Narcolepsy type 2: A rare, yet existing entity.

Because of unspecific diagnostic criteria, there is much controversy around narcolepsy type 2, its existence and its frequency. With this retrospective and purely descriptive study, we aimed to compare the frequency of narcolepsy type 2 compared to the well-described narcolepsy type 1, in a large (n = 3,782) retrospective sample from a single tertiary sleep centre. After 2 weeks washout of sleep-wake active medication, all patients with excessive daytime sleepiness (n = 1,392) underwent 2 weeks actigraphy, polysomnography and multiple sleep latency test, and all diagnoses were made along current diagnostic criteria. Narcolepsy type 1 was diagnosed in 91 patients, and 191 patients without cataplexy had multiple sleep latency test (MSLT) results indicating narcolepsy. After exclusion of shift work syndrome (n = 19), suspected insufficient sleep syndrome (n = 128), delayed sleep phase syndrome (n = 4) and obstructive sleep apnea (n = 34), six patients were diagnosed with narcolepsy type 2, of whom two patients later developed narcolepsy type 1. Altogether, our observations suggest that narcolepsy type 2 exists, but its frequency may be much lower compared to narcolepsy type 1. In addition, they emphasize the importance of scrupulously excluding other potential causes of sleepiness, if possible, with 2-week actigraphy and polysomnography.