REPLY
Subjective Compared with Objective Sleepiness
Selim R. Benbadis, MD, and
Edward Mascha, MD
19 October 1999 | Volume 131 Issue 8 | Page 632
IN RESPONSE:
We appreciate Dr. Schmidt-Nowara's interest and comments. We agree that the lack of correlation between the ESS and the MSLT found in our study may be explained by the dilution effect, whereby some items of the ESS may be strongly correlated with a short sleep latency whereas others may not. However, other investigators also found a poor association between the MSLT and other subjective scales, including the Stanford Sleepiness Scale and a visual analogue scale (1-3). This does not support the contention that the lack of correlation is due to limitations inherent or specific to the ESS (that is, the dilution effect) but rather supports the fact that the discrepancy is between subjective and objective measures of sleepiness.
The methods of the work cited by Dr. Schmidt-Nowara [5] were different from our study and had some limitations: 1) The sleepiness items were qualitative ("yes" for always and often, "no" for infrequently and never) and were not graded [for example, on a scale of 0 to 3], as in the ESS; 2) the "at work" category is arguable, because it is highly dependent on the type of work [for example, falling asleep while driving a truck is not the same as falling asleep while working in a position such as a night watchman]; 3) the association was measured only in terms of sensitivity and specificity; and 4) the cutoff of 8 minutes for the MSLT may be justifiable but is arbitrary and not uniformly accepted. In addition, one particular finding is difficult to explain: Why would falling asleep as a motor vehicle passenger, but not while driving a car, be a reliable predictor of a short sleep latency? This suggests that possible artifactual factors may have played a role.
We certainly agree that sleepiness is a complex and multifaceted phenomenon and that different instruments may be useful for different purposes, but we believe that the "dilution effect" of the ESS does not fully explain the generally modest association found between ESS and MSLT.
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Author and Article Information
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University of South Florida College of Medicine; Tampa, FL 33606 (Benbadis)
The Cleveland Clinic Foundation; Cleveland, OH 44195 (Mascha)
1. Benbadis SR, Mascha E, Perry MC, Wolgamuth BR, Smolley LA, Dinner DS. Association of the Epworth Sleepiness Scale and the multiple sleep latency test in a clinical population Ann Intern Med. 1999;130:289-92.[Abstract/Free Full Text]
2. Schmidt-Nowara WW, Wiggins CL, Walch JK. Sleepiness in an adult population: prevalence, validity, correlates. In: Peter JH, Penzel T, Podszus T, vonWichert P, eds. Sleep and Health Risk. Berlin: Springer-Verlag; 1991.
3. Johns MW. A new method for measuring daytime sleepiness: the Epworth sleepiness scale Sleep. 1991;14:540-5.[Medline]
4. Lavie P. Sleep habits and sleep disturbances in industrial workers in Israel: main findings and some characteristics of workers complaining of excessive daytime sleepiness Sleep. 1981;4:147-58.[Medline]
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