Myasthenia Gravis and Multiple Sclerosis are two different diseases but share some similarities as they are both serious lifelong diseases. Myasthenia Gravis causes serious muscle weakness whereas Multiple Sclerosis affects nerves that communicate with the brain to control other body parts. Here in this article, you have a look at the similarities as well as the differences between Multiple Sclerosis and Myasthenia Gravis. Similarities Between Multiple Sclerosis & Myasthenia Gravis Both affect those under age 40. Both are more common