Am just jumping in here with a couple of things from my own experience:

• Emotionally stressful situations could make some symptoms intensify slightly

• Physiologically (?) stressful situations (I mean like good adrenalin) actually relieved symptoms temporarily, a little like the effect of coffee.

• Stress in the bigger picture did not appear to direct the overall long term course of my own myasthenic weakness. In other words myasthenia appeared to have a life and direction all of its own