Aortic Size Index (ASI) in Turner's Syndrome Surveillance
ASI in Turner's syndrome is a critical measurement calculated by dividing the maximal aortic diameter (in centimeters) by the body surface area (in meters squared), used to assess aortic dissection risk in patients ≥15 years old, with values ≥2.5 cm/m² indicating significantly increased risk of aortic dissection. 1
Definition and Calculation
- ASI is calculated by dividing the maximal aortic diameter (cm) by the body surface area (m²) 1
- This index is specifically recommended for Turner syndrome patients ≥15 years old to account for their typically smaller stature 1
- For children <15 years old, Turner syndrome-specific z-scores are preferred instead of ASI 1, 2
Clinical Significance and Risk Stratification
- An ASI >2.0 cm/m² is considered abnormal 1
- An ASI ≥2.5 cm/m² is associated with significantly increased risk of aortic dissection 1
- Patients with Turner syndrome may experience aortic dissection at relatively small absolute aortic diameters due to their short stature 1
- Type A aortic dissection accounts for approximately 85% of dissections in Turner syndrome patients, while Type B accounts for 15% 1
Surveillance Recommendations Based on ASI
- For patients with stable ASI ≤2.3 cm/m²: TTE or MRI every 2-3 years 1
- For patients with ASI >2.3 cm/m²: At least annual surveillance imaging 1
- For patients with ASI approaching 2.5 cm/m²: More frequent monitoring is appropriate 1
- Surveillance frequency should be adjusted based on aortic diameter, growth rate, hypertension severity, and aortic valve function 1
Surgical Intervention Thresholds
- Surgical intervention should be considered when ASI ≥2.5 cm/m² in patients with additional risk factors 1, 2
- In patients ≥15 years old who are obese or have low body weight relative to height, an absolute aortic diameter >4.0 cm may be more accurate than ASI for determining dissection risk 1
Risk Factors That Modify ASI Interpretation
- Bicuspid aortic valve (BAV) 1, 3
- Aortic coarctation 1
- Hypertension 1
- 45,X karyotype (associated with higher ASI values) 4
- Elongation of the transverse aortic arch (associated with aortic sinus dilatation) 5
Imaging Considerations
- Initial evaluation should include both TTE and cardiac MRI 1, 2
- MRI provides better visualization of the entire aorta compared to echocardiography 4, 6
- Echocardiography shows good concordance with MRI for aortic root and ascending aorta measurements but is less reliable for distal segments 6
Pitfalls and Limitations
- ASI alone may be unreliable in some patients - a case report documented aortic dissection in a Turner syndrome patient with ASI <20 mm/m² 7
- Using absolute aortic diameters without indexing to body size may underestimate dissection risk 1
- Echocardiography has lower feasibility than MRI for visualizing distal aortic segments (88% at descending thoracic aorta level versus 100% with MRI) 6
- Surveillance protocols should consider additional risk factors beyond just ASI 1