Objective: To describe isolated knee monoarthritis as a paraneoplastic syndrome heralding non-small cell lung cancer (NSCLC), and to discuss its clinical characteristics. Methods: Clinical records of all consecutive, new outpatients with isolated knee monoarthritis observed from January 2000 to December 2005 were reviewed. A systematic review of Medline and Cochrane Library databases was performed to identify English-language articles related to rheumatological paraneoplastic syndromes associated with NSCLC. Results: Over 6 years, 6654 new outpatients with different rheumatic disorders were observed. Of these, 296 (4.4%) presented with isolated monoarthritis of the knee. In five out of 296 patients (1.7%) this feature represented the initial manifestation of NSCLC. All five patients were middle-aged men, with a long history of heavy cigarette smoking, who had a non-erosive, isolated knee monoarthritis, with mild articular fluid collection of non-inflammatory type. NSCLC was resectable in all patients, and knee monoarthritis remitted with no relapse confirming its paraneoplastic nature. All five patients are in good condition after a median follow up of 41 months. The literature review revealed that paraneoplastic knee monoarthritis has not previously been reported. Conclusion: Knee monoarthritis may in some cases represent a paraneoplastic syndrome heralding NSCLC at an early stage.
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