How is narcolepsy diagnosed?

The International Classification of Sleep Disorders (ICSD-2), see below, distinguishes three forms of narcolepsy: narcolepsy with cataplexy, narcolepsy without cataplexy, and narcolepsy due to medical condition.
The diagnosis ‘Narcolepsy with cataplexy’ requires the presence of excessive daytime sleepiness 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 narcolepsy with cataplexy and narcolepsy without cataplexy 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 in the majority of patients suffering from narcolepsy without cataplexy.
Narcolepsy due to 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, 2nd edition, 2005
Diagnostic criteria for Narcolepsy

ICSD-2 diagnostic criteria of narcolepsy with cataplexy

A. The patient has a complaint of excessive daytime sleepiness occurring almost daily for at least three months.

B. A definite history of cataplexy, defined as sudden and transient episodes of loss of motor tone triggered by emotions, is present.
Note: to be labeled as cataplexy, these episodes must be triggered by strong emotions -most reliably laughing or joking- and must be generally bilateral and brief (less than 2 minutes). Consciousness is preserved, at least at the beginning of the episode. Observed cataplexy with transient reversible loss of deep tendon reflexes is a very strong, but rare, diagnostic finding.

C. The diagnosis of narcolepsy with cataplexy should, whenever possible, be confirmed by nocturnal polysomnography followed by an MSLT; the mean sleep latency on MSLT is less than or equal to 8 min and two or more SOREMPs are observed following sufficient nocturnal sleep (minimum 6 h) during the night prior to the test.
Alternatively, hypocretin-1 levels in the CSF are less than or equal to 110 pg/ml or one-third of mean normal control values.

D. The hypersomnia is not better explained by another sleep disorder or neurological disorder, mental disorder, medication use, or substance use disorder.

ICSD-2 diagnostic criteria of narcolepsy without cataplexy

A. The patient has a complaint of excessive daytime sleepiness occurring almost daily for at least three months.

B. Typical cataplexy is not present, although doubtful or atypical cataplexy-like episodes may be reported.

C. The diagnosis of narcolepsy without cataplexy must be confirmed by nocturnal polysomnography followed by a MSLT. In narcolepsy without cataplexy, the mean sleep latency on MSLT is less than or equal to 8 min and two or more SOREMPs are observed following sufficient nocturnal sleep (minimum 6 h) during the night prior to the test.

D. The hypersomnia is not better explained by another sleep disorder or neurological disorder, mental disorder, medication use, or substance use disorder.

ICSD-2 diagnostic criteria of narcolepsy due to medical condition

A. The patient has a complaint of excessive daytime sleepiness occurring almost daily for at least three months.

B. One of the following is present:
1. A definite history of cataplexy, defined as sudden and transient episodes of loss of muscle tone (muscle weakness) triggered by emotion, is present.
2. If cataplexy is not present or is very atypical, polysomnographic monitoring performed over the patient’s habitual sleep period followed by an MSLT must demonstrate a mean sleep latency on the MSLT of less than 8 min with two or more SOREMPs, despite sufficient nocturnal sleep prior to the test (minimum 6 h).
3. Hypocretin-1 levels in the CSF are less than 110 pg/ml (or 30% of normal control values), provided the patient is not comatose.

C. A significant underlying medical or neurological disorder accounts for the daytime sleepiness.

D. The hypersomnia is not better explained by another sleep disorder, mental disorder, medication use, or substance use disorder.