Subjective and Objective Indices of Sleepiness (ESS and MWT) are Not Equally Useful in Patients with Sleep Apnea

To understand the relationship between subjective and objective indices of sleepiness, we studied the relationship of the Epworth Sleepiness Scale (ESS) and the Maintenance of Wakefulness Test (MWT) in 41 consecutive patients complaining of snoring and excessive daytime sleepiness. The correlation between ESS and MWT was significant but small (p = -0.39). There was considerable discordance between the two tests. The Lowess fit line between the ESS and the MWT indicates that the ESS falls as the MWT rises to about 4 min. It then stays at a plateau until the MWT rises to about 12 min. Thereafter, it resumes its downward slope as the MWT rises further. Thus, in patients who are severely sleepy on the MWT, the ESS may not be sensitive to different levels of sleepiness. We conclude that the ESS and the MWT are not equally useful in assessing sleepiness in patients with sleep apnea.