Isolated knee monoarthritis heralding resectable non-small-cell lung cancer. A paraneoplastic syndrome not previously described.

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.