Narcolepsy is a rare disease that causes excessive daytime sleepiness. Its diagnosis relies on some standard sleep tests including polysomnogram and the multiple sleep latency test (MSLT).
What is Narcolepsy?
Among sleep disorders, narcolepsy is the second most common cause of excessive daytime sleepiness after sleep apnea. It is a syndrome that consists of several characteristic features. Aside from the excessive sleepiness, there is also the sudden loss of muscle tone with emotions (called cataplexy), vivid hallucinations around the time of sleep onset (hypnagogic hallucinations), and sleep paralysis. Despite these characteristic findings, only one in three people will have all four symptoms.
The Diagnosis of Narcolepsy
If you suspect you may have narcolepsy, you should be evaluated by your doctor and a sleep specialist. There are other causes of excessive sleepiness, including simply not getting enough sleep, and these should be considered. After a careful evaluation and examination, other sleep studies may be recommended.
The standard for the diagnosis of narcolepsy involves an overnight sleep study called a polysomnogram followed by a multiple sleep latency test (MSLT) the next day. Stimulants should be stopped the week before this testing, and antidepressant medications should be stopped three weeks prior.
The polysomnogram will evaluate the nature of your sleep. Importantly, it will identify other potential causes of your excessive sleepiness, including other sleep disorders such as sleep apnea, periodic limb movements of sleep (PLMS), and REM behavior disorder.
In many narcoleptics,the polysomnogram demonstrates spontaneous awakenings, mildly reduced sleep efficiency, and REM sleep that occurs within 20 minutes of the onset of sleep. Normal people do not enter REM sleep until 80 to 100 minutes after going to sleep.
After the first polysomnogram is complete, the next day a multiple sleep latency test (MSLT) is performed. In the MSLT, or nap study, you are given four or five opportunities to nap every two hours. A healthy person will generally fall asleep in 10 to 15 minutes, but a person with narcolepsy may fall asleep in less than five minutes and will often have REM sleep during their naps. If they have sleep onset REM (SOREM) in two or more of their naps, this is highly suggestive of a diagnosis of narcolepsy.
There are a few laboratory tests that might also be used to diagnose narcolepsy. There is a genetic test (though it is not perfect and may also be positive in people who don't have narcolepsy). In addition, if the sleep studies are negative, sometimes it may be useful to test the cerebrospinal fluid for orexin and hypocretin levels, which may indicate the presence of narcolepsy.
Sources:
American Academy of Sleep Medicine. "International classification of sleep disorders." 2nd ed. 2005.
Carskadon, MA et al. "Guidelines for the multiple sleep latency test (MSLT): a standard measure of sleepiness." Sleep 1986; 9:519.
Thorpy, MJ. “Narcolepsy.” Continuum. Neurol 2007;13(3):101-114.
