Daytime sleeping image: benzoix/Freepik

What it does and doesn't tell a sleep physician 

The multiple sleep latency test (MSLT) tests for excessive daytime sleepiness by measuring how quickly you fall asleep in a quiet environment during the day. The MSLT is the standard tool used to diagnose narcolepsy and idiopathic hypersomnia. A MSLT immediately follows an overnight polysomnogram (PSG). An overnight sleep study (PSG) is performed to rule out other sleep disorders such as obstructive sleep apnea and periodic limb movement disorder etc as the cause of a patient’s excessive daytime sleepiness. A PSG is also very helpful in investigating insomnia, narcolepsy, idiopathic hypersomnia and restless limb syndromes.

The MSLT is a full-day test that consists of four or five scheduled naps separated by two-hour breaks. During each nap trial, you will lie quietly in bed and try to go to sleep. Once the lights go off, the test will measure how long it takes for you to fall asleep. You will be awakened after sleeping 15 minutes. A series of sensors will measure whether you are asleep. The sensors also determine your sleep stage. When sleep latency (the time it takes you to fall asleep) is below 10 minutes and there is the presence of sleep-onset REM periods (SOREMPs) in two or more of the MSLT naps suggests a diagnosis of narcolepsy, whereas sleep periods lacking rapid eye movement (NREM Sleep) in the various naps suggests a diagnosis of idiopathic hypersomnia.  However, the importance of this differentiation between REM and NREM and other aspects of the MSLT has been called into question and suggests more thorough testing methods need to be applied. Although the MSLT is currently the only test used to diagnosed IH, the MSLT lacks the ability to document the extended, unrefreshing daytime naps that occur in idiopathic hypersomnia, particularly polysymptomatic hypersomnia.  

Prof. Michel Billiard Professor of Neurology at the University of Montpellier France wrote in his paper Idiopathic Hypersomnia “The diagnostic value of the MSLT is somewhat questionable in subjects with the polysymptomatic form of idiopathic hypersomnia. In these cases, awakening the subject early in the morning for the MSLT precludes documentation of the prolonged night time sleep, and the MSLT protocol precludes the recording and observation of prolonged, unrefreshing daytime sleep episodes.” Dr Billiard suggests the correct testing method should be an overnight PSG test followed by an MSLT and then, “from 7:00 PM onward, a 24-hour continuous polysomnography, either at home with an ambulatory system or in the laboratory on an ad lib protocol”. Put simply Prof Billiard suggests the only way to accurately test how a patients excessive daytime sleep affects them is to test the patient in an overall environment, ie: what happens during the nocturnal sleep episode. How the patient responds in the MSLT and how much and for how long a patient will sleep on an “ad lib” basis – totally unrestricted over a 24hr period.  

There have been various papers published questioning the reliability of the MSLT to accurately test for narcolepsy and idiopathic hypersomnia. The two main issues that are discussed is the subject of the MSLT not being the appropriate test for idiopathic hypersomnia as mentioned by Billiard above and that the specificity of multiple SOREMs for Narcolepsy without cataplexy is not reliable. Apart from multiple SOREMs being found in other sleep disorders including sleep apnea they have also been found in other neurological disorders such as Parkinson disease. Interestingly more than 13% of the normal population can also have multiple SOREMs. This is usually as a result of shift work and/or sleep deprivation. Therefore great caution should be taken by doctors when using the MSLT to diagnose Narcolepsy without Cataplexy and Idiopathic Hypersomnia. It is imperative that all other causes of the symptoms are properly ruled out and proper consideration given to the patients clinical history. 

Test-Retest Reliability of the Multiple Sleep Latency Test in Narcolepsy without Cataplexy and Idiopathic Hypersomnia   - Lynn Marie Trotti, M.D., M.Sc., Beth A. Staab, M.D., and David B. Rye, M.D., Ph.D.

The MSLT: More Objections than Benefits as a Diagnostic Gold Standard Commentary on Goldbart et al. Narcolepsy and predictors of positive MSLTs in the Wisconsin Sleep Cohort.  - Geert Mayer, MD, Gert Jan Lammers, MD, PhD

Challenges in Diagnosing Narcolepsy without Cataplexy: A Consensus Statement  Christian R. Baumann, MD; Emmanuel Mignot, MD, PhD; Gert Jan Lammers, MD, PhD; Sebastiaan Overeem, MD, PhD; Isabelle Arnulf, MD, PhD; David Rye, MD, PhD; Yves Dauvilliers, MD; Makoto Honda, MD, PhD; Judith A. Owens, MD; Giuseppe Plazzi, MD;Thomas E. Scammell, MD