A middle-aged woman suffers from weakness of her ocular and facial muscles, which worsens with repeated use. She has antibodies to acetylcholine receptors in her serum. The likely diagnosis is:

Myasthenia gravis is an autoimmune disease caused by the product of antibodies against acetylcholine receptors. The ocular and facial muscles are usually weakened. The weakness worsens with repeated contractions of the muscles. Treatment with anticholinesterase agents can improve the weakness and fatigability by increasing the quantity of acetylcholine available at the motor. Ref: Ropper A.H., Samuels M.A. (2009). Chapter 53. Myasthenia Gravis and Related Disorders of the Neuromuscular Junction. In A.H. Ropper, M.A. Samuels (Eds),Adams and Victor's Principles of Neurology, 9e.