Introduction to AAA
An abdominal aortic aneurysm is the most common type of arterial aneurysm. Aortic aneurysm is a common condition that affects about 1 in 20 people (2 to 8%) in the developed world and is more common in men (4 to 8% in those older than 50) compared with women (1 to 1.3 percent) [ref]. An aneurysm of the abdominal aorta is a local or diffuse expansion of the diameter of the abdominal aorta measuring greater than 3 cm or dilatation of the aortic diameter by greater than 50% of the normal vessel diameter. AAA account for 15,000 deaths per year in the United States.
Thinning of the wall and development of an aortic aneurysm is mainly acquired and is caused by atherosclerosis in about 95% of cases, other causes (nonspecific aortoarteritis, congenital causes, tuberculous, syphilitic, rheumatic aortic lesions ) accounts for only 5% of cases. In recent years, the number of traumatic aneurysms has increased, especially after car accidents. Among the most common causes of congenital causes of aneurysms of the abdominal aorta are fibromuscular dysplasia, Marfan syndrome. Although an Abdominal aortic aneurysm is often asymptomatic with an uncomplicated course a ruptured aneurysm is often fatal.
What is the Aorta?
The aorta is the largest artery in the human body. Blood enters the aorta after being pumped out from the left side of the heart, then, through numerous arterial branches, supplies oxygenated blood to all of the body’s organs. The aorta departs from the heart in an upward direction, forms an arch, then descends downward, passing through the chest cavity (thoracic aorta) and in the abdomen (abdominal aorta).
The aortic wall consists of three layers: inner (intima), middle (media), outer (adventitia).
Abdominal Aortic Aneurysm (AAA)
An aortic aneurysm is an enlargement of a portion of the aorta or a bulging of its wall. An aneurysm of the abdominal aorta is defined as an increase in the aortic diameter by more than 50% in comparison with the normal vessel which results in bulging of the aortic wall. With the pressure of blood flowing through this vessel, the enlargement or bulging of the aorta can progress. The diameter of the normal aorta in the abdominal region is approximately 2 cm. However, at the site of the aneurysm, the aorta can be expanded to 7 cm or more.
The most common type of aneurysm is an aortic aneurysm that originates in the abdomen (abdominal aortic aneurysm). Less commonly, the aorta that runs through the chest (thoracic aortic aneurysm) is affected.
The wall of the aorta in the area of the aneurysm is much weaker than normal, so it may not withstand the pressure of the blood being pumped through it. This can lead to rupture of the aneurysm. The risk of this complication is directly correlated with the size of the aneurysm.
- 5 years out from a AAA diagnosis, rupture occurs in approximately 2% of AAAs less than 4 cm in diameter and in more than 25% of AAAs larger than 5 cm.
- In some cases, with a small size of the aneurysm, the operation may not be required urgently, but it may be necessary to undergo a followup examination every 6 months.
This disease does not usually manifest itself with symptoms early on. With the progression of the disease and the absence of early treatment, aneurysm rupture can occur , and consequently, massive bleeding, which often is fatal. Timely referral to a specialist, high-quality consultative, diagnostic and surgical assistance can prevent the progression of the disease and ensure the prevention of a life-threatening complication – rupture of an aneurysm.
Why is aortic aneurysm dangerous?
AAA are the cause of 15,000 deaths per year in the United States alone. AAA left unchecked will undergo continuous expansion resulting in increased risk of rupture. Most small aortic aneurysms increase in size by 2.5 mm per year; 4 mm per year is maximum normal growth
The annual risk of rupture for abdominal aortic aneurysms are as follows
- less than 5.5 cm: ≤1.0%
- 5.5 to 5.9 cm: 9.4%
- 6.0 to 6.9 cm: 10.2%
- ≥7.0 cm: 32.5%.
AAAs >5.5 cm have an annual risk of rupture exceeding the elective 30-day operative mortality; hence, this has been the size criterion for elective repair in men. Abdominal aortic aneurysms that measure greater than 5 cm, have a 25% chance of rupture within 5 years. That means that 25 out of 100 patients with aneurysms greater than 5cm will suffer an aortic rupture within 5 years of receiving the diagnosis. For aneurysms measuring less than 4cm in diameter the risk of rupture is approximately 2% up to 5 years post diagnosis. Unfortunately the risk of aortic rupture continues to increase as the size of the aneurysm increases.
The reason for concern is that a ruptured aneurysm can cause massive internal bleeding, which in turn can lead to shock or death.
Abdominal aortic aneurysms can cause other serious health problems. Blood clots (thrombi) are often formed in the aneurysm sac, or parts of the aneurysm. These can detach and move along the branches of the aorta and to other organs and extremities. If one of the blood vessels becomes blocked as a result of this, it can cause severe pain and lead to organ death or loss of a lower limb. Fortunately, if an aortic aneurysm is diagnosed early, treatment can be timely, safe, and effective.
Types of aortic aneurysms
A distinction is made between “true” and “false” aortic aneurysms. A true aneurysm develops as a result of the gradual weakening of all layers of the aortic wall and an outpouching. A false aneurysm also known as a pseudoaneurysm is usually the result of trauma and is a contained rupture of the aorta. A false aneurysm (Pseudoaneurysm) essentially occurs when a blood vessel wall is injured and the leaking blood which collects in the surrounding tissue.
Aneurysms are further subdivided:
- by etiology – acquired (non-inflammatory or inflammatory) and congenital;
- by morphology – into false (of traumatic origin also known as pseudoaneurysm), true;
- in shape – Saccular, Fusiform;
- By clinical symptoms – Uncomplicated course, complicated;
- By location of aneurysm – This is the most commonly used as it has clinical implications. Aneurysm can be broken up into suprarenal and infrarenal aneurysms.
Suprarenal Vs Infrarenal Aneurysm
- Suprarenal aneurysm – refers to an aneurysm which extends to the renal arteries and involves the origins of one or more visceral arteries (superior mesenteric artery or the celiac trunk) but generally does not extend into the chest.
- Pararenal aneurysm – The renal arteries arise from the aneurysmal aorta, but the aorta above the renal arteries at the level of the superior mesenteric artery is not aneurysmal.
- Juxtarenal aneurysm – The aneurysm originates just beyond the take off of the renal arteries. There is no segment of nonaneurysmal aorta distal to the renal arteries, but the aorta at the level of the renal arteries is not aneurysmal.
- Infrarenal aneurysm – The aneurysm originates distal to the renal arteries. Do there is a segment of normal nonaneurysmal aorta beyond the renal arteries. This is the most common type accounting for 95% of cases.
The most common type of abdominal aortic aneurysm is the infrarenal abdominal aortic aneurysm type. These aneurysms generally start somewhere between the renal arteries and the inferior mesenteric artery. Suprarenal and pararenal aneurysms account for 5% of AAA cases. It should be noted that approximately 40% of abdominal aortic aneurysms are associated with iliac artery aneurysms.
Aneurysm Shapes (Morphology):
Saccular aneurysm – expansion of the aortic wall only on one side;
fusiform (fusiform) aneurysm – expansion of the aneurysm cavity from all sides;
mixed aneurysm – a combination of saccular and fusiform forms.
What is a dissecting Aortic Aneurysm?
A dissecting aneurysm, forms as a result of rupture of the inner wall , followed by its dissection away from the remainder of the aortic wall layers and the formation of two channels one being the true original lumen and the second once being false channel for blood flow.
Depending on the location and length of the dissection, there are 3 classes of dissecting aortic aneurysm:
- The dissection begins in the ascending part of the aorta, moves along the aortic arch (50%).
- Dissection occurs only in the ascending part of the aorta (35%).
- The dissection begins in the descending part of the aorta, moves down (more often) or up (less often) along the arch (15%).
Depending on the age of the process, a dissecting aneurysm can be:
acute (1-2 days after the appearance of the endothelial defect);
subacute (2-4 weeks);
chronic (4-8 weeks or more, up to several years).
Causes and risk factors for the development of an abdominal aortic aneurysm
There is generally no single distinct cause for the formation of an abdominal aortic aneurysm but rather several pathologies working in concert. Despite the multifactoral cause of aneurysms the most commonly attributed cause of aneurysm development is considered atherosclerosis. Atherosclerotic aneurysms account for 96% of the total number of all aneurysms. In addition, aortic aneurysms can be both congenital (fibromuscular dysplasia, Erdheim’s cystic medionecrosis, Marfan syndrome, etc.), and acquired (inflammatory and non-inflammatory). Inflammation of the aorta occurs with various infections (syphilis, tuberculosis, salmonellosis, etc.) or as a result of an allergic-inflammatory process (nonspecific aortoarteritis). Non-inflammatory aneurysms most often develop with atherosclerotic aortic lesions. Less commonly, they are the result of trauma to its wall.
Risk factors for aneurysm development
The risk factors associated with aneurysmal disease include:
- Older age
- Male gender
- Cigarette smoking
- Caucasian race
- Family history of AAA
- Other large artery aneurysms (eg, iliac, femoral, popliteal)
A decreased risk of AAA is associated with:
- Female gender
- Non-Caucasian race
The presence of aneurysms in other family members is also a risk factor. It can indicate the role of a hereditary factor in the development of this disease;
Gender: men over the age of 60 (women have fewer abdominal aortic aneurysms).
Symptoms and signs of an abdominal aortic aneurysm
In most patients, abdominal aortic aneurysms do not present with symptoms or clinical manifestations and are oftentimes incidental findings seen on imaging performed for other reasons.
Patients with intact abdominal aortic aneurysm (AAA) may present with or without symptoms.
Asymptomatic – The majority of patients are asymptomatic. A previously unknown AAA may also become apparent as a result of screening or be discovered incidentally on routine physical examination, on imaging studies performed for other indications, or in the course of evaluating other unrelated conditions. Asymptomatic AAAs are difficult to exclude based on physical examination alone in most patients, even when attempted by experienced examiners/clinicians●
Symptomatic but not ruptured – Symptomatic AAA refers to any of a number of symptoms that can be attributed to the aneurysm but are generally nonspecific. The development of symptoms may be a sign that AAA is rapidly enlarging, or has become large enough to compress surrounding structures, or is an inflammatory or infectious aneurysm. Patients with symptomatic AAA most commonly present with abdominal, back, or flank pain, which may or may not be associated with AAA rupture. AAA can also present with other clinical manifestations, such as limb ischemia (acute or chronic), or other systemic manifestations (fever, malaise) in those patient with infections. In patients with abdominal pain, rupture of the aneurysm must be excluded.
Symptomatic and ruptured – The clinical presentation of ruptured abdominal aortic aneurysm is variable with respect to symptoms and time course. The patient may or may not be aware of the diagnosis of AAA prior to their clinical manifestations of rupture. Only 20 to 30 percent of patients who present to an emergency department with rupture have a known history of AAA. The classic presentation of severe pain, hypotension, and a pulsatile abdominal mass occurs in approximately 50 percent of patients. Although the signs and symptoms of ruptured AAA may be obvious, some presentations make ruptured AAA difficult to recognize. Patients with rupture into the retroperitoneum may attribute their symptoms to other causes and delay seeking medical attention. Even after presenting to a physician, a misdiagnosis of ruptured AAA as renal colic, perforated viscus, diverticulitis, gastrointestinal hemorrhage, and ischemic bowel occurs approximately 30 percent of the time.
Diagnosing an abdominal aortic aneurysms
Abdominal aortic aneurysms are often incidental and can most often be diagnosed by ultrasound of the abdomen and the abdominal aorta. Although this method is preferred in patients because of its lack of ionizing radiation and relative ease with which an ultrasound can be performed there are situations where CT or less often MRI is preferred. CT imaging can characterize aneurysms prior to surgery and also better help identify if aneurysms can undergo endovascular minimally invasive aneurysm repair versus open surgical repair. Additionally CT imaging is less operator dependent and therefore is more reproducible as well as sensitive and specific for aneurysms.
Computed tomography – CTA – CT Angiography which is a CT of the abdomen and pelvis with contrast timed for maximum opacification of the aorta is the most sensitive and specific study
Magnetic resonance imaging – MRI can be performed with or without contrast (Time of flight imaging) however given that this exam takes significant time to perform and can sometimes miss other reasons for abdominal pain which can be identified with ultrasound and CT it is not the preferred imaging study.
Ultrasound duplex scanning of the abdominal aorta is a great tool that allows for quick answers but can be dependent on the person performing the ultrasound (Operator dependent) .
Aortic aneurysm treatment
There are several treatments for aortic aneurysm. It is important to know the advantages and disadvantages of each of these techniques. Approaches to the treatment of abdominal aortic aneurysms:
Management of Abdominal Aortic Aneurysms
Abdominal aortic aneurysms (AAAs) are managed according to their diameter and the presence or absence of symptoms. Under most circumstances, patients with symptoms that cannot be definitively attributed to another etiology should be admitted for observation and further vascular evaluation. Asymptomatic aneurysms are evaluated on an outpatient basis, unless they are very large.
Repair of ruptured abdominal aortic aneurysm (AAA) can be offered to most patients, and the timing of the initial evaluation and management of the patient is guided by the hemodynamic status of the patient. The hemodynamically unstable patient (persistent in spite of resuscitation) with known AAA who presents with classic symptoms/signs of rupture (hypotension, flank/back pain, pulsatile mass) should be taken emergently to the operating room for immediate control of hemorrhage, resuscitation, and repair of the aneurysm. Imaging confirmation of the presence of AAA in hemodynamically unstable patients suspected but not known to have the disease is ideal prior to intervention but is not required. Preoperative management of hemodynamically unstable patients, including the concept of “hypotensive hemostasis,” is an area of active investigation. However, in most circumstances, volume resuscitation should be provided to the least amount necessary to maintain mentation and stabilize blood pressure and pulse rate.
For patients with suspected ruptured AAA who are hemodynamically stable, abdominal imaging (preferably computed tomographic [CT] aortography) should be performed urgently to confirm the rupture prior to repair, rule out other potential etiologies as a cause of abdominal pain and hypotension, and determine if an endovascular repair is feasible.
Open surgical versus endovascular repair of ruptured aneurysm
Both open and endovascular techniques can be successfully employed in the treatment of ruptured AAA. Endovascular repair of ruptured AAA may have some advantages over open repair; however, it is not universally available, and the selection of technique is best determined by the available surgical team. Open surgical or endovascular repair of ruptured AAA is accomplished in a manner that is similar to elective repair, with modifications for aortic hemorrhage control, and anticoagulation. Conversion rates from endovascular to open repair for ruptured AAA may be higher compared with elective repair because of unanticipated anatomic features or device-related issues.
Symptomatic (nonruptured) AAA
Aneurysm repair is indicated for patients with symptoms (abdominal/back/flank pain, thromboembolism) that cannot unequivocally be attributed to another etiology, regardless of aneurysm diameter. For patients with symptomatic AAA, the first priority is to determine whether there is any immediate concern that the aneurysm has ruptured or is at high risk for impending rupture, which may be suggested by clinical symptoms or signs, or certain radiologic features (eg, broken calcification, asymmetry) that may indicate instability of the aneurysm.
In the absence of overt rupture, patients with AAA who are thought to be symptomatic or who have possible signs of impending rupture should be admitted for observation and further evaluation. If the patient is a candidate, repair should be accomplished during the same hospitalization.
If the patient is a candidate for repair, a determination needs to be made about whether an open surgical or endovascular approach is more appropriate, primarily based upon an anatomic assessment of aortoiliac anatomy.
The management of asymptomatic AAA is based upon an assessment of the patient’s risk for rupture, compared with the expected risk of perioperative morbidity and mortality associated with repair. When the risk of rupture exceeds the risk of repair, repair is recommended. Conversely, if the risk of repair is greater than the risk of rupture, conservative management and surveillance is recommended.
The assessment of rupture risk depends primarily upon the diameter of the aneurysm at diagnosis and the patient’s medical comorbidities. The annual risk of rupture has been found in randomized trials to be similar to, or lower than, the risk of repair in patients with small- or medium-sized aneurysms (<5.5 cm in diameter). This diameter threshold for AAA repair is not absolute and may depend on the patient’s stature and location of the aneurysm. Other factors that need to be taken into account include the patient’s age and gender, faster expansion rates, and the presence of other peripheral aneurysms.
Patients who are not candidates for repair or refuse repair should create an advanced directive detailing their wishes in the event of rupture. Family members should be made aware of these wishes, given that the patient may not be able to report these wishes at the time of aneurysm rupture.
During the period of observation (watchful waiting), medical therapies are aimed at reducing the rate of aortic expansion and morbidity and mortality from cardiovascular disease. However, no medical therapy other than smoking cessation has proven effective at reducing the rate of AAA enlargement and possibly, by extrapolation, risk for rupture.
During the period of observation, ultrasound surveillance is routinely performed on a schedule that depends primarily upon the diameter of the aneurysm. We generally obtain annual ultrasound; however, a more frequent interval (eg, every six months) may be used depending upon other characteristics of the aneurysm or patient-related factors. Factors that influence the aneurysm surveillance interval are discussed separately.