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).