Comparison of stroke risk estimates

COPENHAGEN, DENMARK. There is no evidence that atrial fibrillation (AF) with no underlying structural heart disease (lone AF), as such, is associated with an increased risk of ischemic stroke. However, if advanced age or specific comorbidities carrying their own independent stroke risk enter the picture, then risk may increase significantly over the risks observed in the general population. Two schemes, the CHADS2 and the CHA2DS2-VASc scores, have been developed in order to estimate the effect of age and comorbid conditions on overall stroke risk.

CHADS2 Score
CHA2DS2-VASc Score
Age 75 years or older - 1 point Congestive heart failure - 1 point
Hypertension - 1 point Hypertension - 1 point
Diabetes - 1 point Age 75 years or older - 2 points
Congestive heart failure - 1 point Age between 65 and 74 years - 1 point
History of stroke or TIA - 2 points History of stroke or TIA - 2 points
- Vascular disease - 1 point
- Diabetes mellitus - 1 point
- Female gender - 1 point

The schemes have been validated in several relatively small studies, but it is still not entirely clear how well they correlate with events in the “real world”. A large Danish study now reports on the correlation between stroke risk as predicted by the two schemes and the actual incidence of thromboembolism (ischemic stroke, pulmonary embolism, and peripheral artery embolism). The annual incidence of thromboembolism was calculated 1, 5 and 10 years after discharge from hospital in a group of 73,538 patients with non-valvular atrial fibrillation. The majority (60%) of patients were 75 years of age or older, 34% had high blood pressure (hypertension), 51% were female, 18% had suffered a previous stroke, and 43% had been prescribed digoxin. None of the patients had been prescribed warfarin at discharge.

At year 5 of follow-up, the actual incidence of thromboembolism corresponding to the risk scores was as follows:

CHADS2 Score
Actual Incidence, %/year
0
1.28
1
3.70
2
5.58
3
10.29
4
14.0
5
12.98
CHA2DS2-VASc Score
Actual Incidence, %/year
0
0.69
1
1.51
2
3.01
3
4.41
4
6.69
5
10.42

It is clear that the stroke risk in afibbers with a core of 0, that is, no accompanying risk factors, is low according to both schemes. However, as stroke risk factors are added, incidence increases significantly. In the CHADS2 score, having experienced a previous thromboembolic event was the most significant risk factor, followed by a combination of diabetes and heart failure, and age 75 years or older. In the CHA2DS2-VASc score, a previous thromboembolism was also the most significant risk factor, followed by age of 75 years or older, and a combination of diabetes and heart failure.

The researchers conclude that afibbers with a CHA2DS2-VASc score of 0 are truly at very low risk and do not require antithrombotic therapy. NOTE: The score considers age of 65 years or older a risk factor with a score of 1. It also assigns a score of 1 to female gender.

Olesen, JB, et al. Validation of risk stratification schemes for predicting stroke and thromboembolism in patients with atrial fibrillation: nationwide cohort study. British Medical Journal, Vol. 342, January 28, 2011
Fang, MC. Anticoagulation in people with atrial fibrillation. British Medical Journal, Vol. 342, February 5, 2011, p. 289

Editor’s comment: It is encouraging to see that male, lone afibbers under the age of 65 years with no accompanying risk factors have a very low risk of thromboembolic events and do not benefit from anticoagulation. The study also lends credence to my long-held belief that AF, in itself, is a minor component in overall stroke risk. For example, the risk of experiencing a stroke more than doubles each decade after the age of 55, irrespective of whether AF is also present. Hypertension alone doubles stroke risk, and diabetes is associated with a 2- to 5-fold increase in stroke risk. Furthermore, it should also be kept in mind that only about 15% of all ischemic strokes are cardioembolic – those caused by a clot originating in the left atrium or atrial appendage. In my opinion, nattokinase would be a superior option to warfarin in preventing the fibrin-rich clots involved in cardioembolic strokes.

Finally, while the CHADS2 and the CHA2DS2-VASc scores were developed to predict the risk of ischemic stroke only, the Danish researchers used them to predict not only the estimated risk of ischemic stroke, but also the risk of pulmonary embolism and peripheral artery embolism, thus increasing the incidence of events associated with a specific risk score beyond what would have been observed if only ischemic stroke had been considered.