Because the cause of myasthenia gravis is largely unknown, risk factors for developing it remain unclear. It seems, though, that more cases of the disorder involve women than men, and it occurs more often among younger women (between 20 and 40 years old) than older.
It is also theorized that an abnormal thymus gland may put you at higher risk. Approximately 10 percent of those with myasthenia gravis have a tumor located on the thymus gland. In greater numbers-about 65 percent-the thymus gland is larger than normal.