Classification of Aortic Aneurysms
Aortic aneurysms are classified by four complementary systems: anatomic location, etiology, morphology, and size-based definitions, with anatomic classification being the primary framework for clinical decision-making and surgical planning. 1, 2
Anatomic Classification
The anatomic classification divides aortic aneurysms by their location along the aorta and is the most clinically relevant system:
Thoracic Aortic Aneurysms (TAAs)
Aortic root aneurysms involve the sinuses of Valsalva, account for approximately 60% of all TAAs, typically affect younger adults (30–50 years) with equal sex distribution, and are frequently associated with heritable thoracic aortic disease or bicuspid aortic valve (20–30% of cases). 2
Supra-coronary (ascending) aortic aneurysms occur above the sinuses of Valsalva, are predominantly caused by atherosclerosis and hypertension in older adults (59–69 years) with a 3:1 male predominance, and may result from medial degeneration. 2
Aortic arch aneurysms represent approximately 10% of TAAs, often coexist with adjacent ascending or descending aneurysms, are usually atherosclerotic in origin, and carry heightened neurological and cardiovascular surgical risk. 2
Descending thoracic aortic aneurysms account for approximately 30% of TAAs, are mainly degenerative with calcification, though trauma, infection, inflammation, or genetic factors can also contribute. 2
Thoracoabdominal Aortic Aneurysms (TAAAs)
- TAAAs extend from the descending thoracic aorta into the abdominal aorta and are classified for endovascular planning by landing zones numbered 0–11, where Zone 0 involves the ascending aorta to the distal end of the innominate artery origin, progressing sequentially through Zone 11 in the infrarenal abdominal aorta. 1, 2
Abdominal Aortic Aneurysms (AAAs)
Infrarenal AAAs are the most common type of aortic aneurysm overall, with the vast majority being atherosclerotic in origin. 3
Juxtarenal AAAs have their proximal extent at or immediately adjacent to the renal artery origins, requiring precise preoperative imaging with highly overlapping axial CT reconstructions (2 mm increments) to differentiate from high infrarenal aneurysms. 4
Suprarenal AAAs extend above the renal arteries but below the superior mesenteric artery. 4
Etiologic Classification
The underlying cause of aortic aneurysms provides important prognostic and therapeutic information:
Degenerative/atherosclerotic disease is the most common cause of TAAs, associated with hypertension in approximately 80% of cases and predominating in older patients. 2
Genetic/heritable thoracic aortic disease accounts for approximately 20% of TAAs and includes both syndromic forms (Marfan syndrome, Loeys-Dietz syndrome, Ehlers-Danlos syndrome) and non-syndromic familial thoracic aortic aneurysms and dissections. 2, 5
Bicuspid aortic valve–associated aneurysms occur in 20–30% of patients with a bicuspid valve, typically affecting the aortic root. 2
Inflammatory aortitis (Takayasu's arteritis, giant-cell arteritis) represents a rare etiologic group. 2, 3
Infectious (mycotic) aneurysms are caused by bacterial infection of the aortic wall; historic syphilitic involvement is now uncommon. 2, 3
Traumatic aneurysms arise from blunt deceleration injuries or are related to coarctation of the aorta. 2
Morphologic Classification
The shape of the aneurysm has implications for rupture risk:
Fusiform aneurysms demonstrate circumferential dilation and are the predominant morphologic pattern of aortic aneurysms. 2
Saccular aneurysms present as focal outpouching, are often linked to penetrating atherosclerotic ulcers, and constitute a high-risk feature for rupture requiring more aggressive management. 2
Size-Based Definition and Risk Stratification
Size criteria define when dilation becomes aneurysmal and guide intervention thresholds:
An aortic aneurysm is defined as a diameter >1.5 times the predicted normal value (i.e., >50% larger), adjusted for age, sex, and body size, corresponding to a z-score >2. 2
Aortic root dilatation thresholds are >40 mm in males, >36 mm in females, or an aortic size index (ASI) >22 mm/m². 2
The aortic height index (AHI) is now preferred over BSA-based correction because it avoids underestimation in overweight individuals. 2
The term "significant aortic dilation" with explicit diameter cut-offs is preferred over "aneurysm" in contemporary guidelines, since surgical indications often arise before reaching classic aneurysmal dimensions. 2
Additional Risk Stratification Features Beyond Diameter
Saccular morphology with a penetrating atherosclerotic ulcer confers high rupture risk independent of absolute size. 2
Uncontrolled resistant hypertension is a major adverse prognostic factor. 2
Rapid aneurysm growth is defined as ≥3 mm/year for the ascending aorta/arch, or ≥10 mm/year (or ≥5 mm in 6 months) for descending thoracic and abdominal aneurysms. 2
Indexed measurements (ASI or AHI) that relate aneurysm size to patient height/body size improve risk assessment, especially in patients at the extremes of body habitus. 2
Clinical Pitfalls
Differentiating high infrarenal from juxtarenal AAAs requires highly overlapping spiral CT reconstructions (2 mm increments rather than 10 mm), as the 10 mm protocol misses 17% of correct classifications and fails to visualize some renal artery origins. 4
Abdominal and thoracic aortic aneurysms have fundamentally different pathologies and etiologies, with AAAs being predominantly atherosclerotic while TAAs frequently involve medial degeneration and genetic influences, necessitating distinct classification schemes. 6, 3
The relative risk of aortic dissection begins to increase appreciably at a diameter of 4.0–4.4 cm and increases dramatically at ≥4.5 cm, emphasizing that intervention thresholds must account for more than absolute size alone. 1