ReviewMeasurement properties of the Epworth sleepiness scale☆
Introduction
Obstructive sleep apnea (OSA) has been estimated to affect 2–4% of middle aged adults [1]. Excessive daytime sleepiness is an important criterion both for establishing the severity of OSA and for determining the response to specific treatment such as nasal continuous positive airway pressure (CPAP) [2]. Objective assessment of daytime sleepiness with tests such as the multiple sleep latency test (MSLT) is both time-consuming and costly and is not offered by many sleep disorders clinics [3]. Consequently, a questionnaire that reliably quantifies the severity of daytime sleepiness and that is responsive to changes in daytime sleepiness over time would greatly assist clinicians and researchers in the management and investigation of OSA.
Since its publication in 1991, the Epworth sleepiness scale (ESS) has been used by several groups of investigators to measure daytime sleepiness in patients with known or suspected OSA [4]. Furthermore, the ESS has also been used to track changes in daytime sleepiness during treatment of OSA [5], [6]. This usage of the ESS as an evaluative instrument that measures change over time may not be appropriate given the original goals of its development and the consequent design of the questionnaire. Furthermore, the use of the ESS as a diagnostic tool may be premature. The objectives of this review are to determine what the ESS actually measures and whether that measurement truly reflects objective sleepiness, and to determine if the ESS can be used to diagnose pathological sleepiness and follow its response to treatment.
Section snippets
What does the ESS measure?
In order to assess the measurement properties of a questionnaire, one must first ask ‘how were the items in the questionnaire selected?’
The ESS was intended to differentiate persons with excessive daytime sleepiness from alert individuals by measuring their sleep propensity, which has been described as the net interaction of the waking drive and the sleep drive [7]. Previous work has shown that sleep propensity is dependent upon the situation in which it is measured [8]. With this
Does the ESS measure objective sleepiness?
Validity testing asks if an instrument truly measures what it purports to measure [15]. In the case of the ESS, which is used primarily to differentiate among respondents and to assess change in level of daytime sleepiness over time, important components of validity are face validity, content validity, and construct validity. Face validity is present when examination of the items in a questionnaire indicates that those items pertain to the attribute that is being measured. High content validity
Should the ESS be used to diagnose pathologic sleepiness and assess the response to treatment?
Adequate questionnaire validity is not sufficient to recommend its use as a diagnostic tool. The instrument must also be shown to be reliable. Reliability refers to the ability of an instrument to provide similar results when administered on repeated occasions to respondents whose measured attributes have remained stable. A high degree of reliability minimizes background ‘noise’ and allows for any changes recorded over time to be confidently interpreted as reflecting a true change in the
Conclusions
In summary, the ESS seems to measure only one factor and has a high degree of internal consistency. Validity testing does not unequivocally support the use of the ESS as a measure of daytime sleepiness or sleep propensity. However, ESS scores do relate to important clinical outcomes such as road traffic accidents and health-related quality of life. Serious reservations exist regarding the reliability and responsiveness of the ESS, given the lack of test-retest data in patients with OSA. Due to
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Competing interest: None declared