A 38 year old medical secretary presents with a 3 month history of diplopia and eyelid drooping. She has noticed that after a long time on the telephone or in meetings her voice becomes quieter. She is otherwise well in herself and has no limb weakness, slurred speech, or muscle fasciculations. Her past medical history includes well controlled Hypothyroidism.
Based on her likely diagnosis, which of the following tests would be most helpful initially to aid diagnosis?