Carotid Stenosis Tool Explanation
The program is based on the results of Cox regression model and estimates the one-year and five-year risks of ipsilateral ischaemic stroke (symptoms lasting longer than 24 hours) on medical treatment in patients with recently symptomatic carotid bifurcation stenosis. The model was derived on patients who had been randomised to medical treatment in the European Carotid Surgery Trial (ECST),1,2 and is detailed elsewhere.3,4 The predictive values of the individual variables included in the model have also been reported previously.5
Risk predictions produced by models such as the ECST model should not be regarded as exact, are not automatically generalisable, and should only be used by experienced clinicians as an aid to clinical decision-making in the wider context of all other relevant information available about the individual patient. The ECST model can be used as a guide to the approximate level of risk of stroke on medical treatment without endarterectomy or angioplasty/stenting. However, the following points should also be borne in mind:
- The model was derived on a population of patients with TIA or non-disabling stroke. The results will not apply to patients with disabling stroke.
- The risk of recurrent stroke is very high indeed during the first few days after a presenting TIA or stroke in a patient with carotid stenosis.6 Few patients were recruited into the randomised trials of endarterectomy during this acute phase. The model has therefore been designed to predict the risk of stroke from day 7 or later. The very early risk of recurrence is not included. Also, because the population on which the model was derived were usually recruited within six months of their last symptomatic event, the model will only accept cases with a time since last event of 180 days or less.
- Patients recruited into randomised trials tend to have a better prognosis than patients in routine clinical practice, which may lead the model to underestimate risk.
- Although antiplatelet treatment (usually aspirin alone) was standard, the majority of patients in the ECST were not treated with statins, which is likely to lead the model to overestimate risk compared with current clinical practice.
- Risk prediction models tend to over-predict at the extremes (i.e. some low risk patients are predicted to be very low risk and some high risk patients are predicted to be extremely high risk). The following limits have therefore been set on the predictions produced by the model: <5% to >20% stroke risk at one year; <10% to >50% stroke risk at five years.
- Not all potentially relevant risk factors were included in the model as they were not available in the ECST. For example, analyses of the North American Symptomatic Carotid Endarterectomy Trial (NASCET) have shown that the risk of stroke on medical treatment is also increased in patients with leukoaraiosis7 and in patients with stenosis of the distal internal carotid artery in addition to carotid bifurcation stenosis.8
- The risk prediction produced by the model is not a prediction of likely benefit from endarterectomy of angioplasty/stenting. The procedural risk and the 1-2% risk of annual risk of stroke after successful treatment7must both be subtracted.
1. European Carotid Surgery Trialists’ Collaborative Group: Randomised trial of endarterectomy for recently symptomatic carotid stenosis: Final results of the MRC European Carotid Surgery Trial (ECST). Lancet 1998;351:1379-1387. [Abstract] [PDF]
3. Rothwell PM, Warlow CP on behalf of the ECST Collaborators: Prediction of benefit from carotid endarterectomy in individual patients: A risk-modelling study. Lancet 1999;353:2105--2110. [Abstract] [PDF]
5. Rothwell PM, Eliasziw M, Gutnikov SA, Warlow CP, Barnett HJM for the Carotid Endarterectomy Trialists’ Collaboration. Endarterectomy for symptomatic carotid stenosis in relation to clinical subgroups and the timing of surgery. Lancet 2004; 363: 915-24. [Abstract][PDF]
6. Lovett JK, Coull A, Rothwell PM, on behalf of the Oxford Vascular Study. Early risk of recurrent stroke by aetiological subtype: implications for stroke prevention. Neurology 2004; 62: 569-74. [Abstract] [PDF]
7. North American Symptomatic Carotid Endarterectomy Trial Collaborators: Benefit of carotid endarterectomy in patients with symptomatic moderate or severe stenosis. N Engl J Med 1998;339:1415-1425. [Abstract] [PDF]
9. Rothwell PM, Eliasziw M, Gutnikov SA, Fox AJ, Taylor W, Mayberg MR, Warlow CP, Barnet HJM for the Carotid Endarterectomy Trialists’ Collaboration. Pooled analysis of individual patient data from randomised controlled trials of endarterectomy for symptomatic carotid stenosis. Lancet 2003; 361: 107-16. [Abstract] [PDF]
10. Rothwell PM, Gibson RJ, Slattery J, Sellar RJ, Warlow CP: Equivalence of measurements of carotid stenosis: A comparison of three methods on 1,001 angiograms. Stroke 1994;25:2435--2439.
11. Rothwell PM, Gibson R, Warlow CP. The interrelation between plaque surface morphology, degree of stenosis and the risk of ischaemic stroke in patients with symptomatic carotid stenosis. Stroke 2000; 31: 615-621. [Abstract] [PDF]