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Post H1N1 narcolepsy
The cause of human narcolepsy with cataplexy remains unknown, however an autoimmune process is most probable that may explain the early loss of hypocretin neurons.
The possibility that narcolepsy could be a rare side effect of H1N1 flu vaccination was first reported by the Swedish and Finnish Medical Product agencies in August 2010, with potential links to Pandemrix vaccination containing the adjuvant ASO3 and a squalene/a-tocopherol. We reported last year within three major world centers of reference for narcolepsy – Montpellier, France; Montreal, Canada; and Stanford University, United States – an unusual increase in abrupt onset narcolepsy-cataplexy diagnosed within a few months of H1N1 pandemic. Cases were reported with a clear temporal link between vaccination and disease onset (mean of 8 weeks), with occasionally an unusual clinical presentation with rapid development and severity of both excessive daytime somnolence and cataplexy. The World Health Organization (WHO) review of data from Finland’s National Institute of Health indicated a 9-fold increased risk of narcolepsy in children and adolescents aged 4 to 19 years following H1N1 vaccination with Pandemrix. Also, the Swedish Medical Products Agency found that the relative risk of narcolepsy was 6.6 higher in vaccinated children and adolescents compared to unvaccinated individuals.
All these results contrast with a recent exciting paper of Fang Han et al., (Annals of Neurol 2011) who reported the narcolepsy onset occurrences over the past 15 years in 629 patients (86% children) diagnosed at the People’s Hospital, Beijing University, China. The occurrence of narcolepsy onset was seasonal, significantly influenced by month (rare in November and most frequent in April, with a 6.7-fold increase from trough to peak) and calendar year. Hence a year-to-year variation was found with a 3-fold increase in narcolepsy onset following the 2009 H1N1 winter influenza pandemic. However in contrast to our results and the Finnish-Swedish studies, this increased incidence cannot be explained by the H1N1 vaccination, reported in less than 5% of Chinese cases (H1N1 vaccine without AS03).
To sum up, narcolepsy onset seems highly correlated with seasonal and annual patterns of upper airway infections including H1N1 influenza per se at least in China. How to explain these findings? We may hypothesize two major issues regarding the pathophysiology of narcolepsy with cataplexy: 1) A specific immune-mimicry component to an H1N1-related antigen, mediated through the presentation by HLA DQB1*0602, and 2) non-specific factors, such as adjuvants, influenza, or streptococcus infections.