Symptoms of myasthenia gravis usually progress to maximum severity within 3 years. After 3 years, patients usually stabilize or improve. Infants with transient neonatal MG may develop acute respiratory failure within a few weeks after birth.
Advances in medical care have reduced the mortality rate from respiratory failure in MG patients to approximately 3 percent. Patients over the age of 40, those with a short history of severe disease, and those with thymoma (tumor of the thymus gland) have a worse prognosis.