13 Introduction Narcolepsy type 2 and idiopathic hypersomnia Narcolepsy type 2 and idiopathic hypersomnia are characterized by excessive daytime sleepiness in absence of cataplexy. Many of the symptoms of narcolepsy type 1 may also occur, including hypnagogic hallucinations, sleep paralysis and daytime vigilance and fatigue complaints [22-24]. The diagnosis of narcolepsy type 2 requires multiple SOREMPS during the MSLT and polysomnography (similar to narcolepsy type 1), whereas only up to one SOREMP is allowed for idiopathic hypersomnia [8]. Excessive daily need for sleep (defined as more than 11 hours) is diagnostically used to identify a subgroup of individuals with idiopathic hypersomnia, a phenotype frequently accompanied by sleep drunkenness [23, 24]. Consensus on the definition of sleep drunkenness is lacking but it is generally described as substantial difficulty with awakening that is accompanied by confusional behaviour, typically lasting for more than 30 minutes [25, 26]. Sleep drunkenness in idiopathic hypersomnia is in contrast with narcolepsy type 1 with typically refreshing nature of sleep. Challenges in current diagnostic criteria Despite these internationally accepted criteria, the diagnostic workup often proves difficult to reliably identify different central disorders of hypersomnolence [27-29]. This is partially because of the rarity of these disorders with estimated prevalences between 25-50 per 100,000 individuals for narcolepsy type 1 [30-32], around 25 per 100,000 individuals for narcolepsy type 2 and roughly 10 per 100,000 for idiopathic hypersomnia [33]. Especially narcolepsy type 2, idiopathic hypersomnia, and chronic sleep deprivation may phenotypically overlap at clinical presentation [28]. Individuals with the same central disorder of hypersomnolence diagnosis may also experience different phenotypes. The challenging diagnostic workup is further emphasized by the poor test-retest reliability of the multiple sleep latency test (MSLT) in the absence of cataplexy, where diagnostic crossover of up to 53% was seen for narcolepsy type 2 and 75% for idiopathic hypersomnia [34, 35]. Multiple opinion papers have recently stressed the importance of identifying new objective biomarkers better reflecting the underlying pathophysiological mechanisms of these disorders to resolve the blurred boundaries between hypersomnolence subtypes [27-29]. With this same goal, the European Narcolepsy Network (EU-NN) launched a collaborative prospective clinical database including all central disorders of hypersomnolence (Box 2) [22]. This unique dataset will be used for the analyses presented in Chapters 2 and 9. 1
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