A retrospective study finds the multiple sleep latency test to be repeatable in diagnosing narcolepsy type 1 but not type 2.

In a retrospective patient study published in the Journal of Clinical Sleep Medicine, researchers set out to examine the repeatability of the multiple sleep latency test (MSLT)—a diagnostic test for individuals with suspected narcolepsy – for narcolepsy type 1 (NT1) and narcolepsy type 2 (NT2).

Often patients haven’t undergone an MSLT, are in different phases of their life than the last time they had an MSLT, or question their diagnosis altogether, according to Chad Ruoff, MD, an author of the study. “Another observation I had,” he says, “is there is little known about the pathophysiology of NT2 [narcolepsy without cataplexy] compared to that of NT1 [narcolepsy with cataplexy]. A lot of these changes in an MSLT diagnosis would go from NT2 to idiopathic hypersomnia (IH) or from IH to NT2.” Because of the frequency of this issue, Ruoff and his team called into question how well the MSLT differentiates between the diagnoses of idiopathic hypersomnia and narcolepsy type 2. And the results showed their hunch was right.

The study results found that cases of NT2 changed to idiopathic hypersomnia 26% of the time, and 57% of the time individuals received a negative MSLT [that is, no diagnosable hypersomnia]. The authors also found that individuals with an original diagnosis of NT1 were 10 to 14 times more likely to receive the same diagnosis compared to the NT2 group (P < 10 -5). “In a clinical setting, a positive MSLT for narcolepsy is a more reproducible and stable feature in NT1 than NT2,” the authors stated.

Ruoff says, “More than ever, based on other studies and our own, we now know that the MSLT is not producing stable findings for NT2 or IH. I think it’s important for anyone performing an MSLT to do it as close to protocol as possible, and if you deviate from that you should put it in the interpretation. When you find a test isn’t as perfect as you thought it was, you have to be more careful when you perform it. Take into account the patient’s normal sleep schedule and perform the MSLT around it. The clinician should also be sure that the overnight polysomnography (PSG) is done prior to the MSLT. All these things make the difference.”

Another approach to diagnosing patients, according to Ruoff, is the use of sleep diaries and actigraphy. Tracking sleep duration prior to an MSLT can shed light on whether the patient has excessive daytime sleepiness simply from self-imposed sleep deprivation. He says, “I think in the future we’ll learn that these things are a vital part in performing a sound PSG/MSLT in looking for hypersomnia.”

When asked what further research needs to be done to better differentiate NT2 and IH, Ruoff notes his original point: that the pathophysiology of NT2, unlike that of NT1, is unknown. “For all we know, NT2 and IH could have the same underlying pathophysiology. So maybe we are artificially differentiating these two conditions even though they may be the same. I would say that maybe we shouldn’t be differentiating them.

“Right now, I believe, the most recent studies done on this topic have been retrospective. What we really need is a large study (funded of course) looking prospectively at the repeatability and stability of the MSLT in NT2 and IH.”

Dillon Stickle is associate editor of Sleep Review.