Correlation between the Epworth Sleepiness Scale and the Maintenance of Wakefulness Test in patients with narcolepsy participating in two clinical trials of sodium oxybate

Background: In evaluating pathologic sleepiness, the Epworth Sleepiness Scale (ESS) assesses subjective sleep propensity; the Maintenance of Wakefulness Test (MWT) is an objective measure of the ability to stay awake. This analysis evaluated the strength of the correlation between ESS and MWT with regard to absolute values in scores.

Methods: Data were analyzed separately from the intent-to-treat populations of two eight-week clinical trials of sodium oxybate for the treatment of narcolepsy, SXB-15 and SXB-22. For all treatment groups, correlations between ESS and MWT were evaluated at baseline, week four, and week eight using the Pearson product-moment correlation coefficient.

Results: Overall, correlations across all treatment groups in each study described an inverse relationship, reflecting the scoring of each measure (ie, whereas higher ESS scores indicate greater sleepiness, higher MWT scores indicate a greater ability to remain awake). Significant correlations of low-to-moderate strength were observed at all time points in both studies. In SXB-15, correlation coefficients were -0.272, -0.365, and -0.343 at baseline (n = 221), week four (n = 212), and week eight (n = 205), respectively, with all P < 0.0001. Similarly, in SXB-22, correlation coefficients were -0.302 (n = 216), -0.418 (n = 211), and -0.432 (n = 196) at the three time points, respectively, also with all P < 0.0001.

Conclusion: Although all correlations showed statistical significance, they were of low-to-moderate strength. These results indicate that ESS and MWT measure features of pathologic sleepiness that may be distinct, but partially overlapping. These data corroborate those of other studies, suggesting that physiologic mechanisms that regulate alertness and sleep propensity may function somewhat independently.