Myasthenia gravis is a rare long-term condition that causes muscle weakness that comes and goes.
It most commonly affects the muscles that control the eyes and eyelids, facial expressions, chewing, swallowing and speaking. But it can affect most parts of the body.
It can affect people of any age, typically starting in women under 40 and men over 60.
Myasthenia gravis is caused by a problem with the signals sent between the nerves and the muscles.