Abdominal aortic aneurysm

Abdominal aortic aneurysm

CT reconstruction image of an abdominal aortic aneurysm (white arrows)
Classification and external resources
Specialty Vascular surgery
ICD-10 I71.3, I71.4
ICD-9-CM 441.3, 441.4
OMIM 100070
DiseasesDB 792
MedlinePlus 000162
eMedicine med/3443 emerg/27 radio/1
MeSH D017544

Abdominal aortic aneurysm (AAA),[1] also known as a triple-a, is a localized enlargement of the abdominal aorta such that the diameter is greater than 3 cm or more than 50% larger than normal.[2] They usually cause no symptoms except when ruptured.[2] Occasionally there may be abdominal, back or leg pain.[3] Large aneurysms can sometimes be felt by pushing on the abdomen.[3] Rupture may result in pain in the abdomen or back, low blood pressure or a brief loss of consciousness.[2][4]

AAAs occur most commonly in those over 50 years old, in men, and among those with a family history.[2] Additional risk factors include smoking, high blood pressure, and other heart or blood vessel diseases.[5] Genetic conditions with an increased risk include Marfan syndrome and Ehlers-Danlos syndrome. AAAs are the most common form of aortic aneurysm.[6] Approximately 85 percent occur below the kidneys with the rest either at the level of or above the kidneys.[2] In the United States screening males with ultrasound who are between 65 and 75 years old and have a history of smoking is recommended.[7] In the United Kingdom screening all men over 65 is recommended.[2] Australia has no guideline on screening.[8] Once an aneurysm is found, further ultrasounds are typically done on a regular basis.[3]

Not smoking is the single best way to prevent the disease. Other methods of prevention include treating high blood pressure, treating high blood cholesterol and not being overweight. Surgery is usually recommended when an AAA's diameter grows to >5.5 cm in males and >5.0 cm in females.[2] Other reasons for repair include the presence of symptoms and a rapid increase in size.[3] Repair may be either by open surgery or endovascular aneurysm repair (EVAR).[2] 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.[2][9][10] There does not appear to be a difference in longer term outcomes between the two.[11] With EVAR there is a higher need for repeat procedures.[12]

AAAs affect between 2% and 8% of males over the age of 65. Rates among women are four times lower. In those with an aneurysm less than 5.5 cm the risk of rupture in the next year is less than 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%. Mortality if ruptured is 85% to 90%.[2] During 2013, aortic aneurysms resulted in 152,000 deaths, up from 100,000 in 1990.[13] In the United States AAAs resulted in between 10,000 and 18,000 deaths in 2009.[6]

Signs and symptoms

Drawing of an abdominal aortic aneurysm.

The vast majority of aneurysms are asymptomatic. However, as abdominal aortic aneurysms expand, they may become painful and lead to pulsating sensations in the abdomen or pain in the chest, lower back, or scrotum.[14] The risk of rupture is high in a symptomatic aneurysm, which is therefore considered an indication for surgery. 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.[15]

Aortic rupture

Main article: Aortic rupture

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.[4] The bleeding can lead to a hypovolemic shock with low blood pressure and a fast heart rate. This may lead to brief passing out.[4]

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.[16] 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.[17] Flank ecchymosis (appearance of a bruise) is a sign of retroperitoneal bleeding, and is also called Grey Turner's sign.[15][18]

Aortic aneurysm rupture may be mistaken for the pain of kidney stones, muscle related back pain.[4]

Causes

The exact causes of the degenerative process remain unclear. There are, however, some hypotheses and well-defined risk factors.[19]

Pathophysiology

A plate from Gray's Anatomy with yellow lines depicting the most common infrarenal location of the AAA.

The most striking histopathological changes of aneurysmatic aorta are seen in tunica media and intima. These include accumulation of lipids in foam cells, extracellular free cholesterol crystals, calcifications, thrombosis, and ulcerations and ruptures of the layers. There is an adventitial inflammatory infiltrate.[17] However, the degradation of tunica media by means of proteolytic process seems to be the basic pathophysiologic mechanism of the 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 the blood pressure.[15] Others 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 tensile strength of the adventitia.[24][25] 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.[26]

Hemodynamics affect the development of AAA. It has a predilection for the infrarenal aorta. The histological structure and mechanical characteristics of infrarenal aorta differ from those of the thoracic aorta. The diameter decreases from the root to the bifurcation, and the wall of the abdominal aorta also contains a lesser proportion of elastin. The mechanical tension in 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.[17] Suitable hemodynamics conditions may be linked to specific intraluminal thrombus (ILT) patterns along the aortic lumen, which in turn may affect AAA's development.[27]

Diagnosis

An abdominal aortic aneurysm is usually diagnosed by physical exam, ultrasound, or CT. Plain abdominal radiographs may show the outline of an aneurysm when its walls are calcified. However, this is the case in less than half of all aneurysms. Ultrasonography is used to screen for aneurysms and to determine the size of any 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 a nearly 100% sensitivity for 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 the aneurysm include MRI and angiography.

An aneurysm ruptures if the mechanical stress (tension per area) exceeds the local wall strength; consequently, peak wall stress (PWS)[28] and peak wall rupture risk (PWRR)[29] 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.[30] This type of biomechanical approach has been shown to accurately predict the location of AAA rupture.[31]

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).[32] If the outer diameter exceeds 5.5 cm, the aneurysm is considered to be large.[33] A ruptured AAA is a clinical diagnosis involving the presence of the triad of abdominal pain, shock and a pulsatile abdominal mass. If these conditions are present, indicating AAA rupture, no further clinical investigations are needed before surgery.[34]

Prevention

Screening

The U.S. Preventive Services Task Force recommends a single screening ultrasound for abdominal aortic aneurysm in males age 65 to 75 years who have a history of smoking.[7] There is an estimated number needed to screen of approximately 850 people.[35] It is unclear if screening is useful in women aged 65 to 75 who have smoked and they recommend against screening in women who have never smoked.[7]

Repeat ultrasounds should be carried out in those who have an aortic size greater than 3.0 cm.[36] In those whose aorta is between 3.0 and 3.9 cm this should be every three years, if between 4.0 and 4.4 cm every two year, and if between 4.5 and 5.4 cm every year.[36]

In the United Kingdom one time screening is recommended in all males over 65 years of age.[2]

Management

The treatment options for asymptomatic AAA are conservative management, surveillance with a view to eventual repair, and immediate repair. There are currently two modes of repair available for an AAA: open aneurysm repair (OR), and endovascular aneurysm repair (EVAR). An intervention is often recommended if the aneurysm grows more than 1 cm per year or it is bigger than 5.5 cm.[37] Repair is also indicated for symptomatic aneurysms.

Conservative

Conservative management is indicated in people where repair carries a high risk of mortality and in patients where repair is unlikely to improve life expectancy. The mainstay of the conservative treatment is smoking cessation.

Surveillance is indicated in small asymptomatic aneurysms (less than 5.5 cm) where the risk of repair exceeds the risk of rupture.[37] As an AAA grows in diameter the risk of rupture increases. Surveillance until the aneurysm has reached a diameter of 5.5 cm has not been shown to have a higher risk as compared to early intervention.[38][39]

Medication

No medical therapy has been found to be effective at decreasing the growth rate or rupture rate of asymptomatic AAAs.[2] Blood pressure and lipids should however be treated per usual.[32]

Surgery

Surgery for an abdominal aortic aneurysm is known as AAA surgery or AAA repair.

The threshold for repair varies slightly from individual to individual, depending on the balance of risks and benefits when considering repair versus ongoing surveillance. The size of an individual's native aorta may influence this, along with the presence of comorbidities that increase operative risk or decrease life expectancy. Evidence; however, does not support repair if the size is between 4 cm and 5.5 cm.[37]

Open repair

Main article: Open aortic surgery

Open repair is indicated in young patients as an elective procedure, or in growing or large, symptomatic or ruptured aneurysms. The aorta must be clamped off during the repair, denying blood to the abdominal organs and sections of the spinal cord; this can cause a range of complications. It is essential to make the critical part of the operation fast, so the incision is typically made large enough to facilitate the fastest repair. Recovery after open AAA surgery takes significant time. The minimums are a few days in intensive care, a week total in the hospital and a few months before full recovery.

Endovascular repair

Abdominal aortic endoprosthesis, CT scan, original aneurysm marked in blue.

Endovascular repair first became practical in the 1990s and although it is now an established alternative to open repair, its role is yet to be clearly defined. It is generally indicated in older, high-risk patients or patients unfit for open repair. However, endovascular repair is feasible for only a proportion of AAAs, depending on the morphology of the aneurysm. The main advantages over open repair are that there is less peri-operative mortality, less time in intensive care, less time in hospital overall and earlier return to normal activity. Disadvantages of endovascular repair include a requirement for more frequent ongoing hospital reviews, and a higher chance of further procedures being required. According to the latest studies, the EVAR procedure does not offer any benefit for overall survival or health-related quality of life compared to open surgery, although aneurysm-related mortality is lower.[40][41][42][43] In patients unfit for open repair, EVAR plus conservative management was associated with no benefit, more complications, subsequent procedures and higher costs compared to conservative management alone.[44] Endovascular treatment for paraanastomotic aneurysms after aortobiiliac reconstruction is also a possibility.[45] A 2014 Cochrane review found tentative evidence of no difference in outcomes between endovascular and open repair of ruptured AAA in the first month.[46]

Rupture

In those with aortic rupture of the AAA, treatment is immediate surgical repair. There appears to be benefits to allowing permissive hypotension and limiting the use of intravenous fluids during transport to the operating room.[47]

Prognosis

AAA Size (cm) Growth rate (cm/yr)[48] Annual rupture risk (%)[49]
3.0-3.9 0.39 0
4.0-4.9 0.36 0.5-5
5.0-5.9 0.43 3-15
6.0-6.9 0.64 10-20
>=7.0 - 20-50

Although the current standard of determining rupture risk is based on maximum diameter, it is known that smaller AAAs that fall below this threshold (diameter<5.5 cm) may also rupture, and larger AAAs (diameter>5.5 cm) may remain stable.[50][51] In one report, it was shown that 10–24% of ruptured AAAs were less than 5 cm in diameter.[51] It has also been reported that of 473 non-repaired AAAs examined from autopsy reports, there were 118 cases of rupture, 13% of which were less than 5 cm in diameter. This study also showed that 60% of the AAAs greater than 5 cm (including 54% of those AAAs between 7.1 and 10 cm) never experienced rupture.[52] Vorp et al. later deduced from the findings of Darling et al. that if the maximum diameter criterion were followed for the 473 subjects, only 7% (34/473) of cases would have succumbed to rupture prior to surgical intervention as the diameter was less than 5 cm, with 25% (116/473) of cases possibly undergoing unnecessary surgery since these AAAs may never have ruptured.[52]

Alternative methods of rupture assessment have been recently reported. The majority of these approaches involve the numerical analysis of AAAs using the common engineering technique of the finite element method (FEM) to determine the wall stress distributions. Recent reports have shown that these stress distributions have been shown to correlate to the overall geometry of the AAA rather than solely to the maximum diameter.[53][54][55] It is also known that wall stress alone does not completely govern failure as an AAA will usually rupture when the wall stress exceeds the wall strength. In light of this, rupture assessment may be more accurate if both the patient-specific wall stress is coupled together with patient-specific wall strength. A non-invasive method of determining patient-dependent wall strength was recently reported,[56] with more traditional approaches to strength determination via tensile testing performed by other researchers in the field.[57][58][59] Some of the more recently proposed AAA rupture-risk assessment methods include: AAA wall stress;[28][60][61] AAA expansion rate;[62] degree of asymmetry;[55] presence of intraluminal thrombus (ILT);[63] a rupture potential index (RPI);[64][65] a finite element analysis rupture index (FEARI);[66] biomechanical factors coupled with computer analysis;[67] growth of ILT;[68] geometrical parameters of the AAA;[69] and also a method of determining AAA growth and rupture based on mathematical models.[70][71]

The post-operative mortality for an already ruptured AAA has slowly decreased over several decades but remains higher than 40%.[34] However, if the AAA is surgically repaired before rupture, the post-operative mortality rate is substantially lower: approximately 1-6%.[72]

Epidemiology

The occurrence of AAA varies by ethnicity. In the United Kingdom the rate of AAA in Caucasian men older than 65 years is about 4.7%, while in Asian men it is 0.45%.[73] It is also less common in individuals of African, and Hispanic heritage.[2] They occur four times more often in men than women.[2]

There are at least 13000 deaths yearly in the U.S. secondary to AAA rupture.[2] The peak number of new cases per year among males is around 70 years of age, the percentage of males affect over 60 years is 2-6%. The frequency is much higher in smokers than in non-smokers (8:1), and the risk decreases slowly after smoking cessation.[74] In the U.S., the incidence of AAA is 2-4% in the adult population.[15]

Rupture of the AAA occurs in 1-3% of men aged 65 or more, the mortality is 70-95%.[33]

History

The first historical records about AAA are from Ancient Rome in the 2nd century AD, when Greek surgeon Antyllus tried to treat the AAA with proximal and distal ligature, central incision and removal of thrombotic material from the aneurysm. However, attempts to treat the AAA surgically were unsuccessful until 1923. In that year, Rudolph Matas (who also proposed the concept of endoaneurysmorrhaphy), performed the first successful aortic ligation on a human.[75] Other methods that were successful in treating the AAA included wrapping the aorta with polyethene cellophane, which induced fibrosis and restricted the growth of the aneurysm. Albert Einstein was operated on by Rudolph Nissen with use of this technique in 1949, and survived five years after the operation, though he eventually died when the aneurysm ruptured.[76] Endovascular aneurysm repair was first performed in the late 1980s and has been widely adopted in the subsequent decades. Endovascular repair was first used for treating a ruptured aneurysm in Nottingham in 1994[77]

Former presidential candidate Bob Dole had an abdominal aortic aneurysm in 2001 and was treated surgically by vascular surgeon Kenneth Ouriel. The operation was successful. In 1993, country music singer Conway Twitty died from AAA, and actor George C. Scott also died of an Abdominal Aneurysm.[78]

Society and culture

In 2001 former presidential candidate Bob Dole underwent surgery for an abdominal aortic aneurysm in which a team of surgeons led by Doctor Kenneth Ouriel inserted a stent graft:

“ Ouriel said that the team inserted a Y-shaped tube through an incision in Dole's leg and placed it inside the weakened portion of the aorta. The aneurysm will eventually contract around the stent, which will remain in place for the rest of Dole's life.[78] ”

— Associated Press

Actor Robert Jacks, who plays Leatherface in Texas Chainsaw Massacre: The Next Generation died from an abdominal aneurysm on August 8, 2001, just one day shy of his 42nd birthday. His father also died from the same cause when Robert was a child.

Research

Risk assessment

There have been many calls for alternative approaches to rupture-risk assessment over the past number of years, with many believing that a biomechanics-based approach may be more suitable than the current diameter approach. Numerical modelling is a valuable tool to researchers allowing approximate wall stresses to be calculated, thus revealing the rupture potential of a particular aneurysm. Experimental models are required to validate these numerical results, and provide a further insight into the biomechanical behaviour of the AAA. In vivo, AAAs exhibit a varying range of material strengths[79] from localised weak hypoxic regions[80] to much stronger regions and areas of calcifications.[81]

Experimental models

Experimental models can now be manufactured using a novel technique involving the injection-moulding lost-wax manufacturing process to create patient-specific anatomically correct AAA replicas.[82] Work has also focused on developing more realistic material analogues to those in vivo, and recently a novel range of silicone-rubbers was created allowing the varying material properties of the AAA to be more accurately represented.[83] These rubber models can also be used in a variety of experimental testing from stress analysis using the photoelastic method[84] to deterimining whether the locations of rupture experimentally correlate with those predicted numerically.[85] New endovascular devices are being developed that are able to treat more complex and tortuous anatomies.[86]

Prevention and treatment

An animal study showed that removing a single protein prevents early damage in blood vessels from triggering a later-stage, complications. By eliminating the gene for a signaling protein called cyclophilin A (CypA) from a strain of mice, researchers were able to provide complete protection against abdominal aortic aneurysm.[87]

Other recent research identified Granzyme B (GZMB) (a protein-degrading enzyme) to be a potential target in the treatment of abdominal aortic aneurysms. Elimination of this enzyme in mice models — both slowed the progression of aneurysms and improved survival.[88][89]

References

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