Very rarely, narcolepsy can be symptomatic of another underlying disorder of the central nervous system, usually a structural lesion involving the region of the hypothalamus. If this is true, there will probably be additional symptoms and signs of endocrine or neurological disorder. Occasionally, epilepsy can be mistaken for narcolepsy, and there have been cases where patients faked narcolepsy (perhaps to get a prescription for drugs).
It's much more common for the reverse to be true: for narcolepsy to be undiagnosed. Indeed, some researchers believe as many as three-quarters of Americans with narcolepsy have not been diagnosed. Narcolepsy is sometimes diagnosed as epilepsy, chronic fatigue syndrome, and schizophrenia.
Recording of patient histories is the key to the diagnosis of excessive daytime sleepiness. A structured history is taken. The Epworth sleepiness scale is often includes in this assessment.
A physical examination is seldom informative. However, obesity and causes of upper airway obstruction may be clues to a diagnosis of obstructive sleep apnoea. Endocrine or neurological signs may point to an underlying neurological cause of sleepiness (other than idiopathic narcolepsy).
Is the patient genuinely sleepy or just tired? The Epworth sleepiness scale is a helpful method of quantifying daytime sleepiness.
Is the patient getting regular and sufficient sleep? Check on bed time, wake time, sleep quality and pattern (for example, shift work)
Does the patient snore heavily or stop breathing during sleep? If so, consider obstructive sleep apnoea
Is there a history of short lived weakness on emotional arousal? If so, consider narcolepsy
Is there a history of uncomfortable, fidgety legs in the evenings? If so, consider restless legs syndrome
Is there evidence of depression (low mood or lack of pleasure in life)? If so, excessive daytime sleepiness might be symptomatic of a mood disorder
Might prescribed or recreational drugs be contributing to sleepiness?
Are there other potentially relevant medical conditions disturbing sleep, such as chronic pain?
Where possible, a history should be taken from the bed partner
Investigations need specialist expertise and equipment and are tailored to the suspected cause of excessive daytime sleepiness. Whether every patient with suspected narcolepsy needs formal evaluation in a sleep laboratory is a matter for debate.
Gathering objective data at the outset by using sleep tests is wise. These tests should include an assessment of overnight sleep quality, typically with polysomnography, and an evaluation of daytime sleepiness on the following day, using the multiple sleep latency test. This test offers the patient four HLA typing and toxicology for amphetamines (in rare cases in which patients are suspected of simulating narcolepsy to obtain a supply) are occasionally helpful. Measurement of hypocretin concentrations in cerebrospinal fluid may become a standard diagnostic tool in narcolepsy, but right now this technique is used only in research.