortic Aneurysm (AAA) is a permanent localised or diffuse dilatation of the abdominal aorta to 1.5 times its normal diameter that involving all three layers of the vessel wall
• normal infrarenal aortic diameters in patients >50y are 1.5 cm in women and 1.7 cm in men
• an infrarenal aorta 3 cm in diameter or more is considered aneurysmal
• ectasia is when aortic diameter is <50% increased

Emergency presentations of AAA include:

• incidental finding, often of a newly discovered asymptomatic AAA
• symptomatic or painful aneurysm due to local inflammatory state or acute dissection (true acute expansion of the aneurysm rarely if ever presents acutely otherwise)
• contained retroperitoneal or compensated rupture (“contained leak”)
• uncontained, decompensated rupture with hypovolaemic shock (“free rupture”)
• distal embolization or local thrombosis
• other atypical presentations

Prevalence and natural history

• AAA is relatively common, primarily affects the elderly and is often fatal
• rare <50 years of age
• 10% rate in men aged 65-79 years
• The pathogenesis of AAA is distinct from that of dissection
• 4600 hospital separations for non-ruptured and ruptured AAA each year between 1998 and 2008. 
• Approximately 1000 deaths annually are attributed to aortic aneurysms.
• The natural history is ongoing expansion, with increased risk of rupture as the aneurysm enlarges
• Aneurysms <5.5 cm expand at an average rate of 2–3 mm each year, with larger aneurysms expanding more rapidly. 
• Aneurysms >5.5 cm diameter in men, and >5.0 cm in women, are at significant risk of rupture and should be considered for repair unless major contraindications exist

12 month AAA rupture risk by diameter
AAA diameter (cm)	Rupture risk (%/year)
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%

Screening and surveillance

• Currently no formal AAA screening guidelines or programs exist in Australia
• Screening with ultrasound in men over the age of 65 years has been demonstrated to reduce aneurysm related mortality in four large trials, including one performed in Western Australia.
• First degree family members of patients with a diagnosed AAA are at higher lifetime risk of developing a AAA of up to 20%,

Suggested surveillance intervals:

Suggested AAA surveillance intervals
AAA diameter (cm)	Surveillance interval (months)
3.0–3.9	24
4.0–4.5	12
4.6–5.0	6
>5.0	3

When to refer

• At initial diagnosis, patients may be referred for general advice and counselling, and in many cases specialist vascular surgeons and units will be happy to coordinate regular surveillance imaging and review. 
• Otherwise, patients should be referred for review when the aneurysm approaches the size threshold for surgery. 
• Patients with a known AAA presenting with abdominal or back pain, syncope or tenderness over the aneurysm, require urgent assessment by a vascular surgeon for potential surgery.
• The benefit of elective aneurysm repair is clear in low and average risk patients with large aneurysms (men >5.5 cm; women >5.0 cm). 
• The optimal management of small AAAs (4.0–5.5 cm) has been clarified by a number of large randomised trials which demonstrate no long term survival benefit with open or endovascular repair.
• Even with low perioperative mortality (<1%) there is no survival benefit with elective repair, due to the very low incidence of rupture in small AAAs (<1%).
• While decision making in AAA management can be complex due to patient age, comorbidities and aneurysm anatomy, it remains a balance between risk of rupture, operative mortality and life expectancy. 
• Common indications for repair of AAAs are
  • Male with AAA >5.5 cm
  • Female with AAA >5.0 cm
  • Rapid growth >1.0 cm/year
  • Symptomatic AAA (abdominal/back pain/tenderness, distal embolisation)

CAUSES

• Strongly associated risk factors:
  • Male gender
  • Age
  • Smoking
  • Hypertension
  • Genetic/familial disposition(e.g. inherited connective tissue disorders such as Marfans and Ehlers-Danlos)
• Identified and postulated causes include:
  • atherosclerotic injury and associated degeneration
  • primary connective tissue degeneration
  • inflammatory arteritis with associated aneurysmal degeneration
  • traumatic injury and pseudoaneurysm
  • mycotic injury and pseudoaneurysm
  • aortic dissection-related aneurysmal degeneration

Anatomy and types of AAA

• True aneurysms of the Abdominal aorta involve all 3 layers of the vessel wall, and are defined as being >3cm in diameter or having increase >50% from baseline
• False aneurysms (or pseudoaneurysms) are deficient in at least one layer
• Fusiform aneurysms have a “bulbous” shape and are usually true aneurysms
• Saccular aneurysms have a lateral out-pouching like a Berry aneurysm and are often false aneurysms
• Most abdominal aortic aneurysms occur in the infra-renal segment (90-95%)
• AAAs to extension above the renal arteries is rare but extension into the iliac arteries is common

Acute events

• Pain:  acute expansion is rarely if ever a cause of pain; pain is typically associated with inflammation or dissection
• rupture
  • contained: leak contained within the retroperitoneal space, may be relatively haemodynamically stable
  • uncontained: free rupture into intra-peritoneal cavity resulting in shock
• thrombosis or distal embolisation

Complications

• death
• major haemorrhage
• aortic branch involvement resulting in ischaemia (e.g. renal, spinal, pancreatitis)
• distal emoblisation (e.g. trash foot)
• rhabdomyolysis

CLINICAL ASSESSMENT

• asymptomatic AAA may be an incidental finding on clinical examination or imaging
• palpation of an expansile mass
  • pass two fingers on either side of the pulsating mass and observe for separation of the fingers with each pulsation
• may be detectable at 4-5cm diameter depending on body habitus
• isolated pain in abdomen, epigstrium or back (contained leaks typically present with back pain)
• ‘classic’ triad of AAA rupture (often not present e.g. guarding, obese, hypotensive or large retroperitoneal haemorrhage)
  • Pain, typically severe and predominantly located in the back
  • Signs if circulatory compromise, often the patient is frankly shocked
  • The presence of a pulsatile mass in the abdomen
• atypical presentations in the elderly
  • back pain (may mimic renal colic)
  • +/- radiation to the legs (mimicking sciatica)
  • chronic severe back pain (contained rupture)
• tender AAA on palpation is an aortic emergency unless proven otherwise
• massive GI haemorrhage raises suspicion for an aorto-enteric fistula (usually in the context of a previous AAA graft that has eroded into the GI tract)

INVESTIGATIONS

• Investigations should be tailored to clinical presentation
• ultrasound
  • best investigation in an unstable patient as can be performed rapidly at the beside
  • can detect aneurysm and free fluid (cannot confirm fluid is due to rupture) if present
  • AAA diameter is measured as the maximum AP diameter in the transverse plane
  • imperfect sensitivity (~95%) and about 100% specific
• CT abdomen with contrast
  • best imaging modality if diagnosis is uncertain
  • shows size and extent of AAA, demonstrates AAA leak and complications
  • imaging in unstable patients is controversial – stability is relative – patients with uncertain diagnoses and borderline stability are more likely to benefit from obtaining a definitive diagnosis with ongoing resuscitation in the CT scan room before undergoing emergency surgery
• MRI
  • highly specific and sensitive
  • only suitable in elective settings

• plain AXR not recommended but may show:
  • mural calcification
  • lateral may be better as right border of AAA often obscured by vertebral column

MANAGEMENT

• Repair options
• Open repair
  • only option for a ruptured AAA
  • involves an abdominal or flank incision, vessels above and below the aneurysm are controlled, and the aneurysm sac is opened with placement of a synthetic graft
  • similar mortality to endovascular repair in the long term (8 to 10 years) for elective repairs
  • ~20% of patients require a subsequent operation
• Endovascular stent grafting
  • less invasive and has lower perioperative morbidity and mortality
  • involves the intraluminal introduction of a covered stent through the femoral and iliac arteries; can be performed under local anaesthesia
  • only possible in ~50% of unruptured AAAs
  • require long-term surveillance owing to a small risk of aneurysm sac reperfusion and late rupture (aka “endoleaks”)
  • 20-30% require a secondary intervention during the next 6 years

PROGNOSIS

• Asymptomatic AAA
  • risk of rupture increases with size, however a AAA of any size can rupture
  • > 6 cm diameter AAA has 5 year survival rate of ~20% and ~50% rupture <1 year of diagnosis
  • mortality rate is decreased by elective repair (~5%) before rupture occurs
• Ruptured AAA
  • 100% fatality unless repaired
  • Of those that survive to hospital with a ruptured AAA, mortality for surgical repair is still high, (up to 50%)
  • Mortality correlates with age, co-morbidities and hypotension