What is a Multiple Sleep Latency Test (MSLT)?

What is a Multiple Sleep Latency Test (MSLT)?

The MSLT is an objective polysomnographic test performed in a sleep laboratory where patients are given multiple scheduled opportunities to sleep throughout the day, measuring how quickly they fall asleep and whether they enter REM sleep, primarily used to diagnose narcolepsy and assess the severity of excessive daytime sleepiness. 1, 2

Test Protocol and Methodology

The MSLT involves putting the patient to bed at set intervals (typically 2-hour intervals) throughout the day with instructions to lie quietly and attempt to sleep. 1 Each test session consists of:

• Five nap opportunities scheduled at 2-hour intervals following overnight polysomnography 3, 4
• EEG monitoring to precisely measure the moment of sleep onset and rapid eye movement (REM) sleep onset during each opportunity 1
• Session termination after 20 minutes if no sleep occurs, or 15 minutes after sleep onset 3
• Measurement of sleep latency (time in minutes to fall asleep) averaged across all test sessions 1, 5

Primary Clinical Indications

The MSLT is indicated as part of the evaluation of patients with suspected narcolepsy and may be useful for suspected idiopathic hypersomnia. 4 The test is essential because:

• Mean sleep latency ≤8 minutes with ≥2 sleep-onset REM periods (SOREMPs) is diagnostic of narcolepsy 2
• Mean sleep latency ≤8 minutes with <2 SOREMPs indicates idiopathic hypersomnia 2
• The number of SOREMPs is the critical distinction between narcolepsy (≥2 SOREMPs indicating REM sleep dysregulation) and idiopathic hypersomnia 2

The MSLT is not routinely indicated for initial evaluation of obstructive sleep apnea, assessment of CPAP treatment response, insomnia, or circadian rhythm disorders. 4

Critical Pre-Test Requirements

Proper preparation is absolutely essential for valid MSLT results, as the test cannot distinguish between sleepiness from insufficient nighttime sleep versus organic causes like narcolepsy. 1

Mandatory Documentation Period

• Wrist actigraphy for 7-14 days (or 1-2 weeks) prior to MSLT to objectively document sleep habits and ensure adequate sleep time 6, 7
• Actigraphy is superior to sleep logs, which tend to overestimate sleep time by approximately 1.5 hours per night 1
• In one study, actigraphy was the only modality showing a significant relationship between average nightly sleep duration and mean sleep latency on the MSLT 1

Medication Management

Discontinue medications that affect sleep-wake regulation before testing: 6

• Stimulants (amphetamines, methylphenidate, modafinil) must be stopped as they mask sleepiness 6
• Sedating medications (benzodiazepines, hypnotics, antihistamines) must be discontinued 6

Overnight Polysomnography

PSG must be performed the night immediately before MSLT to document sufficient total sleep time and rule out other sleep disorders like sleep apnea that could cause daytime sleepiness. 2, 4

Interpretation and Diagnostic Criteria

The MSLT measures two key parameters:

1. Mean sleep latency across all nap opportunities
2. Number of SOREMPs (sleep-onset REM periods)

Diagnostic Thresholds

• Mean sleep latency ≤8 minutes indicates objective excessive sleepiness 2
• ≥2 SOREMPs with mean sleep latency ≤8 minutes = narcolepsy (type 1 or 2) 2
• <2 SOREMPs with mean sleep latency ≤8 minutes = idiopathic hypersomnia 2
• Mean sleep latency >8 minutes suggests insufficient objective sleepiness to support a central hypersomnia diagnosis 2

Test Reliability

The MSLT demonstrates excellent inter- and intrarater reliability, with reliability coefficients ranging from 0.668 to 0.964 for various parameters, and mean interrater reliability of 0.883 for clinical diagnosis of narcolepsy. 8

Critical Pitfalls and Limitations

The MSLT cannot be used in isolation to confirm or exclude narcolepsy and must be interpreted in conjunction with clinical findings. 9, 4 Important caveats include:

• Even with optimal criteria (≥3 SOREMPs plus mean sleep latency <5 minutes), 30% of subjects meeting these criteria did not have narcolepsy 9
• The proportion of naps with SOREMPs can vary substantially between repeat MSLTs in the same narcoleptic patient 9
• Patients may minimize nighttime sleep disruption on sleep logs when MSLT results have job status implications, making objective actigraphy crucial 1
• If a patient doesn't sleep during any of the five nap opportunities, this effectively rules out narcolepsy and other central disorders of hypersomnolence 7

Alternative Confirmation Method

If narcolepsy type 1 is suspected, cerebrospinal fluid hypocretin-1 levels ≤110 pg/mL definitively confirm the diagnosis and cannot be falsely positive from sleep deprivation, unlike MSLT results. 2