What is the classification system used for aortic aneurysms?

Classification Systems for Aortic Aneurysms

Aortic aneurysms are classified using multiple systems depending on the clinical context: anatomic location-based systems for aneurysms, the Stanford and DeBakey systems for dissections, and the Crawford system for thoracoabdominal aneurysms. 1, 2

Anatomic Classification by Location

The thoracic aorta is divided into four anatomic segments that guide aneurysm classification 1:

• Aortic root aneurysms: Involve the aortic valve annulus, valve cusps, and sinuses of Valsalva (annulo-aortic ectasia), typically associated with heritable thoracic aortic disease or bicuspid aortic valve 1
• Ascending aortic aneurysms: Affect the tubular portion from the sinotubular junction to the brachiocephalic artery origin, representing approximately 60% of thoracic aneurysms 1
• Aortic arch aneurysms: Extend from the brachiocephalic artery origin through the head and neck vessel origins, accounting for approximately 10% of thoracic aneurysms 1
• Descending thoracic aortic aneurysms: Begin at the isthmus between the left subclavian artery and ligamentum arteriosum, extending to the diaphragm, representing approximately 30% of thoracic aneurysms 1
• Abdominal aortic aneurysms (AAA): Involve the infradiaphragmatic aorta, most commonly the infrarenal segment 1, 3

Stanford Classification System for Dissections

The Stanford system is the primary classification for acute aortic syndromes and guides immediate management decisions 1, 2:

• Type A: All dissections involving the ascending aorta regardless of the site of origin, requiring emergency surgery with in-hospital mortality of 16-27% even with optimal treatment 1, 2
• Type B: All dissections that do not involve the ascending aorta (including isolated arch dissections), typically managed medically unless complications develop 1, 2

This classification prioritizes treatment approach over anatomic detail, as Type A dissections mandate surgical intervention while Type B dissections are usually managed non-surgically 1, 2.

DeBakey Classification System for Dissections

The DeBakey system provides superior anatomical detail for surgical planning and prognostic assessment 1, 2:

• Type I: Dissection originates in the ascending aorta and propagates distally to include at least the aortic arch and typically the descending aorta, requiring surgery 1, 2
• Type II: Dissection originates in and is confined to the ascending aorta, requiring surgery, with patients likely having better long-term outcomes as they may be left without structural aortic wall lesions after surgery 1, 2
• Type III: Dissection originates in the descending aorta and propagates distally, usually managed non-surgically 1, 2
  • Type IIIa: Limited to the descending thoracic aorta 1
  • Type IIIb: Extending below the diaphragm 1

Crawford Classification for Thoracoabdominal Aneurysms

The Crawford system categorizes thoracoabdominal aneurysms based on anatomic extent and influences risk stratification for paralysis after both open and endovascular repairs 2:

• Type I: Extends from proximal to the sixth rib down to the renal arteries, involving the proximal descending thoracic aorta and upper abdominal aorta 2
• Type II: Extends from proximal to the sixth rib below the renal arteries, involving the entire thoracoabdominal aorta, carrying the highest risk of complications 2
• Type III: Extends from distal to the sixth rib into the abdominal aorta, involving the lower descending thoracic aorta and variable portions of the abdominal aorta 2
• Type IV: Extends from below the diaphragm, involving the entire visceral aortic segment and most of the abdominal aorta, with the lowest adverse event rate among all Crawford types 2

Temporal Classification for Acute Aortic Syndromes

Acute aortic syndromes are classified by time elapsed from symptom onset to diagnosis 1:

• Hyperacute: <24 hours 1
• Acute: 1-14 days 1
• Subacute: 15-90 days 1
• Chronic: >90 days 1

Morphologic Classification

Aneurysms are further classified by morphology 1:

• True aneurysm: Permanent localized dilation with at least 50% increase in diameter compared to expected normal, with all three arterial wall layers present (though may be attenuated) 1
• Pseudoaneurysm (false aneurysm): Contains blood from arterial wall disruption with extravasation contained by periarterial connective tissue, not by arterial wall layers 1
• Fusiform aneurysm: Diffuse circumferential dilation 1
• Saccular aneurysm: Focal outpouching of the arterial wall 1

Defining Aortic Dilatation

Aortic dilatation is defined as diameter >2 standard deviations above the predicted mean (z-score >2) for age, sex, and body size 1:

• Ectasia: Arterial dilatation <50% of normal diameter 1
• Aneurysm: Diameter >50% larger than predicted normal 1
• Practical thresholds: In adults, aortic root dilatation is suspected when diameter exceeds 40 mm in men or 36 mm in women, or when indexed diameter/BSA (aortic size index) >22 mm/m² 1

Common pitfall: The term "dissecting aortic aneurysm" is often used incorrectly and should be reserved only for cases where dissection occurs in an aneurysmal aorta, as dissection and aneurysm are distinct pathologies that may coexist but often occur independently 1.