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?

Lambert-Eaton myasthenic syndrome. Lambert-Eaton syndrome is a paraneoplastic disorder that manifests as muscular weakness, wasting, and fatigability of the proximal limbs and trunk. Also termed myasthenic-myopathic syndrome, the disease is usually associated with small cell carcinoma of the lung, although it may also occur in patients with other malignant diseases. Like myasthenia gravis, the disease seems to have an autoimmune basis because it can be transferred to mice by IgG from patients and it responds to treatment with corticosteroids. The pathogenic IgG autoantibodies recognize voltage-sensitive calcium channels that are expressed both in motor nerve terminals and in the cells of the lung cancer. The calcium channels, which are necessary for release of acetylcholine, are greatly reduced in the presynaptic membrane in these patients, thereby interfering with neuromuscular transmission. Antichannel antibodies are not encountered in the other choices.Diagnosis: Lambert-Eaton myasthenic syndrome