Increase in incidence of elderly-onset patients with myasthenia gravis in Nagano Prefecture, Japan.

OBJECTIVES In European countries and the United States the incidence of elderly-onset myasthenia gravis (MG) has recently been increasing. To investigate whether the incidence of the elderly-onset MG has increased in Nagano Prefecture of Japan, we divided the patients into young and elderly groups, and retrospectively examined their incidence. PATIENTS AND METHODS On the basis of two-step questionnaires sent to hospitals and the patient list of the intractable disease registration system in Nagano Prefecture we studied 213 MG patients diagnosed between 1982 and 2001. This 20-year period was divided into 4 five-year terms, and the incidence of MG in young- (younger than 65) and elderly-onset (65 or older) groups was investigated separately for each term. RESULTS The ratio of the elderly-onset group showed a significantly positive correlation with terms irrespective of associated thymoma (r=0.98, p<0.05). There was a significant difference in the mean onset age among the 4 terms (p<0.005). The standardized incidence of MG gradually increased in both young- and elderly-onset groups as well as in the whole age range. The elderly group showed a particularly high incidence in females (12.01/million/year) and in patients without thymoma (8.78/million/year) in the final five years. CONCLUSIONS We confirmed that the incidence of elderly-onset MG has recently been increasing in the Nagano Prefecture. Since the change of the age distribution in this district is almost identical to that of the whole country, the incidence of MG might have been increasing in Japan as a whole, particularly in the elderly population.