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Subjective Compared with Objective Sleepiness

TO THE EDITOR: Benbadis and colleagues (1) show the difficulty of assessing sleepiness in patients. The authors failed to demonstrate a correlation between a widely used subjective rating scale, the Epworth Sleepiness Scale (ESS), and a standard objective test, the multiple sleep latency test (MSLT). It may be useful to mention research that was first reported at a symposium in 1989 (2). One hundred sixteen consecutive patients assessed for sleepiness by the MSLT also answered a standardized set of questions examining the propensity to fall asleep in six different settings. These items were similar to those in the subsequently reported ESS (3) because both instruments were based on the same pioneering work by Lavie (4). Falling asleep was associated with a significantly lower sleep latency when it occurred in a public place, at work, or in a motor vehicle as a passenger but not in the setting of reading or watching television, relaxing after a meal, or driving a car. Positive responses to the first three conditions were relatively specific (79% to 90%) but had low sensitivity (24% to 43%) for an average sleep latency of less than 8 minutes. The ESS assigns equal weight to each of eight settings, including items shown in our study not to be significantly associated with objective sleepiness. The poor diagnostic efficiency of ESS demonstrated by Benbadis and colleagues can be explained as the dilution of the effect of a few useful items by the addition of others that do not correlate with objective sleepiness. Before concluding that ESS and MSLT measure different aspects of sleepiness, it may be appropriate to extend the investigation of this and other scales and items. Furthermore, in recognition of the inescapable trade-off between sensitivity and specificity, it may be appropriate to design different instruments for different clinical purposes such as screening, diagnosis, and treatment monitoring.