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Objectives: The Distal Motor Function (DMF) sub-score of the NIH Stroke Scale (NIHSS) was measured in the NINDS rt-PA Stroke Trials but is currently not included in the NIHSS. The correlation of DMF with the NIHSS Motor Arm Function (MAF) sub-score, the effect of IV tPA treatment on DMF, and whether adding DMF changes the utility of the NIHSS have not been analyzed.Materials and Methods: MAF and DMF sub-scores were retrieved from the original NINDS rt-PA Stroke Trials for both sides of the body at baseline, 2 hours, 24 hours, 7-10 days, and 3 months after IV tPA treatment. MAF and DMF scores were correlated using Spearman correlation. Clustering of DMF and MAF scores was determined using a Bentler Comparative Fit Index (CFI) to estimate variation in NIHSS when adding DMF. The effect of IV tPA on DMF and MAF was assessed using a linear model comparing changes in scores from baseline to 3 months.Results: MAF and DMF were highly correlated (p < 0.0001) across all time points for both dichotomous and continuous data on both sides. Intravenous tPA accounted for 21% of the change in DMF (p < 0.014, R2 = 0.0157, N = 423) and 39% of the change in MAF (p < 0.093, R2 = 0.0125, N = 547) from 0 to 3 months. On adding DMF to NIHSS, CFI decreased from 0.98 to 0.80 and DMF clustered with MAF, indicating that addition of DMF is unlikely to produce any discrepancy to NIHSS.Conclusions: Including DMF to the NIHSS does not appear to be of additional value. After IV tPA treatment, proximal and distal motor function in upper extremity strongly correlate over time but greater improvement in MAF is noted. Further research is needed on the role of IV tPA on minor strokes with deficits of DMF. [ABSTRACT FROM AUTHOR]