Abdominal Aortic Aneurysm (AAA): Pathogenesis
Different parts of the aorta have different embryologic origins
Atherosclerosis
Hypertension
Age > 65
Progressive deterioration of aorta structural integrity over life span
Connective Tissue Disease
Structurally abnormal protein or protein organization in aorta
Autoimmunity
Infection (e.g Chlamydia, Mycoplasma pneumoniae, Helicobacter pylori, human cytomegalovirus, herpes simplex virus)
Antigens (substance that causes immune response) on virus or bacteria resemble local proteins in abdominal aorta
Antibodies produced in response to infection inappropriately target host cells in the aorta
Antibodies tag cells in the abdominal aorta for destruction by T-lymphocytes
Immune-mediated destruction of aorta
Smoking
Genetics
Unclear mechanisms
Subacute (not clinically detectable) inflammation of aortic tissue
Inflammatory cytokines are released and immune cells are recruited
↑ pressure on aorta and other vessel walls
Infiltration of vessel wall by lymphocytes and macrophages
Production of enzymes that break down elastin & collagen proteins (which provide most tensile strength to aorta)
Aorta susceptible to damage
Degradation of aortic connective tissue
Biomechanical stress on vessels
Authors: Olivia Genereux Davis Maclean Reviewers: Jason Waechter* Amy Bromley* Yan Yu* *MD at time of publication
The exact mechanisms are complex, debatable, and an area of intensive research – the 3 mechanisms and associated pathophysiology presented here are generally thought to be the main causes of abdominal aortic aneurysms
Infrarenal aorta has poorly developed vaso vasorum (dedicated blood supply to vessel wall)
Infrarenal aorta relies solely on nutrient diffusion from aortic blood that crosses abdominal aorta
Infrarenal aortic wall has fewer “lamellar” units (fibromuscular units) than other regions of the aorta
Infrarenal aorta is less elastic & less able to distribute stress
Loss of smooth muscle cells & thinning of tunica media
Destruction of elastin in tunica media
Normal layers of the aortic wall
↓ aortic tensile strength (ability to withstand stretching) Aorta expands and dilates due to internal pressure
Tunica Intima (inner-most tissue layer of aorta)
Tunica Media (layers of elastic
tissue (elastin) and muscle fibers)
Adventitia (thin outermost collagenous layer)
(longitudinal section)
Aortic aneurysms are usually infrarenal (85%)
Abdominal Aortic Aneurysm
Infrarenal aorta more prone to ischemia and has impaired repair potential
Abnormal, irreversible dilation of a focal area of abdominal aorta (area of aorta between diaphragm & aortic bifurcation) to twice the diameter of adjacent normal artery segment
Legend:
Pathophysiology
Mechanism
Sign/Symptom/Lab Finding
Complications
Re-Published February 27, 2021 on www.thecalgaryguide.com