Learn The BasicsNo one knows exactly what causes myasthenia gravis (MG), a rare autoimmune disease characterized by muscle fatigue and weakness.
How does MG affect the body?
In MG, the immune system starts to produce antibodies against different components of the neuromuscular junction, the connection between nerve cells and muscle cells. This is where the nerve signals coming from the brain are transmitted to the voluntary muscles to make them move.
The antibodies produced in people with MG mark certain proteins in the neuromuscular junction for destruction by the immune system. In the absence of these proteins, nerve signals cannot be transmitted to the muscles and muscles cannot contract as they should. The result is muscle weakness and fatigue.
The disease may develop due to several genetic and environmental risk factors.
The genetic component
In most cases, MG is not inherited and develops in people who have no family history of the disease. A very small proportion of people affected by the disease (3-5%) may have other members in their family also affected by MG or another autoimmune disease.
Though no single gene has been identified that is associated with MG, genetic variations that affect the function of the immune system may increase the risk of developing MG.
For example, a genetic marker called HLA-B8, DR3 has been found to be associated with an increased risk of developing MG.
People with MG may also have an increased risk of developing other autoimmune conditions, like systemic lupus erythematosus.
Other demographic factors
Other factors such as age, gender and ethnicity may be associated with the risk of developing MG.
In general, those over 50 have an increased risk of developing MG. However, there are disparities between genders.
Women are generally more likely to develop MG when they are 20 to 30. They are also more likely to have a type of MG called anti-MuSK (muscle-specific kinase) antibody-positive MG.
Men, on the other hand, are more likely to develop the disease when they are much older, in their sixties or seventies.
In some cases, newborn babies whose mothers have MG may be affected by a transient form of the disease. This happens when the mother’s antibodies cross over to the baby. The symptoms may vary from mild ocular symptoms like isolated fatigable droopy eyelids to myasthenic crises involving the respiratory muscles, requiring ventilator support. Neonatal MG usually resolves within a few weeks or months after birth. The duration and severity of MG in the mother are not predictive of the development of neonatal MG.
In rare cases, MG may also affect children, and among them, teenage girls of Caucasian descent have the highest risk.
While MG can affect people of all backgrounds, research has shown that the disease might be slightly more common among people of African descent. This group may also have a higher risk of developing the disease at a younger age and have the anti-MuSK antibody-positive type.
Environmental risk factors
A number of environmental risk factors have been identified as being associated with an increased risk of developing MG.
These include smoking, low levels of vitamin D in the blood and infections, though no specific virus or bacteria have been linked to MG.
Reviewed by Debjyoti Talukdar, M.D., on June 6, 2025.