Abdominal aortic aneurysm

BASIC INFORMATION
DEFINITION
An abdominal aortic aneurysm is a permanent localized dilation of the abdominal aortic artery to at least 50% when compared with the normal diameter. The normal diameter in men is 2.3 cm, and in women it is 1.9 cm. Abdominal aortic aneurysms (AAA) occur most commonly below the renal arteries (infrarenal). The incidence increases with age, being present in 5% of the population > 60 years. They occur five times more frequently in men and in one in four male children of an affected individual. Aneurysms may occur secondary to atherosclerosis, infection (syphilis, Escherichia coli, Salmonella) and trauma, or may be genetic (Marfan’s syndrome, Ehlers-Danlos syndrome).

SYNONYMS
AAA
Symptoms
Most aneurysms are asymptomatic and are found on routine abdominal examination, plain X-ray or during urological investigations. Rapid expansion or rupture of an AAA may cause severe pain (epigastric pain radiating to the back). A ruptured AAA causes hypotension, tachycardia, profound anaemia and sudden death. The symptoms of rupture may mimic renal colic, diverticulitis and severe lower abdominal or testicular pain. Gradual erosion of the vertebral bodies may cause non-specific back pain. The aneurysm may embolize distally. Inflammatory aneurysms can obstruct adjacent structures, e.g. ureter, duodenum and vena cava. Rarely patients with aneurysms can present with severe haematemesis secondary to an aortoduodenal fistula.

Signs
The aorta is retroperitoneal and in overweight patients there may be no overt signs. An aneurysm is suspected if a pulsatile, expansile abdominal mass is felt. The presence of an AAA should alert a clinician to the possibility of popliteal aneurysms. Patients may present with ‘trash feet’, dusky discoloration of the digits secondary to emboli from the aortic thrombus.

EPIDEMIOLOGY & DEMOGRAPHICS
• The incidence of abdominal aortic aneurysms has been rising from 12.2 cases/100,000 persons to 36.2 cases/ 100,000 persons from 1951 to 1980.
• The prevalence ranges from 2% to 5% in men >60 yr.
• AAA is predominantly a disease of the elderly, affecting men > women (4:1).
• Rupture of an abdominal aortic aneurysm is the tenth leading cause of death in men >55 yr (15,000 deaths/yr in the U.S.).
PHYSICAL FINDINGS & CLINICAL PRESENTATION
• Pulsatile epigastric mass that may or may not be tender.
• Discoloration and pain of the feet if the thrombus within the aneurysm embolizes.
• Shock, hypoperfusion, abdominal distention if rupture occurs.
• Rare presentations include hematemesis or melena with abdominal and back pain in patients with aorto-enteric fistulas. Aortocaval fistula produces loud abdominal bruits.
ETIOLOGY
Multifactorial
• Atherosclerotic (degenerative or nonspecific)
• Genetic (e.g., Ehlers-Danlos syndrome)
• Trauma
• Cystic medial necrosis (Marfan’s syndrome)
• Arteritis, inflammatory
• Mycotic, infected (syphilis)
DIAGNOSIS
DIFFERENTIAL DIAGNOSIS
Almost 75% of abdominal aneurysms are asymptomatic and are discovered on routine examination or serendipitously when ordering studies for other complaints. This must be considered in the differential of anyone presenting with abdominal pain or back pain.

IMAGING STUDIES
• Abdominal ultrasound is nearly 100% accurate in identifying an aneurysm and estimating the size to within 0.3 to 0.4 cm. It is not very good in estimating the proximal extension to the renal arteries or involvement of the iliac arteries.
• CT scan is recommended for preoperative aneurysm imaging and estimating the size to within 0.3 mm. There are no false-negatives, and the CT scan can localize the proximal extent, detect the integrity of the wall, and rule out rupture.
• Angiography gives detailed arterial anatomy, localizing the aneurysm relative to the renal and visceral arteries. This is the definitive preoperative study for surgeons.
• MRI can also be used, but it is more expensive and not as readily available.
Management
Like any operation, the management of an asymptomatic aneurysm depends on the balance of operative risk and conservative management. The UK Small Aneurysm Trial showed that patients with infrarenal AAA did best with an operation if the aneurysm was:
* > 5.5 cm diameter
* expanding > 1 cm/year
* symptomatic.
TREATMENT
• Treat atherosclerotic risk factors (diet and exercise for blood pressure, cholesterol, and diabetes, and abstinence from tobacco).
• Definitive treatment depends on the size of the aneurysm (see “Chronic Rx”).
Repair of abdominal aortic aneurysm
Standard therapy is open surgical repair with insertion of a Dacron or Gore-Tex graft.
Endovascular stent
Endovascular stent insertion (via the femoral or iliac arteries) is a non-surgical approach to AAA repair. The EndoVascular Aneurysm Repair studies EVAR (stent versus open surgical repair) and EVAR 2 (stent versus medical therapy in patients unsuitable for open repair) investigated the role of endovascular stents in patients with AAA > 5.5 cm on CT. In EVAR the 30-day mortality rate was 1.7% with stenting versus 4.7% with surgery (p = 0.009) but the long-term mortality rate was similar in both groups at 4 years. In EVAR 2 the 30-day mortality rate with stenting was 9%. Long-term mortality rate was similar in both stent and medical therapy groups. A meta-analysis of three randomized control trials demonstrated a 30-day mortality rate of 2% for stent-graft repair versus 5% for open surgical repair; with reductions in ITU and in-hospital stay with stent-graft repair.
Laparoscopic surgery
An alternative to open-surgical repair or endovascular stenting is laparoscopic repair that is performed with handassisted laparoscopic surgery (HALS, requiring a midline mini-laparotomy) or by total laparoscopic surgery (TLS). In non-randomized controlled trials both methods were associated with reduced length of stay, although the operating times were longer.
Prognosis
After repair, patients with an AAA should return to normal activity within a few months.
ACUTE GENERAL Rx
• Abdominal aortic rupture is an emergency. Surgery is the only chance for survival.
CHRONIC Rx
• Diagnosing, sizing, and repairing the aneurysm in an asymptomatic patient is crucial.
• The most commonly used predictor of rupture is the maximum diameter of the abdominal aortic aneurysm.
• Recent randomized trials found no reduction in mortality from repairing abdominal aortic aneurysms smaller than 5.5 cm in patients at low operative risk.
• For aneurysms 5.5 cm or greater, prosthetic graft replacement is recommended, providing there is no contraindication (e.g., MI within 6 mo, refractory CHF, life expectancy < 2 yr, severe residual from CVA).
• For the high-risk patient deemed inoperable for such major surgery, endovascular stent-anchored grafts under local anesthesia have provided an alternative
approach.
DISPOSITION
• The risk of rupture is 0% per year in aneurysms <4 cm, 0.6%-1%/yr in aneurysms 4.0-5.5 cm, 4.4%/yr in aneurysms 5.5-5.9 cm, 10.2%/hr in aneurysms 6.0-6.9 cm, and 32.5%/yr in aneurysms >7 cm.
• Mortality after rupture is >90%. Of those patients who reach the hospital, it is estimated 50% will survive compared with a 4% mortality rate for elective repair of the nonruptured aorta.