Myasthenia gravis affects people of all ages, but your risk depends heavily on your sex, your age, and whether you already have another autoimmune condition. Women under 55 are diagnosed more often than men in the same age range, while men over 55 see a sharp increase in incidence that continues to climb with age. Overall, the condition affects roughly 20 per 100,000 people in most studied populations, and that number has been rising. Prevalence doubled in Japan between 2006 and 2017, and recent data from both Europe and the U.S. show the highest rates in older adults.
Age and Sex Are the Strongest Risk Factors
Myasthenia gravis follows a distinctive pattern: younger women and older men carry the highest risk. In U.S. claims data, women had higher incidence than men in every age bracket below 55. After age 55, the pattern reversed sharply, with men developing the condition at significantly higher rates that continued to rise with age. A UK study from 2014 to 2018 found the highest incidence in people over 65, at about 51 new cases per million per year for both sexes in that bracket, compared to roughly 18 per million in the general population.
The female predominance in younger adults likely reflects the role of sex hormones in autoimmunity, a pattern seen across many autoimmune diseases. The male surge after 55 is less understood but may involve vascular risk factors, obesity, and reduced physical activity that accompany aging in men. One important caveat: in lower-income populations studied through Medicaid data, women had higher rates regardless of age, suggesting socioeconomic factors also shape who gets diagnosed.
Genetic Factors That Raise Susceptibility
Myasthenia gravis is not directly inherited, but certain immune system genes make it more likely. The strongest genetic link involves a set of genes that help your immune system distinguish your own cells from foreign invaders. In people of Northern European descent who develop the disease before age 40, one particular variant in this gene family (called HLA-B*08) carries an extremely strong statistical association. The link is even stronger in women.
Different genetic signatures appear in different forms of the disease. Late-onset cases in Norwegian patients were linked to a different variant (HLA-DRB1*15:01), and a rare subtype of myasthenia gravis driven by a different antibody was associated with yet another genetic profile found in Dutch and Italian patients. Ethnic background matters here: studies in Northern Chinese populations identified distinct genetic markers tied to cases involving thymus tumors. Having one of these genetic variants doesn’t guarantee you’ll develop the condition, but it does increase your baseline susceptibility.
The Thymus Gland Connection
The thymus, a small gland behind the breastbone that trains immune cells during childhood, plays a central role. About 21% of people with myasthenia gravis have a thymoma, a tumor of the thymus gland. Estimates across studies range from 10% to 30%, but the pooled figure from a systematic review sits at 21%. A larger proportion of patients have thymic hyperplasia, where the thymus is enlarged and overactive without a true tumor.
In both cases, the thymus appears to generate or sustain the misdirected immune response that attacks the communication point between nerves and muscles. This is why surgical removal of the thymus is a treatment option for many patients. If you’ve been diagnosed with a thymoma, your doctor will likely screen you for myasthenia gravis symptoms, and vice versa.
Other Autoimmune Diseases Increase Risk
About 15% of people with myasthenia gravis have at least one other autoimmune disease. Thyroid conditions are the most common overlap, affecting 5 to 10% of patients. Hashimoto’s thyroiditis (underactive thyroid) shows up more frequently than Graves’ disease (overactive thyroid). Rheumatoid arthritis occurs in 4 to 7% of patients, making it the second most common autoimmune companion. Lupus co-occurs at a rate of roughly 2.6%, though some studies report rates as high as 7.7%, particularly in women.
The pattern runs both directions. If you already have an autoimmune thyroid condition, lupus, or rheumatoid arthritis, your immune system has already demonstrated a tendency to attack your own tissues, which modestly raises your odds of developing myasthenia gravis as well.
Infections That May Trigger Onset
Viral infections have been linked to the onset of myasthenia gravis, likely through a process called molecular mimicry, where the immune system confuses viral proteins with the body’s own proteins. Epstein-Barr virus, hepatitis E, West Nile virus, and human parvovirus B19 have all been associated with new cases in published research.
COVID-19 added to this list. The first reported case of myasthenia gravis triggered by SARS-CoV-2 infection appeared in Italy in 2020, and case reports have accumulated since then. The proposed mechanism involves immune hyperactivation following the infection. COVID-19 vaccines have also been reported as potential triggers in rare cases, though the absolute risk remains very small.
Medications That Can Unmask or Worsen It
Several classes of medications can trigger new-onset myasthenia gravis or cause a flare in someone who has it without knowing. Immune checkpoint inhibitors, used in cancer treatment, carry the strongest risk. In one study, 85% of patients with previously controlled myasthenia gravis experienced a flare after starting checkpoint inhibitors, with about half progressing to severe or life-threatening weakness within a week.
Other drugs that can trigger the condition include penicillamine (used for rheumatoid arthritis and Wilson’s disease), certain cancer drugs called tyrosine kinase inhibitors, and interferons. A separate group of medications doesn’t cause the autoimmune process itself but interferes with nerve-to-muscle signaling enough to unmask symptoms in someone with subclinical disease. This group includes certain antibiotics, heart rhythm medications, and anesthetics, particularly in people with kidney problems. Statins can also worsen symptoms in rare cases through a combination of immune effects and direct muscle toxicity.
Newborns of Mothers With Myasthenia Gravis
Babies born to mothers with myasthenia gravis face a temporary form of the condition called transient neonatal myasthenia gravis. This happens when the mother’s antibodies cross the placenta during pregnancy. Recent systematic reviews estimate it affects 15 to 17% of exposed infants, though individual studies report rates as low as 2.5% and as high as 35%.
The condition is temporary because the baby’s body gradually clears the maternal antibodies over weeks to months. Symptoms in affected newborns can include weak cry, poor feeding, and reduced muscle tone. This is not the same as inheriting myasthenia gravis. The baby’s own immune system is not producing these antibodies, and most infants recover fully without long-term effects.
Racial and Socioeconomic Disparities
Research on racial differences in myasthenia gravis is still limited, but what exists points more to disparities in care than in biology. Black patients in the U.S. were found to be less likely to receive standard steroid treatment compared to other groups, and women overall received less treatment than men. These gaps in care can affect outcomes and may partly explain differences in disease severity across populations, though underlying differences in disease frequency between racial groups remain poorly characterized.