Myasthenia gravis (MG) affects roughly 5 to 30 people per 100,000 depending on the country, making it one of the more common autoimmune neuromuscular disorders but still relatively rare. In the United States, an estimated 82,715 adults were living with MG as of 2021, translating to about 320 cases per million adults. Those numbers have been climbing in recent decades, driven by better diagnostic tools and an aging population.
Prevalence and Incidence Worldwide
Globally, the incidence of MG sits around 5.3 new cases per million people per year, though individual studies report a wide range from less than 1 to more than 60 per million. That variation reflects real differences in how thoroughly populations are screened and how healthcare access varies by region. The prevalence, meaning the total number of people living with the condition at any given time, ranges from about 2 to 37 cases per 100,000 people depending on geographic location.
U.S. numbers tend to run higher than global averages. A large study using commercial and Medicare insurance data found an incidence of roughly 69 new cases per million person-years and a prevalence of about 316 per million. Among Medicaid-insured populations, the numbers were slightly lower: about 50 new cases per million and a prevalence of 204 per million. The gap likely reflects differences in access to specialist care and diagnostic testing.
Why the Numbers Keep Rising
MG prevalence has increased over the past several decades, but this doesn’t necessarily mean the disease itself is becoming more common. A significant part of the rise comes from improved blood tests that can detect the antibodies responsible for the condition, catching cases that would have gone undiagnosed in earlier eras. People with MG are also living longer thanks to better treatments, which means they stay in the prevalence pool longer. And because one peak of MG onset occurs in older adults, the aging of populations in high-income countries has pushed the numbers up further.
Who Gets MG: Age and Sex Patterns
MG follows a distinctive pattern based on age and sex. In women, incidence peaks around ages 30 and 50, creating what researchers call a bimodal distribution. In men, rates climb steadily with age, with the highest incidence between ages 60 and 89.
Before age 40, women are affected about three times as often as men. By the fifth decade of life, the ratio evens out. After age 50, men actually become slightly more likely to be diagnosed, with a male-to-female ratio of roughly 3:2. This shift means that early-onset MG (before age 50) looks demographically quite different from late-onset MG (after age 50), which is one reason clinicians treat them as somewhat distinct conditions.
Children and Adolescents
Juvenile MG is uncommon in Western populations, accounting for fewer than 10% of all cases among Caucasian patients. The picture is different in East Asian populations, where up to 50% of MG cases present during childhood, most often with symptoms limited to the eyes. The peak age of childhood onset in these populations falls between 5 and 10 years old.
Ocular vs. Generalized Disease
About half of people with MG first notice symptoms only in their eyes: a drooping eyelid, double vision, or difficulty keeping the eyes open. This form is called ocular MG. For some people it stays limited to the eyes permanently, but a significant proportion progress to generalized MG, which involves weakness in the limbs, neck, or breathing muscles. Estimates of that conversion rate vary widely, from about 20% to as high as 85% depending on the study and the follow-up period. A commonly cited middle estimate is that 20% to 60% of people who start with ocular symptoms will develop generalized disease.
Antibody Types and Diagnosis
Most people with MG produce antibodies that attack a specific receptor at the junction between nerves and muscles. About 85% of generalized MG patients test positive for these antibodies on standard blood tests. A smaller group, roughly 5% to 8%, produces a different antibody that targets a protein involved in clustering those same receptors. Together, these two antibody types account for the vast majority of cases.
That still leaves a subset of patients, sometimes called “double seronegative,” who test negative for both common antibody types. Among this group, about 19% carry a third, less commonly tested antibody. The existence of seronegative patients is one reason MG can be tricky to diagnose, particularly when symptoms are mild or limited to the eyes.
Mortality and Life Expectancy
MG is no longer the death sentence it was in the mid-20th century, when mortality rates exceeded 30%. But it still carries meaningful risk. In one large cohort study, 14% of MG patients had died within five years of symptom onset, rising to 21% at ten years. The median age at death was around 80 to 81 years, which sounds reassuring until you compare it to general population benchmarks. Men with MG died, on average, about three years younger than the national life expectancy (78.3 vs. 81.6 years). For women, the gap was nearly nine years (76.5 vs. 85.2 years).
Patients who experienced a myasthenic crisis severe enough to require a breathing tube fared worse: 27% mortality at five years compared to 12.5% among those who never needed intubation. On the other hand, those who underwent surgical removal of the thymus gland, which plays a role in the autoimmune process, had notably lower mortality (19%) compared to those who kept their thymus intact (35%).
Despite advances in treatment over the past two decades, overall mortality rates in MG have not dropped significantly since the early 2000s. This suggests that while therapies effectively manage symptoms for most people, the disease still poses long-term risks that current treatments don’t fully eliminate.