Definition of Abdominal Aortic Aneurysm
The abdominal aorta is considered aneurysmal when its diameter reaches or exceeds 3.0 cm. 1
Diagnostic Criteria and Measurement
The definition of an abdominal aortic aneurysm (AAA) is based on specific diameter measurements:
- Normal infrarenal abdominal aorta: up to 2 cm in anteroposterior diameter 1
- Ectatic aorta: between 2 and 3 cm in diameter 1
- Aneurysmal aorta: ≥3 cm in diameter 1
Measurement Techniques
When measuring the aorta to determine if it's aneurysmal, several technical considerations are important:
- Measurements should be taken perpendicular to the longitudinal axis of the aorta 1
- For cross-sectional imaging, maximum aortic diameter should be measured perpendicular to the centerline of the aorta using 3-D and multiplanar reformatted images when feasible 1
- There is no universal consensus on whether measurements should include the aortic wall 1:
- Inner-to-inner (ITI) wall measurements can be 3-6 mm smaller than outer-to-outer (OTO) measurements
- Leading-to-leading (LTL) edge measurements fall between ITI and OTO
It's critical to use a consistent measurement technique and document the method employed 1.
Alternative Definitions
Some sources also define an AAA as:
- A localized dilation of the artery that is 1.5 times the diameter of the normal segment 2, 3
- A segmental, full-thickness dilation with a maximal diameter >1.5 times larger than the adjacent normal segment 1
However, the 3.0 cm threshold remains the most widely accepted standard for clinical practice.
Clinical Implications
The importance of correctly identifying an AAA relates to its natural history and management:
- Small AAAs (3.0-5.4 cm in men, 3.0-4.9 cm in women): Periodic surveillance is recommended 1
- Larger AAAs (≥5.5 cm in men, ≥5.0 cm in women): Elective repair is typically considered 1
Surveillance Intervals
For AAAs that don't meet criteria for intervention, surveillance is recommended at the following intervals 1:
- 3.0-3.4 cm: Every 3 years
- 3.5-4.4 cm: Every 12 months
- 4.5-5.4 cm: Every 6 months
Common Pitfalls in Diagnosis
- Tortuous abdominal aorta or transmitted pulsations from the aorta to a nonvascular mass can mimic an AAA on physical examination 1
- Axial measurements not perpendicular to the centerline can overestimate aortic diameter in tortuous aortas 1
- Inconsistent measurement techniques (ITI vs. OTO) can lead to significant discrepancies in diameter assessment 1
By adhering to the 3.0 cm threshold and using consistent, appropriate measurement techniques, clinicians can accurately identify patients with AAA and implement appropriate management strategies to reduce morbidity and mortality.