MSLT

Multiple Sleep Latency Test (MSLT) is a standardized diagnostic procedure used to quantify daytime sleepiness and to evaluate suspected narcolepsy and other hypersomnia disorders. It is typically performed the day after an overnight diagnostic polysomnography (PSG) to exclude other causes of sleepiness such as sleep apnea or periodic limb movements.

The test consists of four or five nap opportunities, scheduled about two hours apart, in a quiet, dark, sleep-conducive environment. Each nap lasts up to 20 minutes or until sleep occurs. Sleep onset time is recorded as the sleep latency for that nap, and if sleep occurs, the first REM period is noted. The main outputs are mean sleep latency (MSL) across naps and the number of sleep-onset REM periods (SOREMPs). A SOREMP is defined as REM sleep beginning within 15 minutes of sleep onset.

Interpretation is integrated with clinical symptoms. A normal MSL is typically greater than about 8 minutes. Narcolepsy is suggested by a reduced MSL accompanied by multiple SOREMPs; specifically, narcolepsy type 1 often requires a mean sleep latency of ≤8 minutes with two or more SOREMPs. Narcolepsy type 2 may show reduced MSL with one or zero SOREMPs and without cataplexy. Other hypersomnias or sleep disorders may yield different patterns, and a normal or inconclusive MSLT does not exclude narcolepsy if clinical symptoms are present.

Factors that can affect results include prior sleep deprivation, irregular sleep-wake schedules, and medications such as stimulants, antidepressants, or sedatives. Appropriate washout and careful interpretation by a clinician are essential.