Epidemiology

MG is an uncommon disease. Incidence has been reported as between 3 and 28 cases per million per year, with an estimated 10 to 20 cases per million per year in the US.[1][30][31] The worldwide prevalence has been estimated as 12.4 people per 100,000 population (a total of around 700,000 cases).[1][31][32]

A Norwegian cohort study reported an annual incidence rate of 8.8 to 16 per million, and prevalence of 131 to 145 per million.[33] Epidemiological data from northern Portugal suggested an MG incidence rate of 6.3 per million per year and a point prevalence of 111.7 per million.[34] The highest prevalence rate was reported in older men (288.1 per million in men aged >65 years).[34] In South Korea, incidence and prevalence rates of 0.55 per 100,000 person-years and 12.99 per 100,000 people, respectively, were reported in 2014.[35]

The prevalence of MG appears to be increasing, particularly in developed countries. This may be due, in part, to an ageing population, improved longevity of patients with MG, and availability of more accurate diagnostic tools. In Ontario, Canada, the age- and sex-standardised prevalence rate increased from 16.3 per 100,000 population in 1996 to 26.3 per 100,000 in 2013, but the incidence rate remained stable during this period.[36]

MG reportedly occurs in all ethnic groups; relative differences in disease prevalence are yet to be determined. One study of ocular MG found that non-white patients tended to present at a younger age.[37] MG is manifest from infancy to old age; more women than men are affected.[31] The incidence is significantly higher in black women, particularly in the US. Women usually present under the age of 40, whereas men typically develop symptoms later, with a median age of onset in the seventh decade.[36][37] MG characterised by antibodies directed against muscle-specific tyrosine kinase (MuSK) predominantly affects women (between 80% to 90% of patients).

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