Brain wave known as a sleep spindle may help ID narcolepsy

Focusing on sleep spindles, a type of brain wave that’s recorded during sleep studies, may help researchers differentiate type 1 narcolepsy from another condition that causes daytime sleepiness, according to a new study from scientists in Japan.

The team of scientists used automated analyses of sleep studies, or polysomnography, to identify features of sleep spindles in people with narcolepsy. These features differed for many people with insufficient sleep syndrome (ISS) — a disorder whose symptoms may overlap with those of narcolepsy, which is marked by uncontrollable urges to sleep at any time of day.

Because treatment approaches are different for narcolepsy and ISS, an accurate diagnosis is important, the researchers noted. Sleep spindles could provide a quantitative, measurable way to help tell the conditions apart.

“These metrics can be obtained from standard [polysomnography] data, may complement subjective measures such as sleep diaries … and may be clinically applicable without additional testing burden,” the researchers wrote, noting that a “differential diagnosis is often difficult” between narcolepsy and ISS.

The study, “A retrospective exploratory study of sleep spindles as an adjunctive diagnostic marker for narcolepsy type 1: a comparison with insufficient sleep syndrome,” was published in the journal Sleep and Biological Rhythms.

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Narcolepsy type 1 causes excessive daytime sleepiness, among other symptoms. During diagnosis, patients may complete a multiple sleep latency test, known as an MSLT, which measures how quickly a person can fall asleep during scheduled daytime naps. People with narcolepsy typically fall asleep quickly in these tests.

However, this MSLT finding isn’t entirely specific to narcolepsy, and false positives are common.

“In clinical practice, we frequently encounter cases in which patients do not have narcolepsy but still meet diagnostic criteria based solely on MSLT results,” the researchers wrote, calling that issue “MSLT-criteria false positivity.”

“Failure to accurately identify ISS with MSLT-criteria false positivity may lead to misdiagnosis and inappropriate long-term use of central nervous system stimulants,” the team wrote.

Sleep spindles occur during non-rapid eye movement sleep stages

Sleep spindles are a type of brain waves that occur during certain non-rapid eye movement (NREM) sleep stages. Scientists believe these patterns reflect learning and memory consolidation during sleep, and may become dysregulated in sleep disorders. Sleep spindles can be categorized as fast or slow, with the different speeds dominating in different brain areas.

Past studies have established that sleep spindles differ between people with narcolepsy type 1 and healthy controls. However, there is little research about how sleep spindles compare between narcolepsy and ISS, the scientists noted.

“In this study, we analyzed [ sleep spindles] recorded during [polysomnography] in definitively diagnosed [type 1 narcolepsy] patients and ISS patients with MSLT-criteria false positivity and explored whether [sleep spindle] parameters could serve as adjunctive markers to differentiate these conditions,” the team wrote.

The small study involved 14 people with type 1 narcolepsy and nine with ISS, ages 10 to 30. The mean age was about 19 in both groups. The research team assessed polysomnography data from the night before each participant completed the MSLT.

First, sleep experts visually examined the sleep spindles and assigned scores. The analysis didn’t reveal any significant differences between the narcolepsy and ISS groups, which the investigators attributed in part to the small number of participants.

However, significant differences emerged when they used an automated program to identify and measure sleep spindles. This is a more objective approach, though the team noted that the “detected events should be interpreted not as [sleep spindles] in the strict sense, but rather as spindle-like events.”

These results showed differences in fast sleep spindles during two sleep stages, N2 and NREM. Throughout NREM sleep, fast spindles occurred less frequently in the narcolepsy group and lasted longer in people with ISS.

Over 80% of narcolepsy patients correctly classified in small study

Based on these differences, the team developed cutoff thresholds to distinguish between narcolepsy and ISS. Both metrics allowed them to correctly classify 86% of the participants with narcolepsy.

Overall, although larger studies are needed to validate these findings, “the results suggest that the duration of fast [sleep spindles] during stage N2 … and the density of fast [sleep spindles] during NREM sleep … may serve as adjunctive markers for differentiating NT1 from ISS,” the team wrote.

The researchers noted certain limitations of their study, specifically related to the diagnostic methods for confirming narcolepsy, the parts of the brain recorded during polysomnography, and the exclusion of people with narcolepsy type 2.

Altogether, “the retrospective, single-center design and small sample size limit generalizability,” the researchers also wrote, noting a need for “future larger prospective cohort studies including
patients with” both narcolepsy types, ISS, and other sleep disorders.

Still, “quantitative evaluation of … single-night [polysomnography] may complement the MSLT and help distinguish [narcolepsy type 1] from ISS in patients with … and false-positive MSLT findings,” the team wrote.