Original contribution
Comparing MSLT and ESS in the measurement of excessive daytime sleepiness in obstructive sleep apnoea syndrome

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Abstract

Objective

The objective of this study is to compare the use of Multiple Sleep Latency Test (MSLT) and Epworth Sleepiness Scale (ESS) in measuring excessive daytime sleepiness (EDS) in patients with different severity of obstructive sleep apnoea syndrome (OSAS).

Method

Two hundred ninety-six consecutive OSAS patients, with their EDS measured by a Chinese version of ESS and a five-nap MSLT, and their severity of OSAS (determined by respiratory disturbance index) by a nocturnal polysomnogram, were classified into mild (RDI 5-15/h, n=59), moderate (RDI 15-30/h, n=71) and severe (RDI >30/h, n=166) groups, respectively. Their ESS, MSLT and other sleep parameters were compared.

Results

The severe group had significantly shorter mean sleep latency (MSL=6.26±4.90 min) than the moderate (8.26±4.57 min) and mild groups had (8.07±4.37 min). There was no significant difference in their ESS scores.

Conclusion

MSLT is better than ESS in the measurement of EDS in relation to the severity of OSAS in clinical patients.

Introduction

Excessive daytime sleepiness (EDS) is one of the commonest symptoms in patients suffering from obstructive sleep apnoea syndrome (OSAS). EDS is known to be a predisposing factor for accidents, interpersonal problems and reduced productivity [1]. In a simulated laboratory test, half of the OSAS patients showed marked impairment in the Divided Attention Driving Test (DADT; [2]), which could be reversed by nasal CPAP treatment [3].

The measurement of the degree of EDS in patients with OSAS is thus important, as the degree of EDS is correlated with the severity of OSAS. It could also be used as an indicator for treatment response. However, there existed problems in the definition of EDS and in the validation of the measurement tools. Amongst the various methods, the Epworth Sleepiness Scale (ESS; [4]) and the Multiple Sleep Latency Test (MSLT) were commonly used. ESS was first introduced by Johns in 1991 [4]. It is a self-administrated, eight-item questionnaire for the participants to describe or estimate how they doze off inadvertently when engaged in activities involving low levels of stimulation, relatively immobile and relaxed. It does not require any instrumental evaluation. It is quick, inexpensive, flexible and able to measure chronic sleepiness [5]. However, its accuracy was dependent on the participants' interpretation and estimation. It was also reported in previous studies that the ESS score was affected by gender [6], psychological variables [7] and the subjective perception of tiredness and lack of energy [8]. MSLT is an objective measurement of tendency in falling asleep and requires EEG evaluation of the participants. However, it is more expensive, as it is performed in a sleep laboratory, which limits its use as a screening tool.

Previous studies have expressed conflicting opinions in suggesting whether the EDS measured by ESS or MSLT is better correlating to the severity of OSAS and in the relationship between ESS score and the mean sleep latency (MSL; see Table 1; [4], [6], [7], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]). Some of these studies were limited either by their small sample sizes [12], [13], [14], [16], heterogeneity of samples [7] or a retrospective estimation of pretreatment ESS score [12].

Therefore, there remained questions on whether ESS and MSLT was better in the measurement of EDS in OSAS patients. This study aimed to examine the relationship amongst ESS score, MSL and severity of OSAS in a large sample of OSAS patients.

Section snippets

Methodology

Three hundred twenty-two consecutive Chinese patients admitted from January 1997 to September 2000, who were newly diagnosed OSAS (with RDI ≥5), were included in the study. Twenty-six of them were excluded because of the concomitant presence of other sleep disorders, including the REM-sleep behavioural and the narcoleptic spectral disorders. Totally, 296 eligible patients (250 males and 46 females, mean age 44.98±8.75, with a range from 20 to 65) were included. A Chinese version of ESS (C-ESS,

Comparing patients with different severity of OSAS

Fifty-nine (19.9%), 71 (23.9%) and 165 (56.1%) patients were classified as suffering from mild, moderate and severe OSAS, respectively, according to their RDI. The n-PSG parameters of the three groups of different severity of OSAS were shown in Table 2. There were significantly more male patients in the groups with severe OSAS than the other two groups. No significant difference in the age distribution, total bed time, total sleep time, sleep efficiency and sleep latency were found amongst the

Discussion

While both MSL and ESS score were significantly correlated to each other and also with some of the n-PSG parameters, only MSL was found to be significantly correlated with the RDI, the longest apnoeic duration and the arousal index. The MSL was found to be significantly shorter in the group of patients with severe OSAS than in those with mild or moderate OSAS, but the ESS score did not show any difference amongst patients with different severities of OSAS. These results suggested that MSLT was

Presentation

Part of this study was presented in the 6th World Congress on Sleep Apnoea, held in Sydney, Australia, on 12–15th March 2000.

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      However, the number of arousals and the degree of sleep fragmentation may not be the only factors causing EDS as they cannot fully explain the results of MSLT [7–10]. In addition, the apnea–hypopnea index (AHI) and the oxygen desaturation index (ODI) are associated with objective EDS to some extent, albeit the correlation of AHI and ODI with mean daytime sleep latency is weak [8–10]. Conversely, our recent study shows that the severity of the desaturations, quantified by the depth and duration, has a strong association with objective EDS [11].

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    Location of study: Sleep Assessment Unit, Department of Psychiatry, Shatin Hospital, Chinese University of Hong Kong, Hong Kong SAR, China.

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