Diagnosing Narcolepsy

If all the common symptoms of narcolepsy are present, diagnosing the disorder is fairly straightforward. However, if sleep attacks are isolated and there is only mild or no cataplexy, making a correct diagnosis is more challenging. It requires excluding carefully insufficient sleep (sleep deprivation), disturbed nocturnal sleep, insomnia, circadian rhythm disorders, sleep-related breathing (sleep apnea) disorders and psychiatric disorders. A general medical check up excluding anemia, hypothyroidism, heart or other general medical issues is essential.

Stanford Sleep Specialists use two main tests to diagnose narcolepsy: the nocturnal polysomnogram and the multiple sleep latency test (MSLT). The polysomnogram continuously records brain waves during sleep, as well as a number of nerve and muscle functions during nighttime sleep. During the test, the narcoleptic will usually fall asleep rapidly, enter REM sleep quickly, and potentially awaken frequently during the night.

For the MSLT, a person is given 4-5 opportunities to sleep every two hours during normal wake times. The specialist uses the test to measure the extent of daytime sleepiness (how fast the patient falls asleep in each nap, also called sleep latency), and also how quickly REM sleep begins, since narcoleptics fall asleep quickly and experience REM sleep early. A positive MSLT (diagnostic for narcolepsy) is obtained when the patient did fall asleep with a mean sleep latency below 8 minutes in the naps, and had at least 2 naps where REM sleep was reached.

In addition to the above, Stanford Sleep Specialists will commonly performed a blood genetic test for narcolepsy if they suspect hypocretin deficiency called Human Leukocyte Antigen (HLA) DQB1*06:02 typing. Almost 99% of subjects where narcolepsy is caused by a lack of hypocretin are DQB1*06:02 positive, but approximately 25% of the normal US population is positive as well so a negative test is used to exclude hypocretin deficiency.

In some selected cases, especially if Human Leukocyte Antigen (HLA) DQB1*06:02 is positive but the case is unclear, a lumbar puncture may be performed, Cerebrospinal Fluid (CSF) drawn, and CSF hypocretin-1 measured. If CSF hypocretin-1 is below 110 pg/ml, the diagnosis is consistent with narcolepsy caused by hypocretin deficiency. This particular test was first developed at Stanford and is the most definitive test for narcolepsy. Other radiological, genetic or biochemical tests may also be ordered in selected cases.