Key Definitions:

• Aneurysm is defined by dilation of the entire vessel wall thickness.
• Dissection occurs when blood enters and separates layers of the vessel wall.

• Both are the result of weak vessel walls, which can result from acquired and/or genetic defects.
• Both can also lead to vessel rupture or other complications.

Review of Aortic Anatomy:

• The aorta arises from the heart as the ascending aorta, then arches posteriorly, and then descends through the trunk, bifurcating to form the common iliac arteries at the pelvis.
  — The descending aorta can be subdivided by the diaphragm: superiorly is the thoracic diaphragm, inferiorly is the abdominal diaphragm.

Aortic aneurysms:

• Vessel wall weakening is due to loss of vascular smooth muscle cells, elastic fibers, and collagen fibers, which are crucial for vessel wall support.
• These deficits may be due to a variety of causes, including atherosclerosis, hypertension, trauma, vasculitis, infections (which cause “mycotic” aneurysms), congenital connective tissue defects, and various genetic factors that predispose an individual to weak vessels.
• Types:
  — Saccular aneurysm consists of asymmetrical outpockets (“sacs”) of the vessel wall.
  — Fusiform aneurysms produce symmetrical dilations – you may recall that “fusiform” means “spindle-shaped,” which describes this morphology.
  — Pseudo-aneurysm/false aneurysm Is not an aneurysm; occurs when a tear in the layers of the vessel wall allows blood to leak through and form a thrombus under the adventitia or surrounding tissues; these are often due to trauma.
• Complications:
  — Ischemia, thromboembolism, dissection, and rupture.

Thoracic aortic aneurysms

• Generally defined as dilations more than 50% of the normal diameter.
• Thoracic aneurysms are associated with genetic disorders that lead to cystic medial necrosis (also called cystic medial degeneration), including Marfan, Loeys-Dietz, and Ehlers-Danlos syndromes.
  — Cystic medial necrosis is characterized by abnormal smooth muscle cells and elastic fibers, with
  “cyst-like” areas of ground substance; because necrosis is not always present, many authors prefer the term “cystic medial degeneration.”
• Thoracic aneurysms are often asymptomatic, but, show that they can compress surrounding tissues and cause chest or back pain, coughing, and dysphagia (difficulty swallowing).

Abdominal aortic aneurysms

• Typically defined as dilations larger than three centimeters.
• They are also usually asymptomatic, but can compress surrounding structures to produce abdominal or back pain. In some patients, a “pulsatile” mass may be present.
• Abdominal aneurysms are associated with smoking, family history of abdominal aneurysm, hypertension, and atherosclerosis.
  — They most commonly occur in males over 65 years; in women, they tend to occur later in life and with worse prognosis.
  — Because the standard definition of an abdominal aneurysm as dilations greater than 3 cm is based on male anatomy, some authors question whether this threshold is appropriate for women, and, whether this definition may lead to underdiagnosis.

• Treatment for aortic aneurysm is geared towards prevention of dissection and rupture, and includes monitoring for aneurysm enlargement, lowering blood pressure, and, where necessary, surgical intervention.

Aortic Dissection

• Dissection occurs when a tear in the tunica intima allows blood to move between the wall layers; indicate that the area where the blood now flows is called a “false channel” or “false lumen.”
• Dissection can produce a sharp “tearing” pain, which may be mistaken for myocardial infarction.
• Complications include aortic valve regurgitation, cardiac tamponade, and internal bleeding, as well as rupture.

4 types of Aortic Dissection
Let’s illustrate four types of aortic dissection, which are categorized via two overlapping systems: the Stanford and DeBakey systems. These systems can be used for treatment and prognostic assessments.
Proximal

• Proximal tears produce Stanford Type A and DeBakey Types I and II dissections:
  — Tears occur in the ascending portion of the aortic arch, and blood can leak through and move along the length of the aorta or remain in the ascending portion.
  Distal
• Tears distal to the left subclavian artery produce Stanford Type B and DeBakey Types IIIa and IIIb: blood can travel as distally as the diaphragm or leave the thoracic cavity and extend all the way down the aorta.

As a simple way to remember this, think of Type A as Proximal, which comes before Type B, which is Distal.

Aortic Rupture

• Can result from aneurysms and/or dissection; it can also occur as the result of trauma, such as motor vehicle accidents.
• Rupture constitutes a medical emergency, since hemorrhaging and shock can be fatal.
• Fluoroquinolones, which are broad-spectrum antibiotics, have been shown to increase the risk of aortic rupture in vulnerable patients (those with hypertension and aneurysms, for example); thus, alternative treatments should be sought in these cases.