Distribution and Incidence

The disease has a worldwide distribution and has been identified as the primary cause of death at the average annual rate of 1.5 per million in the United States. If cases coded as contributory or as a complication are included, then the total would be 2 to 2.5. This seems to be the safest method of reckoning the actual incidence of myasthenia gravis, and previous estimates of 1 in 1,600 of the population probably vastly overestimate the incidence of myasthenia. There is no difference between whites and non-whites, and there is no difference in nationality. The death rate is slightly higher for women than men. In age-adjusted death rates for all ages, there is no appreciable difference in nine geographic regions of the United States. Thus myasthenia gravis seems to be uniformly distributed throughout the United States, and probably is uniformly distributed throughout the world. There is no difference between city and country in the incidence of myasthenia gravis, and the age-specific death rates for the United States, based on a survey of 675 death certificates listing myasthenia gravis as the primary cause of death, showed that 90 percent of the decedents were older than 15 years of age. For this population the age-specific death rate is less than 1 per million until age 35, when there is a steady increase in the incidence of death for myasthenia gravis up to age 75. There appears to be a twin-peaked incidence, with females peaking between the ages of 15 and 24, and men peaking between ages 40 and 60. But if all ages are considered together, the sex ratio is probably close to 1. Morbidity data in surveys of the United States, Canada, England, Norway, and Iceland over a 10-year period using retrospective analysis show that the incidence of myasthenia is probably 0.2 to 0.5 per 100,000, with the prevalence being 3 to 6 per 100,000. In other words, the prevalence is approximately 10 times the incidence.

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