Epidemiological study of myasthenia gravis in Japan

Myasthenia gravis (MG) is an autoimmune-mediated neurological disorder. The relationship between MG and thymic abnormalities is well recognized, and thymectomy is one of the therapies for anti-acetylcholine receptor antibody-positive MG. The major pathogenic factor is anti-acetylcholine receptor antibody followed by anti-muscle-specific kinase antibody, and commercial kits are available to detect these antibodies. Several decades ago, the prognosis of MG was not favorable; therefore, the Ministry of Health and Welfare (predecessor of the Ministry of Health, Labor and Welfare) organized a Taskforce for Intractable Diseases, which included MG, in 1972. The Taskforce carried out consecutive epidemiological studies for MG in 1973, 1987, 2006 and 2018. The four studies found: (i) increasing prevalence; (ii) increasing late- and elderly-onset; (iii) decreasing female dominancy; (iv) decreasing infantile-onset (onset age of 0–4 years); and (v) decreasing frequencies of crisis. The latest epidemiological study in Japan and studies from other countries suggest an increasing number of patients with anti-acetylcholine receptor antibody-positive MG without thymoma in the elderly. It is important to find out the causes of this phenomenon, which will improve the prevention of MG.