318 Chapter 10 than previously described. Multiple respondents said they had encountered fewer potential cataplexy triggers due to the indication for which they were taking opioids, and continued using their cataplexy-suppressing narcolepsy medication. Oxycodone was shown to have the strongest positive effects, followed by codeine. Worsening of excessive daytime sleepiness or disturbed nocturnal sleep was reported by only two respondents and in one this was not consistently present and mixed with symptom improvement. The optimal duration of opioid use to expect effects on narcolepsy symptom severity remains to be studied. Most positive effects on narcolepsy type 1 symptom were reported in questionnaire respondents using oxycodone for several weeks. For other types of opioids this effect was not as strong. This is in line with our systematic literature review; studies reporting opioid use for several weeks generally showed most striking effects. Thannickal et al. [125] only noted significant hypocretin-producing neuron count changes in mice that received morphine (also a selective μ-receptor opioid) for at least two weeks. All respondents reported that the narcolepsy symptom relief when taking opioids disappeared almost immediately when the opioid was discontinued. Differences in effects of opioids on narcolepsy type 1 symptoms are probably attributable to the heterogeneity in opioid type, dosage, frequency, route of administration and duration of use. Previously published case reports and series on opioid use in narcolepsy were subject to similar weaknesses. Patterns of mixed treatment efficacy have however also been reported when studying other types of narcolepsy medication [374]. Despite the strong effects described, the level of evidence remains low. Only one combined open-label and randomised controlled trial has been published so far, reporting beneficial effects of short-term codeine use on self-reported but not objective sleepiness complaints [130]. More standardised clinical trials – preferably double-blind randomised controlled trials – are needed to understand better the relationship between opioid use and narcolepsy symptom severity changes. Based on the observed effects in our questionnaire we believe that stronger μ-receptorbinding opioids such as oxycodone, fentanyl or morphine would be promising targets, especially as they received little attention in previously published research. We consider the concurrent use of sodium oxybate in 4 of 16 respondents who had also used opioids to be an alarming sign as this combination of drugs is strongly contraindicated. The interaction between these drugs is known to cause severe side effects, including respiratory and central depression [360362]. Luckily, none of these side effects was reported by our respondents. We would nonetheless like to emphasise the importance of providing adequate
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