Month of birth and the risk of narcolepsy: A systematic review and meta-analysis

Study Objectives: The aim of this study is to evaluate the relationship between the month of birth (MOB) and the risk of narcolepsy. Methods: We conducted a systematic review of the electronic databases PubMed, Embase, and Cochrane CENTRAL from their inception to September 30, 2021. We also added data on narcolepsy from the National Health Insurance Research Database in Taiwan. Then we extracted the relative risk (RR) ratios of narcolepsy in each month of birth to those of the general population and transformed them from MOB to season. A random-effects model was used to calculate pooled RR ratios from the meta-analysis and 95% confidence interval (CI). Results: The meta-analysis analyzed 7 studies and included 3,776 patients from 8 areas (Canada, China, France, Germany, Hong Kong, Netherlands, Taiwan, and United States). The RR ratio was highest in March (1.11; 95% CI, 0.99–1.26) and August (1.11; 95% CI, 0.98–1.26) and lowest in April (0.90; 95% CI, 0.78–1.03). However, none of the MOBs reached statistical significance. Moreover, the narcolepsy risk patterns on the 3 continents (Asia, Europe, and North America) were different. In North America, the highest and lowest significant risks were found in March (1.47; 95% CI, 1.20–1.79) and September (0.75; 95% CI, 0.56–0.99). In Asia, the lowest notable risk was in April (0.80; 95% CI, 0.66–0.97). In Europe, the risk of narcolepsy was not significantly related to any MOB. In terms of seasons, only spring MOBs in North America had a significantly higher risk (1.21; 95% CI, 1.06–1.38). Conclusions: The findings indicated that the risk of narcolepsy and MOB differed across the 3 continents. This study indicates the important role of environmental factors in narcolepsy.