Diagnosis

These instruments record activity as the patient passes, or fails to pass, through the various sleep stages. One study using polysomnography reported that both healthy patients and those with narcolepsy perform equally during the first 5 to 10 minutes of the test, but after that, patients with narcolepsy show evidence of drowsiness and even indications of sleep. In general, however, polysomnography is most useful for ruling out other disorders, such as sleep apnea, in people with narcolepsy.

Testing Spinal Fluid for Hypocretin

Testing the patient's spinal fluid to detect deficiencies in hypocretin may be a useful method for diagnosing narcolepsy. Low levels may indicate narcolepsy. (Low levels, however, can also occur with brain injury or Guillain-Barre syndrome.) Some researchers believe that measuring hypocretin levels may identify people with early or mild symptoms of narcolepsy (such as cataplexy without altered consciousness). This would help avoid inaccurate diagnoses such as epilepsy or psychosis, which require potent drugs that have significant side effects and are not helpful for patients with narcolepsy.

Investigative Diagnostic Procedures

Transcranial Magnetic Stimulation. An investigative test 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.

Ruling out Other Disorders

Ruling out Psychologic Disorders. In one study, 40% of patients who actually had narcolepsy had been diagnosed incorrectly with some psychological or psychiatric problem. Certainly, patients with narcolepsy have emotional difficulties because of the condition and it is often difficult, particularly for a nonspecialist, to detect the physical problem. Even worse, hypnagogic hallucinations may result in diagnoses of schizophrenia or bipolar disorder, which are treated with potent antipsychotic drugs that have severe side effects and are useless for narcolepsy.

Ruling out Epilepsy. Narcolepsy can easily be mistaken for epilepsy, a group of disorders that cause seizures. Case studies have reported a misdiagnosis of epilepsy in patients who were actually experiencing cataplexy and sleep paralysis.