304 Chapter 10 severity. As opioids are not prescribed for narcolepsy, our questionnaire targeted opioid use for indications other than narcolepsy and asked whether people experienced changes in their narcolepsy symptoms while using these opioids. No information was given to respondents on the rationale for initiating this study nor our hypotheses, in order to avoid influencing their responses. People with narcolepsy type 1 who visited our tertiary sleep-wake clinic (Sleep-Wake Centre SEIN) between 2010 and 2021, were randomly invited by telephone to complete a digital questionnaire on recent opioid use. All approached individuals were encouraged to participate in the study, irrespective of current narcolepsy medication and/or previous opioid use. Narcolepsy type 1 diagnosis was confirmed according to the third edition of the International Classification of Sleep Disorders (ICSD-3) criteria [8]. Diagnostic procedures could have been performed in our clinic or externally. Clinical characteristics derived from this screening process were statistically compared between opioid users and non-opioid users. Student’s t-tests were used for age, Epworth sleepiness scale scores, multi sleep latency test (MSLT) sleep latency and sleep-onset rapid eye movement periods (SOREMPs), chi-square tests for sex, cataplexy presence, human leukocyte antigen (HLA) DQB1*0602 positivity and polysomnography (PSG) SOREMP presence, and a MannWhitney U test for cataplexy frequency and hypocretin levels. In the questionnaire, respondents were asked for recent use of any drug from a list of all opioids available in the Netherlands (substance and brand names, Appendix A). Only opioid use in the previous three years was included, to minimise possible recall bias effects. If opioids were previously used, the type of opioid, route of administration, dosage, frequency and duration of use and medical indication were sought. Respondents were thereafter asked whether they had noticed any changes in their narcolepsy symptom severity while using opioids. The narcolepsy symptoms we investigated included excessive daytime sleepiness, disturbed nocturnal sleep, cataplexy, hypnagogic/ hypnopompic hallucinations and sleep paralysis. Individuals were first asked whether individual symptoms were applicable to them and, if so, whether they experienced strong improvement, slight improvement, no changes, slight worsening or strong worsening when they were using opioids. All respondents who indicated they had recently used opioids also had a semi-structured telephone interview to verify and better understand their questionnaire answers. We asked respondents to make a timeline of possible changes in their narcolepsy symptom severity from before the indication for which they started using an opioid (I), to when the indication started for which they later received an opioid (II) to when the opioid was initiated
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