How do you test for narcolepsy?
The main test used to help diagnose suspected narcolepsy is an overnight sleep study (called a polysomnogram) followed the next day by a Multiple Sleep Latency Test (MSLT).
A polysomnogram records brain waves, eye movements, muscle tone, and breathing across a night of sleep. This evaluates the amount and quality of night-time sleep and can look for any evidence of an additional sleep disorder (such as sleep apnea or periodic limb movements in sleep).
The MSLT is a series of five scheduled naps spread across the day and it is done the day after the polysomnogram so a doctor can determine whether the prior night’s sleep could be affecting the daytime naps. Every 2 hours, the person is allowed to sleep for 20 minutes or more.
These tests can provide clear evidence of sleepiness and detect abnormal patterns of REM sleep. They also help determine whether other disorders might be contributing to a person’s symptoms.
Before undergoing these tests, a person should ensure they get sufficient sleep on a regular schedule and discontinue any medications that can affect sleep. You may be asked to keep a sleep diary for one to two weeks before the tests or wear an activity tracker that records body movements to estimate sleep so that your doctor can be certain that insufficient sleep is not a contributing factor.
What sleep test findings diagnose narcolepsy?
In most people with narcolepsy, the overnight polysomnogram is often normal. But one-third of patients will enter REM sleep within 15 minutes after falling asleep, which greatly supports the diagnosis of narcolepsy with cataplexy.
The polysomnogram may also show that a person falls asleep quickly – in less than five minutes or has greater-than-normal amounts of light non-REM sleep, but these are not decisive for a diagnosis of narcolepsy. A polysomnogram can also help identify additional sleep disorders such as sleep apnea or periodic limb movements that could contribute to poor quality sleep.
An MSLT that reveals that the person falls asleep rapidly (in less than eight minutes on average) when allowed to nap and enters REM sleep during two or more naps is considered highly suggestive of narcolepsy. In contrast, most people without narcolepsy take much longer to fall asleep during naps, and if they do fall asleep, they rarely enter REM sleep. The two major indicators of narcolepsy that MSLT can highlight is:
- People with narcolepsy can fall asleep quickly, even after a good night’s sleep
- People with narcolepsy have impaired regulation of REM sleep. Normally, REM sleep occurs only during the night, but in people with narcolepsy, it can occur at any time of day.
Sometimes the MSLT results are unclear, and repeat testing may be necessary.
Are there any other tests other than sleep studies that help diagnose narcolepsy?
A genetic marker (called HLA-DQB1*06:02) has been identified in people with narcolepsy that is associated with the disorder. But this gene is only a predisposing factor to developing narcolepsy and is fairly common in the general population, so testing for it will not confirm a diagnosis of narcolepsy but may be helpful if symptoms are atypical.
Levels of hypocretin-1 may also be checked occasionally in spinal fluid. This test is very specific for narcolepsy, as hypocretin levels are low in almost no other condition, but requires a lumbar puncture (spinal tap) to collect the fluid. It may help with the diagnosis of narcolepsy in children, in adults who have unusual cataplexy, or in people who cannot discontinue medications that interfere with the MSLT.
What are the different types of narcolepsy?
Narcolepsy is divided into two major types depending on levels of hypocretin-1
- Type 1 narcolepsy: Cataplexy or low levels of hypocretin-1
- Type 2 narcolepsy: No cataplexy, normal levels of hypocretin-1, and less severe symptoms.
Secondary narcolepsy can also result from an injury to the hypothalamus. Individuals with this disorder often have severe neurological problems and sleep for long periods (more than 10 hours) each night.
- Narcolepsy: Testing. Feb 21, 2018. https://healthysleep.med.harvard.edu/narcolepsy/diagnosing-narcolepsy/narcolepsy-testing
- Narcolepsy Fact Sheet National Institute of Neurological Disorders and Stroke 2021. https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Fact-Sheets/Narcolepsy-Fact-Sheet
- The Early Signs of Narcolepsy. April 21, 2020. Encroe Research Group. https://encoredocs.com/2020/04/21/the-early-signs-of-narcolepsy/
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