Narcolepsy type 2 is an unstable diagnosis and idiopathic hypersomnia has the potential for remission - long-term and cross-sectional observations from the Bern Sleep Wake Registry.

Study Objectives The boundaries between narcolepsy type 1 (NT1), narcolepsy type 2 (NT2) and idiopathic hypersomnia (IH) and the evolution of these diseases over time are poorly understood. This retrospective cohort single centre study with a cross-sectional follow-up aims to assess changes of diagnosis and test findings over time. Methods Retrospective and cross-sectional data collection: We assessed demographic characteristics, symptoms, sleep-wake tests and diagnosis changes in patients with NT1, NT2 or IH seen at the Bern Sleep ̶ Wake Epilepsy centre between 2000 and 2021. In addition, we obtained information about changes in symptoms and diagnosis by contacting patients seen between 2005 and 2017 by telephone. The two data sources were combined to analyse the frequency of diagnosis changes. Results 214 patients (NT1 n = 122, NT2 n = 37, IH n = 55) were included. In 17% (30/179) of patients with follow-up, the diagnosis changed during the observational period. The most instable diagnosis was NT2 (17/30, 57%), most commonly transitioning to NT1 (n= 5) and IH (n= 9). Five patients with IH went into remission. NT1 was mostly stable (104/107). We confirmed a poor retest reliability of the MSLT in patients with NT2 and IH. Conclusions Frequent diagnosis changes in patients with NT2 and IH challenge the current diagnostic criteria, in addition, IH can remit.