Aortic aneurysm & Dissection
Key Definitions
The following are key definitions in aortic vascular pathology:
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 vessel wall histology:
arteries &
veins
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.
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.
By region
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.
- 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 have 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
Treatment for aortic aneurysm is geared towards prevention of dissection and rupture, and includes monitoring for aneurysm enlargement, reducing blood pressure, and, where necessary, surgical intervention.
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
We 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 Dissection
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 Dissection
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 hemorrhage and cardiogenic 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.
For references, please see the full tutorial on Aortic Aneurysm & Dissection
- Radiographic Image Reference: https://commons.wikimedia.org/wiki/File:Descending_(Type_B_Stanford)_Aortic_Dissection.PNG
