NASCET Carotid Stenosis
Percent internal carotid artery stenosis by the NASCET method used to guide carotid revascularisation.
NASCET quantifies internal carotid artery stenosis by comparing the residual lumen at the narrowest point with the diameter of the normal ICA distal to the bulb, the reference used in the landmark surgical trials.
📐 Formula
% stenosis = (1 − residual lumen / distal ICA) × 100
🎯 When to Use
- Grading extracranial ICA stenosis on CTA, MRA or catheter angiography.
- Selecting symptomatic patients for carotid endarterectomy or stenting.
- Comparing across studies where the NASCET convention is required.
⚙️ How It Works
- Measure the minimum residual lumen diameter at the stenosis.
- Measure the diameter of the normal ICA well distal to the bulb, where the walls are parallel.
- Percent stenosis = (1 − residual / distal normal) × 100.
- This differs from the ECST method, which uses the estimated original vessel diameter and yields higher values.
📊 Interpreting the Result
- < 50% — mild stenosis.
- 50–69% — moderate stenosis (surgical benefit is modest, greatest in men and with recent symptoms).
- 70–99% — severe stenosis; endarterectomy clearly benefits symptomatic patients.
- 100% — occlusion; no residual flow, and revascularisation is not indicated.
⚠️ Limitations & Pitfalls
- Near-occlusion collapses the distal ICA, making the ratio unreliable and falsely low.
- Requires a truly normal distal segment; disease there invalidates the denominator.
- Measurement is projection- and technique-dependent.
📚 References
- North American Symptomatic Carotid Endarterectomy Trial Collaborators. Beneficial effect of carotid endarterectomy in symptomatic patients with high-grade carotid stenosis. N Engl J Med. 1991;325(7):445-453.
- Barnett HJM, Taylor DW, Eliasziw M, et al. Benefit of carotid endarterectomy in patients with symptomatic moderate or severe stenosis. N Engl J Med. 1998;339(20):1415-1425.
⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.