The primary symptom of myasthenia gravis is muscle weakness that can vary in type and intensity. It often worsens after activity and improves with rest. In milder cases, myasthenia gravis affects the muscles of the eyes and face, commonly causing drooping eyelids, impaired vision, and changes in facial expression.
In more severe cases, the disease can affect the muscles in the limbs and the muscles that control breathing, chewing, swallowing, and speaking, leading to impaired speech, shortness of breath, and difficulty eating and drinking. Sometimes, severe muscle weakness can cause respiratory failure and require immediate medical care.
About 15 to 20 percent of people with myasthenia gravis will experience at least one myasthenic crisis—a medical emergency in which the muscles that control breathing become so weak that a person requires a ventilator to breathe. Anything from stress, infection, or surgery, to an adverse reaction to a medication can trigger a myasthenia crisis. With that said, up to 50% of myasthenic crises will not have a clear cause.