Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. Fainting also called as syncope is quite common among myasthenia patients. Most often it is caused by side effects of drugs like Pyridostigmine or other MG drugs. Other probable cause may be due to cardiac involvement, like sick sinus syndrome. If you have repeated attacks, then consult a neurologist and get evaluated. Wishing you good health. Thanks.