Abdominal aortic aneurysm (AAA)

Reviewed and revised 5 September 2015, including amendments suggested by Dr Jason Chuen (@OzVascDoc)

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 embolisation or local thrombosis
• other atypical presentations

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

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

PATHOPHYSIOLOGY

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

Pathogenesis involves:

• infiltration of the vessel wall by lymphocytes and macrophages
• destruction of elastin and collagen in the media and adventitia by proteases, including matrix metalloproteinases
• loss of smooth-muscle cells with thinning of the media
• neovascularization

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

• see management

Bedside

• ECG (do not delay resuscitation or transfer to operating theatre if ruptured AAA)
• VBG and glucose

Laboratory

• FBC
• UEC
• group and hold or cross match

Imaging

• 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

Rupture

• attend to ABCs
• insert 2 x large bore peripheral IV cannulae, attached to giving sets with hand pumps or rapid infusion systems
• target SBP ~90 mmHg to maintain end organ perfusion (analogous to the permissive hypotension approach in penetrating trauma)
• cross-match 6+ units of blood, consider activation of massive transfusion protocol
• obtain FBC, UEC, coagulation profile, VBG and ECG
• provide titrated analgesia (e.g. fentanyl or morphine)
• arrange urgent transfer to the operating theatre for definitive treatment (unless palliation is appropriate)
• refer to vascular surgery, anaesthesia and  ICU
• insert IDC and arterial line (unless this will cause a delay to OT – CVC in ED is not necessary)
• a contained rupture still requires urgent repair, less acutely than an uncontained rupture and with less need for resuscitation

Acute pain

• refer all cases to vascular surgery urgently
• acute pain is an emergency, even in a stable patient the absence of rupture
• assume that any pain in a patient with an AAA is due to an aortic emergency, until proven otherwise

Incidental asymptomatic AAA

• refer to vascular surgeon if surgical candidate, urgency depends on AAA size and presence/ absence of symptoms
  • <2cm diameter is normal
  • 2-5.5 cm diameter and asymptomatic requires follow up with regular ultrasound
  • > 5.5 cm diameter requires operative intervention regardless of symptoms in patients suitable for surgery

Decision to operate on AAA, and by which procedure, depends on:

• emergency or elective/ prophylactic
• anatomy (not all patients have anatomy amenable to endovascular repair)
• presence of symptoms
• AAA diameter and growth rate
• Operative risk (i.e. age and comorbidities)
• Patient preferences and values

Indications for repair

• Male with AAA >5.5 cm
• Female with AAA >5.0 cm
• Rapid growth >1.0 cm/year
• Symptomatic AAA (e.g. rupture, abdominal/back pain/tenderness, distal embolisation)

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

References and Links

LITFL

• CCC — Renal failure post AAA repair
• CCC — Hypotension post AAA repair
• CCC — Detecting myocardial ischaemia post AAA repair
• CCC — AAA repair hot case
• CCC — Permissive hypotension

Journal articles

• Al-Hashimi, M, Thompson J. Anaesthesia for elective open abdominal aortic aneurysm repair. Contin Educ Anaesth Crit Care Pain (2013) 13 (6): 208-212. [Free Full Text]
• Frederick JR, Woo YJ. Thoracoabdominal aortic aneurysm. Ann Cardiothorac Surg. 2012 Sep;1(3):277-85. PMC3741772.
• Gawenda M, Brunkwall J. Ruptured abdominal aortic aneurysm: the state of play. Dtsch Arztebl Int. 2012 Oct;109(43):727-32. PMC3498473.
• Kasipandian V, Pichel AC. Complex endovascular aortic aneurysm repair. Contin Educ Anaesth Crit Care Pain (2012) 12 (6): 312-316. [Free Full Text]
• Kent KC. Clinical practice. Abdominal aortic aneurysms. The New England journal of medicine. 371(22):2101-8. 2014. [pubmed]
• Leonard A, Thompson J. Anaesthesia for ruptured abdominal aortic aneurysm. Contin Educ Anaesth Crit Care Pain (2008) 8 (1): 11-15. [Free Full Text]
• Robinson D, Mees B, Verhagen H, Chuen J. Aortic aneurysms – screening, surveillance and referral. Australian family physician. 42(6):364-9. 2013. [pubmed]
• Upchurch GR Jr, Schaub TA. Abdominal aortic aneurysm. Am Fam Physician. 2006 Apr 1;73(7):1198-204. PMID: 16623206. [Free Full Text]