Normal Aortic Anatomy & Indexing - Professor Atef Allam

1. Normal Aortic Anatomy and Indexing

The aorta's diameter varies by age, sex, and body size. Indexing aortic size to body surface area (BSA) or height improves risk stratification, especially in smaller individuals (e.g., women, Asians).

                Normal Diameters (2022 ACC/AHA Guidelines):
                Ascending aorta: ≤3.5 cm (men), ≤3.3 cm (women)
Descending aorta: ≤2.8 cm
Abdominal aorta: ≤3.0 cm

            

Aortic Size Index (ASI)

ASI = Max aortic diameter (cm) / BSA (m²)

ASI ≥2.75 cm/m² (ascending aorta) indicates higher rupture risk
Height-indexing: Some studies suggest aortic diameter/height (cm/m) >1.8–2.0 as a risk threshold

Clinical Implications:

Marfan syndrome: Intervention considered at ASI ≥2.5 cm/m² even if absolute diameter is <5.0 cm
Small-stature patients: Absolute diameter thresholds may underestimate risk; indexing is critical

2. Aortic Imaging Recommendations

Modalities & Frequency

Echocardiography (TTE/TEE):

First-line for ascending aorta (e.g., bicuspid valve, Marfan)
Surveillance: Every 6–12 months if high-risk (e.g., growth >0.3 cm/year)

                CT Angiography (CTA):
                Gold standard for dissection, aneurysms, pre-op planning
Low-dose protocols reduce radiation (e.g., ECG-gated CTA)

            

MRI/MRA:

Preferred for young patients, serial monitoring (no radiation)
4D Flow MRI: Assesses wall shear stress, predicts dissection risk

PET-CT:

For aortitis (e.g., FDG uptake in vasculitis)

                2022 ESC Imaging Guidelines:
                Baseline imaging for all first-degree relatives of aortic dissection patients
Annual imaging if aorta >4.0 cm or genetic aortopathy

            

3. Change in Size Threshold for TAA Repair & Supporting Evidence

Population	Old Threshold	2022 ACC/AHA Threshold	Evidence
General population	≥5.5 cm	≥5.5 cm	IRAD registry data
Bicuspid aortic valve	≥5.0 cm	≥5.0 cm	Increased dissection risk at smaller sizes
Marfan syndrome	≥5.0 cm	≥4.5–5.0 cm	GEN-TAC registry
Loeys-Dietz syndrome	≥4.5 cm	≥4.2–4.5 cm	High rupture risk

Key Studies Supporting Lower Thresholds:

German Marfan Registry (2021): 25% of dissections occurred at <5.0 cm
IRAD Substudy (2022): Bicuspid valve patients had dissections at 4.6–5.0 cm

4. AAA & Thoracoabdominal Aneurysm (TAAA) Guidelines

AAA (2023 SVS Update)

                Intervention Threshold:
                Men: ≥5.5 cm (EVAR preferred if anatomy permits)
Women: ≥5.0 cm (higher rupture risk at smaller sizes)

            

Surveillance:

3.0–3.9 cm: Every 3 years
4.0–5.4 cm: Every 6–12 months

TAAA (2023 ESVS Guidelines)

Open Repair: Still gold standard for extensive Crawford I-III aneurysms
F/B-EVAR: Preferred for high-risk patients if ≥4 experienced centers
Spinal Cord Protection: CSF drainage + staged repair reduces paraplegia risk

Clinical Summary

Aortic disease management is increasingly personalized, integrating imaging, genetics, and advanced endovascular techniques. Key shifts include lower size thresholds for high-risk genetics, TEVAR dominance in TBAD/descending aneurysms, and strict BP control + activity restrictions to prevent progression.