Multiple Sleep Latency Test and Maintenance of Wakefulness Test
David Moore
S. Justin Thomas
LEARNING OBJECTIVES
On completion of this chapter, the reader should be able to:
1. Recognize the potential consequences of excessive sleepiness.
2. Identify the difference between subjective and objective evaluation of sleepiness.
3. Discuss the indications for performing the multiple sleep latency test (MSLT) and the maintenance of wakefulness test (MWT).
4. Describe the general considerations regarding the MSLT and MWT.
5. Describe the specific protocols for performing and interpreting the MSLT and MWT.
KEY TERMS
Excessive daytime sleepiness (EDS)
Narcolepsy
Idiopathic hypersomnia
Sleep latency
Sleep-onset rapid eye movement period (SOREMP)
The American Academy of Sleep Medicine (AASM) has defined excessive daytime sleepiness (EDS) as sleepiness occurring in a situation when an individual would be expected to be awake and alert (1). Excessive sleepiness is a complex issue that has had a profound impact on society. The cost of accidents due to excessive sleepiness has been estimated at approximately $43.15 to $56.02 billion during 1988 alone (2). Although these figures were criticized as overestimated (3), even a fraction of this amount would be a costly figure. Furthermore, it is important to recognize that when these accidents include the loss of life, the cost is incalculable.
Sleepiness on the job may cause accidents in the workplace. It is estimated that 42% to 49% of commercial vehicle accidents are caused by sleepiness or inattention. Twenty-five percent of Americans perform shift work (most of these in the transportation industry) and 4% work nights (2). Considering that humans are not biologically wired to be nocturnal, attempting to work during one’s biologically natural sleep time produces a tendency to be sleepy while at work. Additionally, shift workers often have to take extreme measures to sleep efficiently during the day. This leads to a generally sleepy working population at risk for accidents. Ironically, most sleep technologists have worked or currently work nights. It is not unusual to hear stories of fellow sleep technologists who have had trouble with sleepiness, including falling asleep while driving home.
The total number of motor vehicle accidents and ensuing fatalities due to sleepiness reported in the literature varies greatly from 1% to 41.6% (2, 4). The variation in these reports may be due to several factors. Until recently, the issue of sleepiness in relation to accidents has not been routinely assessed, and in many cases, the individuals involved are unaware of falling asleep or will not admit to falling asleep at the wheel at the risk of assuming responsibility for an accident. Unlike alcohol-related accidents, there is no blood test to determine one’s level of sleepiness when an accident occurs, and therefore, sleepiness is often merely speculation. However, the association between sleepiness and motor vehicle accidents is undeniable. In clinical situations, large numbers of patients with diagnosed sleep disorders report having had motor vehicle accidents (2). Furthermore, when polled, nine out of ten police officers had pulled over at least one motorist under the suspicion of driving under the influence when the person was actually sleepy. The National Highway Traffic Safety Administration reported that between 2009 and 2013, drowsy driving resulted in an average of 72,000 crashes, 44,000 injuries, and 800 deaths per year (5).
As sleepiness becomes a recognized problem in society, the role of the sleep center in documenting pathologic sleepiness has increased in importance. However, measuring sleepiness is a difficult and often an elusive task. Several scales, such as the Epworth Sleepiness Scale and the Stanford Sleepiness Scale, have been developed to subjectively measure sleepiness. Because of their subjective nature, these scales are used in conjunction with the sleep history and more objective measures to aid in the diagnosis of sleep disorders. Objective measures of
sleepiness are the multiple sleep latency test (MSLT) and the maintenance of wakefulness test (MWT).
sleepiness are the multiple sleep latency test (MSLT) and the maintenance of wakefulness test (MWT).
The MSLT measures one’s tendency to fall asleep. It is used as part of the evaluation of patients with suspected narcolepsy and may be used in evaluating patients with suspected idiopathic hypersomnia (1). The MSLT consists of five nap opportunities, each separated by 2 hours. A shorter four-nap test may be performed; however, this test is not reliable for the diagnosis of narcolepsy unless at least two sleep-onset rapid eye movement (REM) periods (SOREMPs) have been recorded (1). The MWT measures one’s ability to resist the urge to fall asleep. The MWT may be used to evaluate treatment response or to assess patients whose inability to remain awake may constitute a public or personal safety issue (1). The MWT consists of four 40-minute trials, each separated by 2 hours. Currently, the MSLT and MWT are the only objective measures available to the clinician to quantify sleepiness in a patient. They should be used in conjunction with other clinical information to determine the correct diagnosis and best course of treatment. The importance of quantifying sleepiness lies in the impact that sleepiness has on the patient and society as a whole. In this regard, technologists play an important role in the diagnosis of the disorders of excessive sleepiness.
INDICATIONS
The MSLT is indicated in the diagnosis of narcolepsy and may be indicated in the diagnosis of idiopathic hypersomnia. The mean sleep latency (MSL), calculated as the arithmetic mean of the individual sleep latencies for the total number of naps or trials, is used as a component in the diagnosis of the disorders of excessive sleepiness. The difference in the MSL between controls and narcoleptics is statistically significant, with 84% of narcoleptics having an MSL less than 5 minutes (6). REM latency is defined as the time from sleep onset to the first epoch of REM sleep. SOREMPs are defined as a REM onset during an MSLT, with two or more SOREMPs being highly indicative of narcolepsy (6). However, other sleep disorders may produce similar results in an MSLT and, therefore, must be ruled out in the previous night’s polysomnogram. Complications arise when attempting to use the MSLT to diagnose idiopathic hypersomnia. The MSL for idiopathic hypersomnia falls between that of narcoleptics and normal controls and is similar to that of sleepier normal controls (1).
The MWT may be indicated in two instances: determining treatment success, whether it is with medication or continuous positive airway pressure (CPAP), or with one’s ability to remain awake as a measure of safety. However, in both of these cases, normative data are sparse. Because both the MSLT and the MWT assess sleepiness but from two different angles, it has been suggested that they may be performed during the same day to assess more accurately the individual’s level of sleepiness than if they were performed independently (7). This is an interesting proposition that warrants more investigation.
GENERAL CONSIDERATIONS AND PROCEDURES
Before the start of the MSLT or the MWT, several factors must be taken into account. A 2-week sleep diary is helpful in assessing the patient’s sleep schedule before the study. Furthermore, actigraphy is a powerful tool in verifying the sleep diary. Used appropriately, actigraphy in coordination with a sleep diary will validate the patient’s reported sleep, ensure adequate sleep is achieved, and rule out other disorders such as delayed phase syndrome. Once a history of adequate sleep has been obtained and circadian sleep disorders are ruled out as a possible differential diagnosis, an overnight polysomnogram must be performed before the MSLT to rule out other sleep disorders that would affect sleep architecture and produce excessive sleepiness. The prerequisites for the MWT are less stringent, and the need for a sleep diary or a prior polysomnogram is based on the clinician’s judgment (1).
For both tests, a patient’s normal sleep schedule should be considered. Shift workers may need a daytime polysomnogram before a nocturnal MSLT or MWT. Of course, one issue with such a schedule is that competition with the circadian rhythm may be affecting the MSLT or MWT. It may be quite normal, even after sleeping during the day, to experience sleepiness at 4 a.m. (8) and have a lower than normal sleep latency on a nocturnal MSLT.
Medication should also be considered before the performance of the MSLT or MWT. Numerous medications affect levels of sleepiness, including alcohol, caffeine, antihistamines, stimulants, barbiturates and other anticonvulsants, benzodiazepines, and narcotics (9, 10, 11, 12, 13, 14, 15, 16). Furthermore, some medications may need to be stopped for a period so that a valid test could be conducted. Stimulants should be stopped 2 weeks before the performance of the MSLT. REM suppressants should also be stopped if narcolepsy is suspected so that the suppression of possible SOREMPs is prevented (1).
Selective serotonin reuptake inhibitors (SSRIs) suppress REM sleep, and therefore, discontinuation of this medication should be considered before the performance of an MSLT. From a technical standpoint, SSRIs may produce atypical eye movements throughout sleep. Every attempt should be made, in consultation with the
patient’s prescribing primary care physician or psychiatrist, to discontinue these drugs if feasible, practical, and safe in order to fully interpret the data obtained.
patient’s prescribing primary care physician or psychiatrist, to discontinue these drugs if feasible, practical, and safe in order to fully interpret the data obtained.
These considerations may not be necessary for the MWT, particularly if the purpose is to document the effectiveness of the stimulant. Although not required, a urine drug screen may be performed for any substance that might not have been reported. These general procedures should be followed before any MSLT or MWT. Omission of any of these preliminary considerations may lead to questions regarding the validity of the test and final diagnosis.
The recording montage for both the MSLT and the MWT is generally less complicated than a standard polysomnographic (PSG) montage, and in fact, it is more comfortable for the patient and also common practice to remove the unnecessary leads following the overnight polysomnogram. However, there are essential elements to each montage (Table 43-1). They include a frontal electroencephalogram (EEG [F3-M2 and/or F4-M1]), a central EEG (C3-M2 and/or C4-M1), an occipital EEG (O1-M2 and/or O2-M1), a left and right electrooculogram (EOG), a mental/submental electromyogram (chin EMG), and an electrocardiogram (8, 17). For the MSLT, extra EOG leads may also be added to provide a clearer picture of lateral and vertical eye movements (a total of four EOG leads, including one above and one below the outer canthus of each eye). Although not routine, a snoring microphone and/or respiratory channels may be added if indicated (8).
Table 43-1 Multiple Sleep Latency Test and Maintenance of Wakefulness Test Montages | ||||||||||||||||
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