How is narcolepsy diagnosed?
Diagnosis of narcolepsy is based on the medical history and physical examination. Sleep studies may be helpful. Doctors may diagnose narcolepsy by measuring how quickly the patient falls asleep, and how often rapid eye movements (REM)
are present at or near the onset of sleep.
A specific study called a multiple sleep latency test is a necessary part of the evaluation for narcolepsy. This test must be performed after the person has had an adequate night's sleep. A multiple sleep latency test consists of four 20-minute opportunities to nap, which are offered every two hours throughout the day. Patients with narcolepsy fall asleep in approximately five minutes or less, and move into REM sleep during at least two of the four naps. Normal well-rested sleepers take about 12 to 14 minutes to fall asleep for a daytime nap, and don't fall into REM sleep.
In some cases overnight polysomnography is a valuable means for determining the basic cause of sleepiness. The patient arrives at the sleep center about two hours before bedtime without having made any changes in daily habits. The patient is hooked up to a battery of monitoring devices including electroencephalogram, or EEG, electrocardiogram or ECG, electromyogram, and electrooculogram. These instruments record activity in these organs as the patient passes, or fails to pass, through the various sleep stages. Polysomnography is most useful for ruling out other disorders, such as sleep apnea in people with narcolepsy.
Transcranial magnetic stimulation is an investigative test that uses an instrument that magnetically stimulates part of the brain to produce cataplexy. In one study of patients with narcolepsy, such stimulation caused loss of muscle tone in certain areas when patients were off their medication, but had no effect when they were in treatment.