Abdominal Aortic Aneurysm (AAA): Causes, Warning Signs, Survival, and Treatment

Abdominal aortic aneurysm (AAA) is a localized enlargement of the abdominal aorta such that the diameter is greater than 3 cm or more than 50% larger than normal.  An AAA usually causes no symptoms, except during rupture.^[1] Occasionally, abdominal, back, or leg pain may occur. Large aneurysms can sometimes be felt by pushing on the abdomen.  Rupture may result in pain in the abdomen or back, low blood pressure, or loss of consciousness, and often results in death. AAAs occur most commonly in men, those over 50 and those with a family history of the disease.  Additional risk factors include smoking, high blood pressure, and other heart or blood vessel diseases.  Genetic conditions with an increased risk include Marfan syndrome and Ehlers–Danlos syndrome.  AAAs are the most common form of aortic aneurysm.^[4] About 85% occur below the kidneys, with the rest either at the level of or above the kidneys.  In the United States, screening with abdominal ultrasound is recommended for males between 65 and 75 years of age with a history of smoking. In the United Kingdom and Sweden, screening all men over 65 is recommended.  Once an aneurysm is found, further ultrasounds are typically done on a regular basis.

Abstinence from cigarette smoking is the single best way to prevent the disease.^[1] Other methods of prevention include treating high blood pressure, treating high blood cholesterol, and avoiding being overweight.  Surgery is usually recommended when the diameter of an AAA grows to >5.5 cm in males and >5.0 cm in females. Other reasons for repair include the presence of symptoms and a rapid increase in size, defined as more than one centimeter per year.^[2] Repair may be either by open surgery or endovascular aneurysm repair (EVAR).^[1] As compared to open surgery, EVAR has a lower risk of death in the short term and a shorter hospital stay, but may not always be an option. There does not appear to be a difference in longer-term outcomes between the two. Repeat procedures are more common with EVAR.

AAAs affect 2-8% of males over the age of 65.  They are five times more common in men.^[13] In those with an aneurysm less than 5.5 cm, the risk of rupture in the next year is below 1%.^[1] Among those with an aneurysm between 5.5 and 7 cm, the risk is about 10%, while for those with an aneurysm greater than 7 cm the risk is about 33%.^[1] Mortality if ruptured is 85% to 90%.^[1] During 2013, aortic aneurysms resulted in 168,200 deaths, up from 100,000 in 1990.^[5][14] In the United States AAAs resulted in between 10,000 and 18,000 deaths in 2009.^[4]

Signs and symptoms

The vast majority of aneurysms are asymptomatic. However, as the abdominal aorta expands and/or ruptures, the aneurysm may become painful and lead to pulsating sensations in the abdomen or pain in the chest, lower back, legs, or scrotum.^[15]

Complications

The complications include rupture, peripheral embolization, acute aortic occlusion, and aortocaval (between the aorta and inferior vena cava) or aortoduodenal (between the aorta and the duodenum) fistulae. On physical examination, a palpable and pulsatile abdominal mass can be noted. Bruits can be present in case of renal or visceral arterial stenosis.

The signs and symptoms of a ruptured AAA may include severe pain in the lower back, flank, abdomen or groin. A mass that pulses with the heart beat may also be felt.^[6] The bleeding can lead to a hypovolemic shock with low blood pressure and a fast heart rate, which may cause fainting.^[6] The mortality of AAA rupture is as high as 90 percent. 65 to 75 percent of patients die before they arrive at the hospital and up to 90 percent die before they reach the operating room.^[17] The bleeding can be retroperitoneal or into the abdominal cavity. Rupture can also create a connection between the aorta and intestine or inferior vena cava.^[18] Flank ecchymosis (appearance of a bruise) is a sign of retroperitoneal bleeding and is also called Grey Turner’s sign.^[16][19]

Causes

The exact causes of the degenerative process remain unclear. There are, however, some hypotheses and well-defined risk factors.^[20]

• Tobacco smoking: More than 90% of people who develop an AAA have smoked at some point in their lives.^[21]
• Alcohol and hypertension: The inflammation caused by prolonged use of alcohol and hypertensive effects from abdominal edema which leads to hemorrhoids, esophageal varices, and other conditions, is also considered a long-term cause of AAA.
• Genetic influences: The influence of genetic factors is high. AAA is four to six times more common in male siblings of known patients, with a risk of 20–30%. The high familial prevalence rate is most notable in male individuals. There are many hypotheses about the exact genetic disorder that could cause higher incidence of AAA among male members of the affected families. Some presumed that the influence of alpha 1-antitrypsin deficiency could be crucial, while other experimental works favored the hypothesis of X-linked mutation, which would explain the lower incidence in heterozygous females. Other hypotheses of genetic causes have also been formulated.^[16] Connective tissue disorders, such as Marfan syndrome and Ehlers-Danlos syndrome, have also been strongly associated with AAA. Both relapsing polychondritis and pseudoxanthoma elasticum may cause abdominal aortic aneurysm.
• Atherosclerosis: The AAA was long considered to be caused by atherosclerosis, because the walls of the AAA frequently carry an atherosclerotic burden. However, this hypothesis cannot explain the initial defect and the development of occlusion, which is observed in the process.^[16] Another hypothesis is that plaque buildup can cause a feed-forward dysfunction in the signaling among neurons that regulate pressure in the aorta. This feed-forward process leads to an over-pressuring condition that ruptures in the aorta.^[25]
• Other causes of the development of AAA include: infection, trauma, arteritis, and cystic medial necrosis.^[18]

Pathophysiology

The most striking histopathological changes of the aneurysmatic aorta are seen in the tunica media and intima layers. These changes include the accumulation of lipids in foam cells, extracellular free cholesterol crystals, calcifications, thrombosis, and ulcerations and ruptures of the layers. Adventitial inflammatory infiltrate is present.^[18] However, the degradation of the tunica media by means of a proteolytic process seems to be the basic pathophysiologic mechanism of AAA development. Some researchers report increased expression and activity of matrix metalloproteinases in individuals with AAA. This leads to elimination of elastin from the media, rendering the aortic wall more susceptible to the influence of blood pressure.^[16] Other reports have suggested the serine protease granzyme B may contribute to aortic aneurysm rupture through the cleavage of decorin, leading to disrupted collagen organization and reduced tensile strength of the adventitia.^[26][27] There is also a reduced amount of vasa vasorum in the abdominal aorta (compared to the thoracic aorta); consequently, the tunica media must rely mostly on diffusion for nutrition, which makes it more susceptible to damage.

Hemodynamics affect the development of AAA, which has a predilection for the infrarenal aorta. The histological structure and mechanical characteristics of the infrarenal aorta differ from those of the thoracic aorta. The diameter decreases from the root to the aortic bifurcation, and the wall of the infrarenal aorta also contains a lesser proportion of elastin. The mechanical tension in the abdominal aortic wall is therefore higher than in the thoracic aortic wall. The elasticity and distensibility also decline with age, which can result in gradual dilatation of the segment. Higher intraluminal pressure in patients with arterial hypertension markedly contributes to the progression of the pathological process.^[18] Suitable hemodynamic conditions may be linked to specific intraluminal thrombus (ILT) patterns along the aortic lumen, which in turn may affect AAA’s development.

Diagnosis

An abdominal aortic aneurysm is usually diagnosed by physical exam, abdominal ultrasound, or CT scan. Plain abdominal radiographs may show the outline of an aneurysm when its walls are calcified. However, the outline will be visible by X-ray in less than half of all aneurysms. Ultrasonography is used to screen for aneurysms and to determine their size if present. Additionally, free peritoneal fluid can be detected. It is noninvasive and sensitive, but the presence of bowel gas or obesity may limit its usefulness. CT scan has nearly 100% sensitivity for an aneurysm and is also useful in preoperative planning, detailing the anatomy and possibility for endovascular repair. In the case of suspected rupture, it can also reliably detect retroperitoneal fluid. Alternative less often used methods for visualization of an aneurysm include MRI and angiography.^{[citation needed]}

An aneurysm ruptures if the mechanical stress (tension per area) exceeds the local wall strength; consequently, peak wall stress (PWS),^[30] mean wall stress (MWS),^[31] and peak wall rupture risk (PWRR)^[32] have been found to be more reliable parameters than diameter to assess AAA rupture risk. Medical software allows computing these rupture risk indices from standard clinical CT data and provides a patient-specific AAA rupture risk diagnosis.^[33][34][35] This type of biomechanical approach has been shown to accurately predict the location of AAA rupture.^[34][35][36]

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  Aortic measurement on abdominal ultrasonography in the axial plane between the outer margins of the aortic wall^[37]

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  Ultrasonography in the sagittal plane, showing axial plane measure (dashed red line), as well as maximal diameter (dotted yellow line), which is preferred

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  A ruptured AAA with an open arrow marking the aneurysm and the closed arrow marking the free blood in the abdomen

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  Sagittal CT image of an AAA

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  Biomechanical AAA rupture risk prediction

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  An axial contrast-enhanced CT scan demonstrating an abdominal aortic aneurysm of 4.8 by 3.8 cm

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  The faint outline of the calcified wall of an AAA as seen on plain X-ray

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  Abdominal aortic aneurysms (3.4 cm)

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  An aortic aneurysm as seen on CT with a small area of remaining blood flow

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  Ultrasound showing a previously repaired AAA that is leaking with flow around the graft^[38]

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  Ultrasonography of an aneurysm with a mural thrombus

Classification

Abdominal aortic aneurysms are commonly divided according to their size and symptomatology. An aneurysm is usually defined as an outer aortic diameter over 3 cm (normal diameter of the aorta is around 2 cm),^[41] or more than 50% of normal diameter.^[42] If the outer diameter exceeds 5.5 cm, the aneurysm is considered to be large.^[40] Ruptured AAA should be suspected in any person older than 60 who experiences collapse, unexplained low blood pressure, or sudden-onset back or abdominal pain. Abdominal pain, shock, and a pulsatile mass is only present in a minority of cases.^{[citation needed]} Although an unstable person with a known aneurysm may undergo surgery without further imaging, the diagnosis will usually be confirmed using CT or ultrasound scanning.^{[citation needed]}

The suprarenal aorta normally measures about 0.5 cm larger than the infrarenal aorta.

Differential diagnosis

Aortic aneurysm rupture may be mistaken for the pain of kidney stones, or muscle related back pain.