How is narcolepsy diagnosed?

The International Classification of Sleep Disorders (ICSD-3), see below, distinguishes three forms of narcolepsy: NT1 (narcolepsy type 1 with cataplexy), NT2 (narcolepsy type 2) without cataplexy, and narcolepsy secondary to another medical condition.
The diagnosis NT1 (formerly ‘narcolepsy with cataplexy’) requires the presence of excessive daytime sleepiness (EDS) and cataplexy, both of which must be evaluated by careful history taking, and confirmed with polysomnographic studies, including a multiple sleep latency test (MSLT) and/or hypocretin-1 measurement in the cerebrospinal fluid (CSF). Hypocretin measurement is particularly helpful in cases in which the presence of cataplexy is doubtful, or in patients in which the interpretation of the MSLT is hampered, i.e. in patients using anti-depressants and in children.

The only clinical difference between NT1 and NT2 is the absence of cataplexy in the latter. The polysomnographical criteria for both are the same. An important difference, however, is that hypocretin-1 levels in the CSF are normal or intermediate (<200pg/ml) in the majority of patients suffering from NT2 (formerly ‘narcolepsy without cataplexy’).

Narcolepsy secondary to another medical condition may be accompanied by cataplexy or not. It is clinically undistinguishable from the other two subtypes but is presumed to be caused by a coexisting medical disorder. Examples are Parkinson’s disease, Niemann-Pick type C, Prader-Willi syndrome, and (rare) vascular, neoplastic, or inflammatory lesions confined to the lateral hypothalamic area.
International Classification of Sleep Disorders, 3rd edition, 2014
Diagnostic criteria for Narcolepsy
Narcolepsy type 1 (NT1, formerly ‘Narcolepsy with Cataplexy):

A. Daily periods of the irrepressible need to sleep or daytime lapses into sleep, occurring for at least 3 months
B. The presence of one or both of the following:
1. Cataplexy and mean sleep latency of 8 minutes or less and at least 2 sleep onset rapid eye movement periods (SOREMPs) on a multiple sleep latency test (MSLT)
• A SOREMP (within 15 min of sleep onset) on the preceding nocturnal polysomnogram (PSG) may replace one of the SOREMPs on the MSLT
– If narcolepsy type 1 is strongly suspected clinically but MSLT criteria (see B1, above) are not met, a possible strategy is to repeat the MSLT
2. Cerebrospinal fluid (CSF) hypocretin-1 concentration, measured by immunoreactivity, is either 110 pg/mL or less, or less than ⅓ of mean values obtained in normal subjects using the same standardized assay
Narcolepsy type 2 (NT2, formerly ‘Narcolepsy without Cataplexy’):
A. Daily periods of irrepressible need to sleep or lapses into daytime sleep, occurring for at least 3 months
B. Mean sleep latency of 8 minutes or less and at least 2 sleep onset rapid eye movement periods (SOREMPs) on a multiple sleep latency test (MSLT)
• A SOREMP (within 15 min of sleep onset) on the preceding nocturnal polysomnogram (PSG) may replace one of the SOREMPs on the MSLT
C. Absence of cataplexy
• If cataplexy develops later, then the disorder should be reclassified as narcolepsy type 1
D. Cerebrospinal fluid (CSF) hypocretin-1 concentration is either not measured, or found to be either greater than 110 pg/mL, or greater than ⅓ of mean values obtained in normal subjects using the same standardized assay
• If the CSF hypocretin-1 concentration is tested at a later stage and found to be either 110 pg/mL or less, or less than ⅓ of mean values obtained in normal subjects with the same assay, then the disorder should be reclassified as narcolepsy type 1
E. Hypersomnolences and/or MSLT findings are not better explained by other causes such as insufficient sleep, obstructive sleep apnoea, delayed sleep phase disorder, or the effect of medication or substances or their withdrawal