The month-of-birth pattern in narcolepsy is moderated by cataplexy severity and may be independent of HLA-DQB1*0602.

Objectives: A March peak and a September trough in the birth pattern of narcolepsy patients with clear-cut cataplexy was recently reported. The objectives of the present study were to determine whether the month-of-birth pattern would (a) vary with the presence and severity of cataplexy and (b) differ for patients positive and negative for HLA-DQB1*0602.

Design: Cross-sectional survey with data obtained from the clinical trials assessing the safety and efficacy of modafinil in the treatment of narcolepsy.

Setting: Sleep clinics throughout the United States.

Patients: A group of 530 narcolepsy patients diagnosed based on the International Classification of Sleep Disorders using clinical histories, nocturnal polysomnography, and Multiple Sleep Latency Tests.

Interventions: NA.

Measurements And Results: A surplus of March births and a fall-off in September births was found in narcolepsy relative to the general population. This finding was only observed when cataplexy was moderate or severe. The month-of-birth pattern was similar for HLA-DQB1*0602 positive and negative patients. A March birth and HLA-DQB1*0602 positivity were independent risk factors in a logistic regression analysis.

Conclusions: Environmental events during development may influence narcolepsy severity or the likelihood of developing the disease.