A 70-year-old man who had been treated for small cell carcinoma of the lung develops marked weakness of his legs and arms. He also complained of mouth dryness, double vision, and drooping upper eyelids. On physical examination there is diffuse muscle weakness and wasting. A muscle biopsy is normal. Laboratory studies demonstrate serum IgG autoantibodies that recognize voltage-sensitive calcium channels in motor nerve terminals. Which of the following is the most likely diagnosis?