Narcolepsy - Classification

Classification

The 2001 International Classification of Sleep Disorders (ICSD) divided primary hypersomnia syndromes primarily between narcolepsy, idiopathic hypersomnia and the recurrent hypersomnias (like Klein-Levin syndrome), and it further divided narcolepsy into that with cataplexy and that without cataplexy. This ICSD version defined narcolepsy as “a disorder of unknown etiology that is characterized by excessive sleepiness that typically is associated with cataplexy and other REM-sleep phenomena, such as sleep paralysis and hypnagogic hallucinations,” and it also laid out specific diagnostic criteria for narcolepsy, with 2 different sets of minimal criteria, as shown below.

Minimal diagnostic criteria set #1:

• “Daytime naps or lapses into sleep” that “occur almost daily for at least 3 months.”
• Cataplexy.

Minimal diagnostic criteria set #2:

• A “complaint of excessive sleepiness or sudden muscle weakness.”
• Associated features that include: sleep paralysis; disrupted major sleep episode; hypnagogic hallucinations; automatic behaviors.
• Polysomnography with one or more of the following: “sleep latency less than 10 minutes;” “REM sleep latency less than 20 minutes;” an MSLT with a mean sleep latency less than 5 minutes; “two or more sleep-onset REM periods” (SOREMPs).
• ”No medical or mental disorder accounts for the symptoms.” (see hypersomnia differential diagnosis)

In the absence of clear cataplexy (as per the second set of diagnostic criteria), it becomes much more difficult to make a firm diagnosis of narcolepsy. “Various terms, such as essential hypersomnia, primary hypersomnia, ambiguous narcolepsy, atypical narcolepsy, etc., have been used to classify these patients, who may be in the developing phase of narcolepsy.”

Since the 2001 ICSD, the classification of primary hypersomnias has been steadily evolving, as further research has shown more overlap between narcolepsy and idiopathic hypersomnia. The 3rd edition of the ICSD is currently being finalized, and its new classification will label narcolepsy caused by hypocretin deficiency as “type 1 narcolepsy,” which is almost always associated with cataplexy. The other primary hypersomnias will remain subdivided based on the presence of SOREMPs. They will be labeled: “type 2 narcolepsy,” with 2 or more SOREMPs on MSLT; and “idiopathic hypersomnia,” with less than 2 SOREMPS.

However, “there is no evidence that the pathophysiology or therapeutic response is substantially different for hypersomnia with or without SOREMPs on the MSLT.” Given this currently understood overlap of idiopathic hypersomnia and narcolepsy, the 5th edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-V) is also updating its classification of the primary hypersomnias. It reclassifies narcolepsy without cataplexy as major somnolence disorder (MSD). Additionally, MSD will encompass all syndromes of hypersomnolence not explained by low hypocretin, including idiopathic hypersomnia (with and without long sleep time) and long sleepers (patients requiring >10 hours sleep/day).

Further complicating these updated classification schemes, overlap between narcolepsy with cataplexy and idiopathic hypersomnia has also been reported. A subgroup of narcoleptics with long sleep time, comprising 18% of narcoleptics in one study, had symptoms of both narcolepsy with cataplexy and idiopathic hypersomnia (long sleep time and unrefreshing naps). It is felt that this subgroup might have dysfunction in multiple arousal systems, including hypocretin and GABA (see idiopathic hypersomnia causes).