Abdominal aortic aneurysm

Abdominal aortic aneurysm (AAA) is a focal dilatation of the abdominal aorta to more than 1.5 times its normal diameter. AAAs are classified by location as either suprarenal or infrarenal aneurysms. Men of advanced age are at increased risk for their formation; smoking and hypertension are also major risk factors. AAAs are frequently asymptomatic and therefore detected incidentally. Symptomatic AAAs can manifest with lower back pain, a pulsatile abdominal mass, and a bruit on auscultation. Abdominal ultrasound is the best initial and confirmatory test to diagnose AAAs and determine their extent. Observation, close follow-up, and reduction of cardiovascular risk factors are indicated for small aneurysms, whereas pronounced (> 5.5 cm) or rapidly expanding aneurysms require surgery. Surgical treatment involves open resection of the aneurysm with graft placement or, increasingly, endovascular stent placement. The prognosis is markedly worse if dissection or aneurysm rupture occurs. AAA rupture typically presents with sudden onset of severe tearing back or abdominal pain, a painful pulsatile mass, and hypovolemic shock, and should be managed with emergent surgery. All men between 65 and 75 years of age with a history of smoking should be screened once with an ultrasound to exclude an AAA. See also thoracic aortic aneurysm for more information.

• Localized dilation of all three layers of the abdominal aortic wall (intima, media, and adventitia) to ≥ 3 cm ^[1]

• Peak incidence: 60–70 years (rare in patients < 50 years)
• Sex: ♂ > ♀: ∼ 2:1

Epidemiological data refers to the US, unless otherwise specified.

• Risk factors
  • Advanced age
  • Smoking (most important risk factor)
  • Atherosclerosis
  • Hypercholesterolemia and arterial hypertension
  • Positive family history
  • Trauma

• Localization
  • Infrarenal: below the renal arteries
    • Most common location ^[2]
    • One-third of aneurysms extend into the iliac arteries. ^[1]
  • Suprarenal: above the renal arteries
• Shape
  • Saccular (spherical) ^[3]
  • Fusiform (spindle-shaped)

• Inflammation and proteolytic degeneration of connective tissue proteins; (e.g., collagen and elastin and/or smooth muscle cells) in high-risk patients → loss of structural integrity of the aortic wall →; widening of the vessel → mechanical stress (e.g., high blood pressure) acts on weakened wall tissue → dilation and rupture may occur.
• The aneurysmatic dilatation of the vessel wall may cause disruption of the laminar blood flow and turbulence.
• Possible formation of thrombi in the aneurysm → peripheral thromboembolism

References:^[4][5]

Aortic aneurysms are usually asymptomatic or have nonspecific symptoms. They are often discovered incidentally on ultrasound or CT scan. Rupture or dissection of the aneurysm is a life-threatening condition (see “Ruptured AAA”).

• Lower back pain
• Pulsatile abdominal mass at or above the level of the umbilicus
• Bruit on auscultation
• Peripheral thrombosis and distal atheroembolic phenomena (e.g., blue toe syndrome and livedo reticularis)
• Decreased ankle brachial index

The diagnosis of AAA is confirmed by imaging showing aortic diameter > 3 cm. Unstable patients should be taken directly to the OR for emergency surgery if ruptured AAA is suspected (see ruptured AAA). There are no laboratory findings specific to AAA. ^[1]

Imaging should not delay treatment if AAA is suspected in hemodynamically unstable patients.

Imaging ^[6][7]

• Duplex ultrasound
  • Indications
    • Best initial and confirmatory test in:
      • Asymptomatic patients
      • Patients with abdominal pain and no known AAA or risk factors for AAA
    • To determine the presence, size, and extent of an aneurysm
    • Screening and surveillance
  • Supportive findings
    • Dilatation of the aorta ≥ 3 cm ^[1]
    • Thrombus may be present (hyperechoic)
  • Disadvantages: Abdominal ultrasound has low sensitivity for aneurysmal leaks, branch artery involvement, and suprarenal involvement, and its findings are insufficient for procedural planning. ^[8][1]

If a large (> 5.5 cm) aneurysm is seen on ultrasound in a patient presenting with abdominal pain, refer the patient for treatment immediately.

• CT angiography abdomen and pelvis with IV contrast
  • Indications
    • Imaging modality of choice in symptomatic patients and for preintervention planning
    • To help confirm the diagnosis when ultrasound is not possible in asymptomatic patients
    • More detailed evaluation of the location, size, and extent of the aneurysm, involvement of branch vessels, and presence of thrombus or rupture
  • Supportive findings
    • Dilatation of the aorta ≥ 3 cm and possibly branch vessels ^[1]
    • Thrombus may also be present (hypodense, nonenhancing)

• MR angiography abdomen and pelvis with and without IV contrast
  • Indications
    • Preintervention planning when CT angiography is not possible
    • To help confirm diagnosis when ultrasound and CT angiography are not possible in asymptomatic patients
  • Supportive findings: similar to CT angiography
• Arteriography (aortography abdomen)
  • Indications
    • To help confirm diagnosis or for preintervention planning if the patient has significant contraindications to CTA and MRA
    • More detailed assessment of the aortic lumen
  • Supportive findings: contrast column in the lumen of the aneurysm and branch vessels ^[6]
  • Disadvantage: may mask the actual diameter of the aneurysm (because a mural thrombus does not appear on arteriography)

• See differential diagnoses of acute abdomen in acute abdomen.
• Other types of aortic aneurysm (e.g., thoracic aortic aneurysm): See table below.

Abdominal vs. thoracic aortic aneurysm
Characteristics	Abdominal aortic aneurysm	Thoracic aortic aneurysm
Location	Below the renal arteries (most common)	Ascending aorta (most common)
Epidemiology	Advanced age Predominantly men More common than TAA	Advanced age Predominantly men
Etiology	Smoking (most important risk factor) Atherosclerosis Hypercholesterolemia and arterial hypertension	Arterial hypertension Bicuspid aortic valve Tertiary syphilis [9] Connective tissue diseases (e.g., Marfan syndrome, Ehlers-Danlos syndrome) Trauma Smoking
Clinical features	Pulsatile abdominal mass Bruit on auscultation Lower back pain	Feeling of pressure in the chest Thoracic back pain
Diagnostics	Abdominal ultrasound (best initial and confirmatory test)	Chest x-ray and CTA of chest

The differential diagnoses listed here are not exhaustive.

Approach ^[1]

• Unstable patients (e.g., in case of rupture): emergency repair within 90 minutes (see “Ruptured AAA”)
• Symptomatic patients with impending rupture or leaking AAA; : urgent aneurysm repair within hours Consider optimizing conditions for a successful outcome: If this delays surgery for a few hours, the patient should be monitored in an ICU in the meantime.
  • Ensure blood product availability.
  • Management of comorbidities (e.g., heart failure, acute kidney injury)
• Asymptomatic patients: elective aneurysm repair or aneurysm surveillance
• All patients: reduction of cardiovascular risk factors ^[1]
  • Appropriate medical management; of other atherosclerotic risk factors (e.g., hypertension, diabetes, hyperlipidemia)
  • Smoking cessation

Invasive treatment: AAA repair

• Indications ^[1]
  • Emergency repair: unstable patients
  • Urgent repair: impending rupture or leaking AAA
  • Elective repair
    • Fusiform aneurysm with maximum diameter ≥ 5.5 cm and low or acceptable surgical risk
    • Small fusiform aneurysm expanding ≥ 1 cm per year
    • Saccular aneurysm ^[1]
    • Aneurysm with maximum diameter 5.0–5.4 cm in women
    • Small aneurysm (4.0–5.4 cm) in patients requiring chemotherapy, radiotherapy, solid organ transplantation: individual approach

Procedures ^[1]

The long-term survival and complication rates of endovascular and open surgical repair are similar, and these procedures each have their advantages and disadvantages.

• Endovascular aneurysm repair (EVAR)
  • Indications: minimally invasive procedure that is preferred over open surgical repair for most aneurysms, especially in patients with a high operative risk
  • Procedure: Under fluoroscopic guidance, an expandable stent graft is placed via the femoral or iliac arteries intraluminally at the site of the aneurysm.
  • Disadvantage: Reintervention rates are higher for EVAR than for OSR.
• Open surgical repair (OSR)
  • Indications
    • Mycotic aneurysm or infected graft
    • Persistent endoleak and aneurysm sac growth following EVAR
    • Anatomical contraindications for EVAR
  • Procedure: A laparotomy is performed and the dilated segment of the aorta is replaced with a tube graft or Y-prosthesis (bifurcated synthetic stent graft).

Preoperative assessment for elective repair ^[1]

• Calculation of mortality risk: used to weigh operative risk against life expectancy for patients being considered for elective AAA repair

Elective AAA repair postoperative mortality risk score [10]
	Parameter	Points
Planned intervention	EVAR	0
	OSR (infrarenal)	2
	OSR (suprarenal)	4
Aneurysm size (mm)	< 65	0
	≥ 65	2
Age	≤ 75 years	0
	> 75 years	1
Sex	Male	0
	Female	1
Comorbidities	History of MI or cerebrovascular disease	1
	COPD	2
Serum creatinine (mg/dL)	< 1.5	0
	≥ 1.5	2
Interpretation 0–4 points: low risk 5–7 points: medium risk 8–10 points: high risk 11–14 points: prohibitively high risk

• Preoperative management of comorbid conditions
  • Cardiac consult in patients with cardiac diseases
    • Optimize heart failure therapy.
    • Consider coronary revascularization.
  • 12-lead ECG in all patients
  • Echocardiography in patients with worsening dyspnea or dyspnea of unknown origin
  • Pulmonary function studies, including ABG, in patients with COPD, tobacco use, exertional dyspnea
• Additional considerations: Life expectancy should also be considered when planning elective repair. ^[11]

Perioperative care

• IV antibiotic prophylaxis ^[1]
  • First-generation cephalosporin, e.g., cefazolin
  • If the patient is allergic to penicillin: vancomycin
• Anticipate and treat acute blood loss anemia
  • Ensure blood product availability.
  • Indications for blood transfusion ^[1]
    • Hemoglobin is ≤ 7 g/dL
    • Hemoglobin is < 10 g/dL and there is ongoing blood loss
• Central venous access and arterial line monitoring during the procedure
• Consider postoperative admission to ICU:
  • In patients with significant cardiac, pulmonary, or renal comorbidities
  • In patients requiring mechanical ventilation
  • After significant arrhythmia or hemodynamic instability during procedure
• Multimodal pain management
  • E.g., morphine
  • Consider epidural analgesia after OSR
  • See also acute pain management.
• VTE prophylaxis

Surveillance after repair ^[1][7]

Postoperative surveillance following EVAR is important because it can help to detect possible endoleaks, sac growth, device migration, and device failure. Because of the risk of an anastomotic aneurysm or aneurysmal dilation in the visceral aorta or iliac arteries, regular follow-up is recommended after OSR.

• CT angiography abdomen and pelvis with IV contrast
  • After 1 month, 12 months, then annually
  • After 6 months if an abnormality is seen on the 1-month scan
• MR angiography abdomen and pelvis without and with IV contrast
  • Indication: contraindications to CT angiography, avoidance of radiation
  • Artifacts might be visible depending on stent material and orientation.
• Duplex ultrasound
  • Indication: may be used for annual follow-up if the 12-month scan is unremarkable
  • Abdominal and pelvic CT angiography with IV contrast should still be performed every 5 years.

Conservative treatment: AAA surveillance without repair

• Small (< 5.5 cm), asymptomatic AAA can typically be observed with interval surveillance ultrasound. ^[12]
  • To identify the expansion rate and thus decrease the risk of rupture
  • Frequency depends on the size of the aneurysm.

Follow-up frequency for AAA surveillance [1]
Maximum diameter of the abdominal aorta	Recommended follow-up interval
2.5–2.9 cm	Repeat ultrasound after 10 years.
3–3.9 cm	Ultrasound every 3 years
4–4.9 cm	Ultrasound every 12 months
5.0–5.4 cm	Ultrasound every 6 months

Regular monitoring is essential because aneurysm size and expansion rate are strong predictors for the risk of rupture.

• Abdominal aortic aneurysm rupture
• Embolism: caused by thrombotic material from the aneurysm
• Aortic dissection
• Postoperative complications ^[13]
  • Ischemia of the bowel, kidneys, and spinal cord
  • Anterior spinal artery occlusion
  • Prosthetic graft infection
  • Aortoenteric fistula
  • Complications following EVAR ^[1]
    • Endoleak
    • Access site complications, e.g., bleeding, hematoma, false aneurysm
    • Graft limb thrombosis

We list the most important complications. The selection is not exhaustive.

• Risk factors
  • Rapidly expanding aneurysm
  • Large diameter aneurysm
  • Smoking, tobacco use
• Clinical features
  • Hypovolemic shock (especially in free ruptures)
  • Sudden onset of severe, tearing back or abdominal pain with radiation to the flank, buttocks, legs, or groin
  • Painful pulsatile mass
  • Grey Turner sign and/or Cullen sign (if there is an extensive retroperitoneal hematoma)
  • Nausea, vomiting
  • Syncope
  • Hematuria
• Diagnostics ^[1]
  • Ruptured AAA is a clinical diagnosis; consider imaging only if the diagnosis is uncertain and the patient is hemodynamically stable
  • Ultrasound
    • Dilatation of the aorta ≥ 3 cm
    • Periaortic fluid
    • Free intra- or retroperitoneal fluid (depending on location of rupture)
  • CT angiography abdomen and pelvis with IV contrast: only indicated if an alternative diagnosis seems more likely
    • Sign of impending rupture: high-attenuation crescent within mural thrombus ^[14]
    • Signs of rupture: retroperitoneal hematoma, retroperitoneal stranding, indistinct aortic wall, extravasation of contrast
  • Laboratory findings that may be seen:
    • CBC: ↓ hemoglobin, ↓ hematocrit, ↓ red blood cell count
    • Metabolic acidosis in cases of shock
• Treatment: emergency EVAR or OSR ^[1]
  • Hemodynamic support: IV fluid resuscitation with permissive hypotension (e.g., SBP 70–90 mm Hg) ^[1]
• Prognosis: high mortality rate (∼ 81%) ^[15]

Unstable patients should be taken to the OR immediately for emergency surgery if ruptured AAA is suspected.