Cancer polyarthritis is a very rare condition. Here, we present a case of cancer polyarthritis, who also had mononeuritis simplex as a manifestation of paraneoplastic neuralgic syndrome. A 71-year-old man, who initially presented symmetrical polyarthritis and unilateral posterior interosseous nerve palsy, was subsequently diagnosed to have gastric cancer. Total gastrectomy was performed, and his polyarthritis and the palsy simultaneously disappeared within 2 weeks after the resection. His gastric cancer was found to be metastasized to his liver 16 months after the total gastrectomy; however, the polyarthritis and the palsy did not recurrent throughout his course. The polyarthritis in this case was diagnosed as a cancer polyarthritis from its features. On the other hand, the isolated posterior interosseous nerve palsy in this case met the diagnostic criteria for paraneoplastic neurological syndrome. This case was also unique in that the manifestations of paraneoplastic syndromes did not recur even after the metastasis of the primary cancer, suggesting that some specific clones in the cancer were responsive to the manifestations of paraneoplastic syndromes. Our case suggested that relapse of the manifestations of paraneoplastic syndromes may not always herald the recurrence of primary or metastatic tumour, and other tumour markers and signs should be periodically followed to search for the recurrence of the tumours.
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