Original Article
Interconversion of the National Institutes of Health Stroke Scale and Scandinavian Stroke Scale in Acute Stroke

https://doi.org/10.1016/j.jstrokecerebrovasdis.2009.02.003Get rights and content

Introduction

The National Institutes of Health Stroke Scale (NIHSS) and Scandinavian Stroke Scale (SSS) are both validated measures of neurologic impairment and have been used in many acute stroke trials. Methods for interconverting SSS and NIHSS are needed.

Methods

Conversion equations were developed using linear regression (both unadjusted, and adjusted for age and sex) using a random 50% of the data at both baseline and 90 days. The remaining 50% of data were used to test the accuracy of the models produced.

Results

Data from 5 acute stroke trials (2004 patients) were included. Fitted models at baseline were NIHSS = 25.68 – 0.43 ∗ SSS (R2 = 0.57, prediction error [PE] –0.2, P = .20), and SSS = 50.37 – 1.63 ∗ NIHSS (R2 = 0.59, PE 0.2, P = .35). The 90-day models were NIHSS = 22.99 – 0.39 ∗ SSS (R2 = 0.82, PE –0.3, P = .001), and SSS = 56.68 – 2.20 ∗ NIHSS (R2 = 0.80, PE –0.4, P = .08). Adjustment did not materially improve the R2 values.

Conclusion

Total scores for NIHSS and SSS may be interconverted with good precision; the mathematic conversion equations may prove useful in clinical practice and in comparison of data from observational studies and randomized trials.

Section snippets

Data

Data from 5 acute stroke trials included in Virtual International Stroke Trials Archive,6 where both NIHSS and SSS had been recorded at baseline and day 90, were included. Information on age, sex, side of stroke, tissue plasminogen activator use, stroke type, and functional outcome at 90 days (modified Rankin Scale and Barthel Index) was also provided.

Statistical Methods

Conversion equations were developed using linear regression (both unadjusted, and adjusted for age and sex) using 50% of the data selected at

Results

Data from 5 completed acute stroke trials were included, with a mean time to treatment of 7 hours. In all, 2004 patients had baseline data collected on both the SSS and NIHSS. A total of 1628 patients had day-90 data available for both scales, and 1505 patients had both baseline and day-90 data. The patients included are reasonably representative of patients in stroke trials, with a mean age of 66 (SD 11.8) and slightly more men (57%) than women.

Discussion

The results show that NIHSS and SSS can be derived from each other in both directions with reasonable to good degrees of reliability. Mathematic models using data measured at baseline had an acceptable level of fit between the actual and predictive values. In particular, the variation was always less than one point on either scale. The models produced with data collected at 90 days postrandomization had a higher goodness of fit, although converting SSS to NIHSS predicted higher values of NIHSS

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Dr. Gray is supported, in part, by The Stroke Association (UK) and Medical Research Council (UK). Prof. Bath is Stroke Association Professor of Stroke Medicine.

Presented as a poster at the European Stroke Conference in Glasgow in May 2008.

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