Glossary of Terms

A sudden, generally brief (<2 minutes) loss of muscle tone, with retained consciousness, triggered by strong emotions.1,2
A useful mnemonic to recall the 5 major narcolepsy symptoms; it stands for Cataplexy, Hypnagogic hallucinations, Excessive daytime sleepiness, Sleep paralysis, and Sleep disruption.3
Clinical interview
Comprehensive assessment of excessive daytime sleepiness and other symptoms suggestive of narcolepsy generally starts with a clinical interview and includes observation and queries on the frequency, nature, and impact of the sleepiness.4,5  It is helpful to include the patient's spouse, family member, or bed partner as part of the clinical interview to corroborate or refute the patient's report of sleepiness.4,5 A complete clinical interview assessing the presence of narcolepsy symptoms, and especially history of cataplexy, is essential for making a diagnosis of narcolepsy with cataplexy.1,4,5 A complete interview would include medical, psychiatric, family, and psychosocial history.
CSF hypocretin-1 levels
Hypocretin is thought to be a wake-promoting and sleep-wake state stabilizing neuropeptide.6,7  Narcolepsy type 1 is caused by hypocretin deficiency, thought to be due to a selective loss of hypothalamic hypocretin-producing neurons.1,6,8,9 Measurement of a cerebrospinal fluid (CSF) hypocretin-1 level <110 pg/mL is a highly specific and sensitive finding for the diagnosis of narcolepsy type 1.1,10 
Epworth Sleepiness Scale
The Epworth Sleepiness Scale (ESS) is a validated subjective scale that measures an individual’s propensity to doze or fall asleep during 8 common daily activities to determine the level of daytime sleepiness.11,12  The ESS takes only a few minutes to administer.11 Questionnaires such as the ESS are helpful to characterize the level of daytime sleepiness or propensity to sleep.1,11
Excessive daytime sleepiness (EDS)
The inability to stay awake and alert during the day, resulting in periods of irrepressible need for sleep or unintended lapses into drowsiness or sleep.1 Patients may not complain of excessive daytime sleepiness specifically; rather, they may report manifestations such as fatigue, tiredness, or difficulty concentrating.4  For all patients with excessive daytime sleepiness, narcolepsy should be considered in the differential diagnosis.1,4
Hypnagogic hallucinations
Vivid dream-like experiences that occur at wake-sleep transitions.1-3
Multiple sleep latency test (MSLT)
The MSLT is a series of four or five 20-minute daytime nap opportunities scheduled at 2-hour intervals, beginning 1.5 to 3 hours after the end of the PSG, in which patients are instructed to try to fall asleep.13 The MSLT is a validated, objective measure of the physiologic ability or tendency to fall asleep in standardized conditions.13 In diagnosing narcolepsy, the MSLT is also used to quantify the number of sleep-onset REM periods (SOREMPs), or the number of nap episodes in which the patient enters REM sleep during nap trials.1,13,14
Narcolepsy Symptom Screener
The Narcolepsy Symptom Screener contains questions about narcolepsy symptoms to help patients recognize and record their symptoms for discussion with their healthcare provider.
Polysomnography (PSG)
A PSG, or sleep study, is an overnight laboratory assessment, typically performed at the patient's habitual bedtimes, in which various physiological assessments are used to characterize the patient's sleep patterns, the continuity of sleep, and the presence of sleep disorders such as obstructive sleep apnea that may be responsible for the patient’s symptoms.4,15 
REM sleep
Rapid eye movement (REM) sleep is the distinct stage of sleep that is associated with low voltage, fast electroencephalogram (EEG) activity and complete loss of muscle tone (REM muscle atonia) as well as the characteristic rapid eye movements for which it is named. Dreams occur most frequently in REM sleep.16
Sleep attack
Sudden, irresistible sleep onset that can occur even in active situations such as eating, walking, and talking in patients with excessive daytime sleepiness in narcolepsy.1,17
Sleep disruption
Interruption of sleep by frequent awakenings and poor quality sleep.1,18
Sleep laboratory testing
Sleep laboratory testing, consisting of overnight PSG followed by MSLT, which is recommended to assess the severity of sleepiness, diagnose other concomitant sleep disorders, and confirm the narcolepsy diagnosis.1
Sleep paralysis
Disturbing temporary inability to move voluntary muscles or speak at sleep-wake transitions.1-3,19
Sleep-onset REM period (SOREMP)
Short (within 15 minutes) latency to REM sleep after sleep onset, as assessed using overnight PSG or MSLT.1