Aortic Aneurysm (AA) – Thoracic, Abdominal Types of Aneurysms

What is the aorta?

The aorta is the artery that carries oxygen-rich blood out of the left ventricle of the heart. As the largest artery in the body, it has many branches which helps to distribute the oxygenated blood to organs and tissues.

Typically the aorta starts from the top of the heart (ascending aorta), curves (arch of the aorta) and continues downwards (descending aorta). The part of the descending aorta that runs through the chest (thoracic cavity) is known as the thoracic aorta while the part that runs below the diaphragm, through the abdomen is known as the abdominal aorta.

Anatomy, Parts of the Aorta

The coronary arteries, both the right and left coronary artery, originate from the aorta just as it leaves the heart (ascending portion). The aorta runs downwards and divides into the common iliac arteries which continue through to the lower limbs.

Any pathology that affects the aorta can have serious effects or even be potentially fatal due to the nature of this artery. Some of the pathologies that may affect the aorta include :

  • Aneurysm where there is ballooning of a portion of the artery wall.
  • Atherosclerosis where there is a hardening and narrowing of the aorta due to plaques that build up in the vessel wall.
  • Coarctation where there is narrowing of a portion of the aorta.
  • Dissection where the wall of the aorta tears and blood fills in the wall.
  • Rupture where there is a tear or rupture of the aorta, often from trauma.

What is an aneurysm?

An aneurysm is a balloon or bulge at one area of the artery wall. When the artery wall weakens at a spot along the course of the vessel, it is prone to ballooning due to  blood collecting within the sac-like pouch. This ballooning continues to add stress to the already compromised vessel wall and can rupture. This can be fatal.

A aneurysm can affect any artery but tends to occur more frequently in the aorta or cerebral artery. Aneurysms do not always appear like a balloon extending from the wall. At times it may appear like a bulge or widening of the artery at the affected area.

An aneurysm is caused by weakening of the artery wall which may be a result of the following contributing factors or disorders.

  • Genetic – example Marfan syndrome
  • Congenital (since birth) weakening of the wall
  • Hypertension (high blood pressure)
  • Arteriosclerosis or atherosclerosis
  • Trauma – injury to the vessel wall
  • Family history
  • Previous aneurysm
  • Smoking
  • Infection – vasculitis
  • Pharmaceutical or narcotic drug use

What is an aortic aneurysm?

An aneurysm is an abnormal dilatation of a blood vessel which is localized to one portion of the vessel. It is often seen as a bulge or ballooning of the vessel which may be congenital (present since birth) or acquired (develops during life). The aorta, the largest artery in the body that arises from the left ventricle of the heart is a common site of an aneurysm – aortic aneurysm. This may involve the thoracic aorta, the part of the aorta which arises from the heart, curves and run downwards in the thoracic cavity, or the abdominal aorta, the part of the aorta which runs through the abdomen. Most aortic aneurysms develop in the abdominal aorta and are known as abdominal aortic aneurysms (AAA).

Types of Aortic Aneurysms

Location

An aortic aneurysm (AA) may only affect the thoracic aorta (TA), abdominal aorta (AAA) or coexist in both segments of the aorta (TAA / thoracoabdominal aneurysm).

Shape and Size

Aneurysms may also be categorized according to its shape and size. Saccular aneurysms involve a bulging of only portion of the vessel wall, whereas fusiform aneurysms involve the entire circumference of a vessel wall. A saccular aneurysm may vary from 5 to 20 centimeters (cm) while a fusiform aneurysm can reach 20 centimeters in diameter.

Distribution

The wall of a blood vessel is composed of 3 layers – intima (inner), media (middle), adventitia (outer). This is discussed in detail under blood vessels structure. Aneurysms may be categorized as true, involving all 3 layers, or false, involving only the outer layer (adventitia). With a false aneurysm, blood collects between the media and adventitia thereby causing the adventitia to bulge.

How does an aneurysm occur?

The exact cause of an aneurysm is unclear. It is known to occur more frequently in the elderly and is therefore associated with degenerative changes. However, this does not arise in every person with the same predisposing factors. The wall the aorta, like many other arteries, is perpetually under strain due to the high pressure of the blood traveling within it. The wall, particularly the inner lining (intima), is constantly regenerating in order to maintain the structural integrity.

It appears that certain risk factors, like atherosclerosis and hypertension (high blood pressure) in particular, shift this balance between degradation and repair/synthesis. The wall of the aorta is weakened usually at a localized segment and with time, the flowing blood ‘stretches’ that affected portion of the vessel. Being an elastic vessel, the artery wall bulges outwards thus forming an aneurysm.

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