[A case of small cell lung cancer that developed during therapy for myasthenia gravis]

A 56-year-old man had been complaining of progressive proximal muscle weakness and bilateral ptosis before his first admission to our hospital. He received an injection of edrophonium chloride, which resulted in remarkable improvement of muscle strength. Electromyographic studies revealed a compound muscle action potential that decreased after repetitive stimulation at a high rate (15 Hz). He was regarded as having myasthenia gravis (MG) rather than Eaton-Lambert syndrome because of these findings. Eighteen months after successful treatment of MG with oral anticholinesterase medication, he complained of an abdominal mass. The mass was found to be a tumor that had metastasized from a primary small cell carcinoma of the lung. Cases of MG with small cell carcinoma of the lung seem to be very rare, and the details of the relationship between them remain unknown. In this patient, MG may have developed by paraneoplastic mechanisms. This hypothesis is interesting, since it has been demonstrated recently by molecular biological techniques that small cell carcinomas of the lung express nicotinic acetylcholine receptors.