What is an aortic aneurysm?

What is an Aortic Aneurysm

An aortic aneurysm is a permanent, abnormal dilation of the aorta—the body's main artery—defined as an increase in diameter of at least 50% greater than expected for that aortic segment in unaffected individuals of the same age and sex. 1

Definition and Measurement

The formal definition requires the aortic diameter to be more than 2 standard deviations above the mean for age, sex, and body size (z-score >2). 1 In practical clinical terms:

• Male adults: Aortic root dilation is suspected when diameter exceeds 40 mm 1
• Female adults: Aortic root dilation is suspected when diameter exceeds 36 mm 1
• Indexed measurement: An aortic size index (ASI) >22 mm/m² suggests dilation 1

Anatomical Classification

Aortic aneurysms are described by four key characteristics: size, location, morphology, and cause. 1

Location-Based Types

Thoracic aortic aneurysms (TAA) occur in the chest portion of the aorta:

• 60% involve the ascending aorta and/or aortic root 1
• 10% involve the aortic arch 1
• 30% involve the descending thoracic aorta 1

Abdominal aortic aneurysms (AAA) occur in the abdominal portion, most commonly below the renal arteries. 1, 2

Underlying Pathophysiology

The fundamental mechanism involves medial degeneration with fragmentation of elastic fibers in the aortic wall. 1 This structural weakening allows blood pressure to cause the arterial wall to bulge outward like a balloon. 1

The thoracic and abdominal aorta have marked structural differences due to different embryologic origins, leading to distinct pathologic patterns in each segment. 1

Epidemiology and Risk Factors

Thoracic aortic aneurysms occur in approximately 10.4 to 13.8 cases per 100,000 person-years. 1

Major Risk Factors:

For thoracic aneurysms:

• Hypertension (present in 80% of cases) 1
• Genetic conditions (involved in 20% of cases) 1
• Bicuspid aortic valve (20-30% develop aortic root aneurysms) 1
• Connective tissue disorders: Marfan syndrome, Loeys-Dietz syndrome, vascular Ehlers-Danlos syndrome, Turner syndrome 1
• Chronic obstructive pulmonary disease 1

For abdominal aneurysms:

• Male sex (much more common than in women) 1
• Age over 60 years 3
• Smoking history (strongest modifiable risk factor) 1, 2, 4
• Hypertension 3
• Atherosclerosis 3

Clinical Presentation

Most aortic aneurysms are asymptomatic and discovered incidentally on imaging performed for other reasons. 1, 3 Approximately 30% of AAAs are detected as a pulsatile abdominal mass on routine physical examination. 3

When Symptoms Occur:

Thoracic aneurysms may cause:

• Chest pain 1
• Hoarseness (from left recurrent laryngeal nerve compression) 1
• Stridor (from tracheal/bronchial compression) 1
• Dyspnea (from lung compression) 1
• Dysphagia (from esophageal compression) 1
• Neck and jaw pain (with arch aneurysms) 1
• Back or interscapular pain (with descending aneurysms) 1
• Aortic valve regurgitation leading to heart failure 1

Abdominal aneurysms may cause:

• Abdominal pain 3
• Back pain 3

Catastrophic Presentations:

Rupture presents with sudden severe pain and hypotension, often leading to death. 1 Most people who experience AAA rupture die before reaching the hospital. 1

Dissection represents an acute aortic syndrome where the intimal layer tears, creating a false lumen. 1

Natural History and Growth

The average expansion rate of thoracic aortic aneurysms is 0.10 to 0.42 cm per year. 1

Risk of rupture increases with size:

• Aneurysms 6.0 to 6.5 cm carry a 7% annual risk of rupture 1
• When the aorta reaches 57.5 mm, yearly rates are: 3.6% rupture, 3.7% dissection, and 10.8% death 1
• Risk roughly doubles with every 1 cm of growth over 5 cm 1

Degenerative TAAs grow faster in women than men and carry a three-fold higher risk of acute aortic events. 1

Common Pitfalls

Younger patients (<51 years) with aortic aneurysms are more likely to:

• Present with symptoms (46% vs 6.7% in older patients) 4
• Have larger aneurysms at presentation (6.9 cm vs 6.0 cm) 4
• Have proximal aortic involvement (46% vs 18%) 4
• Have identifiable genetic syndromes (23% vs 0%) 4
• Be cigarette smokers (83% vs 51%) 4

In genetic aortopathies, microscopic changes resemble degenerative changes but occur at a much earlier age. 1