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Forty- Versus 20-Minute Trials of the Maintenance of Wakefulness Test Regimen for Licensing of Drivers

Published Online:https://doi.org/10.5664/jcsm.27393Cited by:25

ABSTRACT

Study Objectives:

Objective assessment of the ability to maintain wakefulness, although very important, is still equivocal. A recent study from our lab has shown that the Maintenance of Wakefulness Test (MWT), when performed with the 20-minute protocol (MWT20), is unreliable in assessing patients who are highly motivated to maintain wakefulness. In this study, we sought to examine whether the 40-minute protocol (MWT40) is a better tool in assessing such individuals.

Methods:

One hundred sixty-four consecutive subjects referred to our sleep lab by the Medical Institute for Driving Safety were studied. All subjects underwent a full-night polysomnogram followed by an MWT, 4 trials of 40 minutes each. All subjects knew that if they failed the wakefulness test their driving license would be revoked.

Results:

Forty-one subjects out of 164 (25%) fell asleep at least once. Of 39 subjects with severe obstructive sleep apnea, (respiratory disturbance index > 40/h), 19 fell asleep (48.7%). Of 13 subjects with a minimum oxygen saturation level below 65%, 7 fell asleep (53%). In the MWT20, only 7% of patients with severe obstructive sleep apnea fell asleep at least once.

Conclusions:

We conclude that the MWT40 is superior to the MWT20 in detecting difficulties maintaining wakefulness in a highly motivated population. However, our results yield a significantly lower detection of difficulties maintaining wakefulness than those reported in healthy subjects, suggesting that the MWT40 is also highly affected by motivation. We believe that, for a highly motivated population (such as for a driver’s license validation), different average sleep-latency threshold should be used than in general population.

Citation:

Arzi L; Shreter R; El-Ad B; Peled R; Pillar G. Forty- versus 20-minute trials of the Maintenance of Wakefulness Test regimen for licensing of drivers. J Clin Sleep Med 2009;5(1):57-62.

INTRODUCTION

Prevention of motor vehicle crashes is a challenge for authorities worldwide. Many motor vehicle crashes, in particular fatal ones, are caused by drivers who suffer from excessive daytime sleepiness (EDS) and fall asleep while driving. EDS may result from various sleep disorders, such as obstructive sleep apnea (OSA), insomnia, jet lag, sleep deprivation, narcolepsy, and others. EDS impairs the individual’s daytime performance and may lead to motor vehicle crashes and work-related accidents.15 A case-control study by Teran-Santos et al6 of groups of drivers who were involved in motor vehicle crashes, determined that drivers who suffer from OSA (more than 10 apneas or hypopneas per hour of sleep) have a higher likelihood of being involved in motor vehicle crashes. Treatment of OSA with continuous positive airway pressure) can reduce the likelihood of these crashes.79 Therefore, diagnosing OSA and treating these patients is an important public health issue.

It is difficult to objectively and accurately assess daytime sleepiness and maintenance of wakefulness.10,11 Although the quality of sleep and the severity of sleep disorders can be objectively quantified, there is no clear correlation between any of these variables and the degree of daytime sleepiness, as assessed by the currently available tests.1215 These tests consist of subjective questionnaires such as the Epworth Sleepiness Scale (ESS)1618 and objective methods such as Multiple Sleep Latency Test (MSLT),19,20 the Maintenance of Wakefulness Test (MWT),19,21 and the more recently introduced Psychomotor Vigilance Test.22,23

During the MSLT, the patient is instructed to try to fall asleep during five 20-minute daytime trials. During the MWT, the subject is instructed to stay awake in soporific circumstances during four 20- or 40-minute trials. Thus, the MSLT quantifies the drive to sleep whereas the MWT evaluates the ability to maintain wakefulness. It is quite common for people who fall asleep during the MSLT to manage to maintain wakefulness during the MWT.24 Hence, logically, the MWT is the preferred test for examining the ability of drivers to remain alert, in cases of suspected EDS.2528

The diagnostic criteria for the MWT were determined in 1997 by Doghramji et al.29 In research conducted on 64 healthy subjects, a score of less than 2 standard deviations from the average was considered diagnostic of a severe problem in the ability to maintain wakefulness. The threshold value for the MWT20 protocol was lower than 10.9 minutes and, for the MWT40, was lower than 19.4 minutes. In a similar trial by Banks et al,30 the threshold value for the MWT40 was 26.1 minutes. However, these tests were performed in experimental settings with no potential real-life factors that may affect the results.31,32 The clinical use of the MSLT and MWT have recently been thoroughly reviewed by Arand et al.19 Among other conclusions, they suggest that the MWT is affected by many factors besides daytime sleepiness and that it doesn’t discriminate well between normal populations and patients with sleep disorders.19

In a recent study from our lab, we reported that, under conditions of high motivation (i.e., when failing the MWT may lead to losing one’s license to drive), only 5 out of 54 potentially sleepy drivers fell asleep in any of the 5 trials of the MWT20.33 The calculated average MWT scores were higher than in any of the previously published papers. Some of the patients in that study had severe untreated OSA, and others had been involved in a previous sleep-related crash. From the subgroup of 32 subjects with severe untreated OSA, only 1 fell asleep. We, therefore, concluded that motivation had a significant effect on the MWT20 and that the test is not reliable in the evaluation of sleepiness in highly motivated subjects. Thus, in the current study, we sought to assess whether the MWT40 regimen is a more reliable tool to quantify difficulties maintaining wakefulness in a motivated population. A study by Bonnet et al34 reported that financial reward could not motivate 12 healthy volunteers to increase their sleep latency during the MWT40 protocol, suggesting that the ability to stay awake during the MWT40 is less likely to be affected by motivation and is a better measure of alertness than the MSLT. In the current study, we utilized a different motivator (the threat of losing a license to drive) and sought to compare the MWT40 (tested on a new dataset of patients) with the MWT20 regimen (using previously reported data), in a population of potentially sleepy patients.

METHODS

Subjects

The study was consisted of 164 newly tested consecutive subjects who were referred to the Technion sleep lab from June 2006 until January 2007 by the Medical Institute for Driving Safety of the Ministry of Transportation. The subjects were referred for evaluation of their ability to maintain wakefulness for the purpose of renewal of their license to drive. Many of the subjects were drivers who were previously diagnosed with a sleep disorder. Among them, 39 subjects had OSA, 10 who were already being treated with nightly continuous positive airway pressure. Other subjects were suspected of having EDS due to variety of reasons, including snoring, obesity, or previous involvement in a sleep-related motor vehicle crash. Seventeen of the subjects were professional drivers, 1 was an aviation pilot. All subjects were referred to our center for the purpose of undergoing an MWT, and all were informed that, if they failed the test (i.e., fell asleep), they may lose their license to drive. Data from these patients were compared with data in a previously reported paper with similar recruitment method and similar population, tested by the MWT20 protocol.33

Study Protocol

All participants filled out the Technion sleep questionnaire35,36 and the ESS. On the night before the MWT, the subjects underwent a medical interview and then a complete full-night polysomnogram. On the following day, the MWT40 was performed.

Test Procedures

Overnight Polysomnography

Each patient was studied in a private room. The recording included 2 electroencephalography (EEG) channels (C3-A2, O2-A1), 2 electrooculography (EOG) channels, an electromyography (EMG) channel (submental), leg movements (tibialis anterior EMG), an electrocardiography (ECG) channel, airflow (both thermistor and nasal pressure), respiratory effort (thoracic belt), oxygen saturation (pulse oximeter), and quantitative snoring intensity (by a dB-meter placed 1 meter above the bed). A technician viewed the patients during the night by a closed-circuit monitor. Bedtime was between 22:00 and 23:00, and the patients were awakened between 06:30 and 07:00, to start the MWT at 08:00.

The results of the recording were scored by an experienced technician based on the Chicago criteria with the following definitions: apnea was defined as a cessation of airflow for 10 seconds or more. Hypopnea was defined as any notable decline in airflow that was accompanied by an arousal (defined as an appearance of 3-second alpha wave or frequency change in the electroencephalographic or an increase in submental electromyographic signal) or a decline in the oxygen saturation level (at least 3%). The respiratory disturbance index (RDI) was calculated by dividing the total number of respiratory events by the total sleep time. The minimum oxygen saturation was the lowest saturation value due to a respiratory event (excluding artifacts) and the maximum snoring intensity was the maximum recorded volume in decibels that was due to snoring (excluding speech, cough, or other noises).

The MWT

Each of the subjects was placed in a private, dimly lit, and quiet room and was told to sit down in bed (in a reclining position, with the back at about 45° to the bed surface) and to maintain wakefulness for 40 minutes. The subject was not allowed to perform any unusual action to maintain wakefulness (such as read, talk, or make repetitive movements).The recording included 2 EEG channels, 2 EOG channels, an EMG channel placed on the submental muscle, an ECG channel, and quantitative snoring intensity. The trial was stopped if the subject fell asleep or if the subject maintained wakefulness for 40 minutes. For the purpose of terminating a study, sleep was defined as 3 consecutive 30-second epochs of stage 1 sleep or any epoch of any other sleep stage. For the MWT score, sleep latency was calculated as the latency to the first epoch of any sleep stage.

During the test, 4 such trials were performed at 08:00, 10:00, 12:00, and 14:00. Subjects were given breakfast and lunch between the trials. Caffeine-containing drinks were not allowed, nor were any drugs other than those medically prescribed.

For each trial, the sleep latency was recorded (the time was 40 minutes if the patient maintained wakefulness). The calculated score of the test was the average score of all 4 trials.

RESULTS

Table 1 summarizes the demographic and nocturnal polysomnography variables of the 164 subjects. The average age was 50 ± 14 years, the average body mass index was 35 ± 6.5 kg/m2, and the average RDI was 25 ± 23. About a quarter of the participants had substantially disturbed nocturnal sleep, such as total sleep time of less than 5 hours with no rapid eye movement sleep detected, or a sleep latency of greater than 4 hours. More than 20% of the participants had severe OSA, with an RDI of up to 85 per hour.

Table 1 Demographics and Nocturnal Variables

VariableMeanSDRange
Age, y50.6614.1020–78
Weight, kg107.6022.1150–165
BMI, kg/m235.176.4917.76–52.08
Sleep latency, min28.5234.510–285
Sleep period time, min443.9633.10338–528
TST, min343.9869.59117–480
RDI, no./h25.4623.050–85
Min Sao2, %84.0411.651–100
Sleep stage, % of TST
261.6614.127–95
3–417.0912.50–56
REM20.987.80–41

Abbreviations: BMI, body mass index; TST, total sleep time; RDI, respiratory disturbance index; REM, rapid eye movement.

Forty-one out of the 164 subjects (25%) fell asleep in 1 or more of the MWT40 trials: 15 (9.1%) fell asleep in only 1 trial, 11 (6.7%) in 2 trials, 5 (3%) in 3 trials, and 10 (6.1%) fell asleep in all 4 trials (Figure 1). In the MWT20 study33 that was conducted on a similar population, only 5 out of 54 subjects fell asleep in any of the trials (9.2%, p < 0.05) (Figure 2).

Figure 1
Figure 1

Number of falling-asleep cases among 164 subjects who underwent the Maintenance of Wakefulness Test 40-minute protocol (MWT40): 123 (75%) never fell asleep, 15 (9.1%) fell asleep during 1 trial, 11 (6.7%) during 2 trials, 5 (3%) during 3 trials, and 10 (6.1%) during all 4 trials.

Figure 2
Figure 2

Number of falling-asleep cases among 54 subjects who underwent the Maintenance of Wakefulness Test 20-minute protocol (MWT20): 59 (90.7%) never fell asleep, 2 (3.7%) fell asleep once, and 3 (5.6%) fell asleep 2 or more times (up to 5 trials). Data retrieved from Shreter et al.33

By grouping the subjects according to their RDI (Table 2), it can be seen that, in the severe OSA group (RDI > 40), 19 out of 39 subjects (48.7%) fell asleep at least in 1 of the MWT40 trials, and 11 (28%) fell asleep more than once. When we look at the 3 lower severity categories, the percentage of subjects who fell asleep is significantly smaller (p < 0.05 between the subjects with an RDI > 40 and other groups). In the MWT20 trial,33 there was no clear association between RDI score and the number of subjects who fell asleep (Table 3).

Table 2 Results Sorted by RDI in the 40-Minute MWT Protocol

RDI< 55 – 1920–39≥ 40
Subjects
    Total, no.33503839
    Fell asleep, no. (%)5 (15.15)10(20.00)5(13.16)19(48.72)
        Once2418
        Twice1324
        3 times2327
    Stayed awake, no.28403320
Sleep latencya38.1837.6938.0934.93

Abbreviation: RDI, respiratory disturbance index.

aMean sleep latency on the Maintenance of Wakefulness Test, 40-minute protocol (MWT40).

Table 3 Results Sorted by RDI During the MWT20a

RDI< 55 – 1920 – 39≥ 40
Subjects, no.
    Total14141214
    Fell asleep, no. (%)1(7.14)2(14.29)1(8.33)1(7.14)
    Stayed awake, no.13121113
Sleep latency, min19.9219.9019.6419.67

Abbreviation: RDI, respiratory disturbance index.

aData retrieved from Shreter et al.33

aMean sleep latency on the Maintenance of Wakefulness Test, 20-minute protocol (MWT20).

In Tables 2, 4, and 5, we sorted and grouped our data according to different medical and nocturnal parameters. Because data were missing on a few of our subjects in Table 2, data from 4 patients were excluded; in Table 4, data from 5 patients are excluded; and, in Table 5, data from 3 patients are excluded.

Table 4 Results Sorted by Minimum Oxygen Saturation Level During the MWT40

Saturation, %95–10085–9465–84< 65
Subjects
    Total, no.20834313
    Fell asleep, no (%)3(15.00)16(19.28)12(27.91)7(53.85)
        Once2544
        Twice0621
        ≥ 3 times1562
Stayed awake, no.1767316
Sleep latency, mina38.9437.4836.136.18

aMean sleep latency on the Maintenance of Wakefulness Test, 40-minute protocol (MWT40).

Table 5 Results Sorted by BMI in MWT40

BMI, kg/m2< 2525 – 3031 – 40> 40
Subjects
    Total, no.10189637
    Fell asleep, no. (%)2(20.00)3(16.67)24(25.00)11(29.73)
        Stayed awake, no.8157226
Sleep latencya38.1838.0136.5337.64

aMean sleep latency on the Maintenance of Wakefulness Test, 40-minute protocol (MWT40).

Abbreviation: BMI refers to body mass index.

Table 4 shows the data sorted according to the minimum oxygen saturation level recorded during the polysomnogram. The results show that, when the minimum saturation was 95% to 100%, only 3 out of 20 subjects (15%) fell asleep in any of the MWT trials, whereas, in the group with severe OSA (minimal oxygen saturation < 65%), 7 out of 13 subjects fell asleep (53%, p < 0.05).

Dividing the subjects into subgroups according to their body mass index (Table 5) demonstrates that there was an increased tendency to fall asleep during the MWT40 with increasing BMI, although the differences were not statistically significant.

DISCUSSION

Our study shows that the MWT40 protocol is superior to the MWT20 protocol in detecting difficulties to maintain wakefulness in a highly motivated albeit sleepy population. Although fewer than 10% of potentially sleepy subjects fell asleep in any of the MWT20 trials, with the MWT40, this proportion more than doubled, reaching 25%.

The purpose of this study was to evaluate the ability of the MWT40 to detect failure to maintain wakefulness in people with suspected EDS and high motivation to show normal daytime alertness. Evaluating daytime sleepiness has always been a challenge. In our study, the challenge was even greater because our subjects were highly motivated to maintain wakefulness. Although the MWT20 protocol indicated low resolution ability of the test (only 10% of potentially sleepy subjects fell asleep in any trial, with no association between falling asleep during the daytime test and any nocturnal sleep parameters), with the MWT40 regimen this figure increased. Although it is very challenging to quantify the ability to maintain wakefulness, and although we do not have a gold standard to which we can compare our results, our study showed that, with the MWT40, more subjects fell asleep, with a significantly higher proportion of patients with severe OSA falling asleep. These findings suggest that the MWT40 protocol is superior to the MWT20 protocol in detecting subjects unable to stay awake. Furthermore, in our current study, we found a significant negative correlation between RDI or nocturnal sleep disruption and the ability to maintain wakefulness, which was not found with the MWT20 protocol. It is a commonsense assumption that, with longer trials, it becomes harder for sleepy patients to maintain wakefulness, even when highly motivated.

Obviously the MWT40 is not perfect. In a study of healthy subjects reported by Doghramji et al,29 the average mean sleep latency was 35.2 ± 7.9 minutes and the lower normal limit was below 19.4 minutes. In contrast, the mean sleep latency of our subjects who were suspected of having EDS was 37 ± 6.6 minutes. Only 8 of our subjects had a sleep latency shorter the reported low-normal cutoff value of 19.4 minutes, 4 of whom had severe OSA (all were untreated), 1 had restless legs syndrome, and three had no previous diagnosis. The fact that our subjects were sitting in a reclining position (rather than lying down) further emphasizes these results. Thus, we believe that high motivation of the subjects may still contribute significantly to their ability to maintain wakefulness. Our subjects’ results indicate that they are even more alert than healthy subjects without sleep disorders but also without high motivation to stay awake. Although Bonnet et al 34 found that financial reward could not motivate 12 healthy volunteers to increase their sleep latency in the MWT40 protocol, we believe that losing a driver’s license is a stronger motivator than a monetary one, and healthy volunteers may be differently affected than patients by motivation. Although the mean MWT sleep latency in our highly motivated subjects is not shorter than the mean sleep latency reported in normal subjects, as determined by Doghramji et al,27 we still believe that the average sleep latency is a good parameter for identifying people with difficulty maintaining wakefulness. We think that greater importance should be attributed to the number of trials in which subjects fall asleep. Comparing numbers of trials containing sleep among the general population, people with sleep disorders, and highly motivated populations and correlating the MWT results with outcomes (i.e., motor vehicle crashes) needs to be supported by future research.

Our study had several limitations. One of them is the lack of a control group of healthy subjects and another is the lack of outcome data. We have scored hypopneas based on the Chicago criteria, which probably has resulted in higher RDIs than if we had scored according to the new American Academy of Sleep Medicine guidelines. The main general limitation is the lack of a “gold standard” test, an objective way to determine the “real” tendency of these subjects to fall asleep. Comparing the results of the MWT40 with the results of the MWT20 in different cohorts is also not ideal. However, although different subjects were studied, their general characteristics were very similar and did not differ between the cohorts (i.e., they were both referred by the Medical Institute for Driving Safety of the Ministry of Transportation to evaluate their ability to maintain wakefulness for the purpose of renewal of their license to drive, and they were of similar age: 50.7 versus 49.2 years, NS; had similar body mass index: 35.2 vs 34 kg/m2, NS; and had similar RDIs: 25.5 versus 26.3 per hour, NS. Only the time of study really differed between the cohorts, without any meaningful change in referral characteristics. It therefore seems that there is a substantial effect of the test protocol itself. Thus, we believe that our results preclude using the MWT20 protocol for the purpose of driver’s license validation in the future. Whether the Psychomotor Vigilance Test or any other objective measure of the ability to maintain wakefulness is better than the MWT40 remains to be determined. In addition, it is difficult to quantify motivation. Although we believe that losing one’s license to drive highly motivates a subject to stay awake, individual differences are always an issue.

In conclusion, despite these limitations, we believe that our study shows that, for the evaluation of drivers with suspected EDS, the MWT40 is a more appropriate test than the MWT20. The test, however, is still affected by motivation and, as such, is not completely reliable. The evaluation of EDS is still challenging. Setting a threshold for study failure or developing other methods should be further explored.

DISCLOSURE STATEMENT

This was not an industry supported study. Dr. Pillar has received research support from Cephalon and Actelion and has consulted for Itamar. The other authors have indicated no financial conflicts of interest.

ABBREVIATIONS

BMI

Body Mass Index

ECG

electrocardiography

EEG

electroencephalography

EMG

electromyography

EOG

electrooculography

ESS

Epworth Sleepiness Scale

MSLT

Multiple Sleep Latency Test

MWT

Maintenance of Wakefulness Test

OSA

Obstructive Sleep Apnea

PSG

Polysomnography

PVT

Psychomotor Vigilance Test

RDI

Respiratory Disturbance Index

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