Background/Purpose: Cohort studies are essential to provide better understanding of the prognosis of acute monoarthritis, but few studies have focused on this issue. Our objective was to study the joint distribution and 2-year outcome of patients with new-onset monoarthritis, with a particular focus on outcome according to the joint involved.

Methods: 1119 patients (age 18-75 years) with arthritis of less than 16 weeks duration were included in the NOR-VEAC study from 2004 to 2010. Patients with crystal arthritis, septic arthritis, osteoarthritis or arthritis due to trauma were excluded. 364 patients (33%) had monoarthritis (clinical synovitis of one joint only) at baseline. Follow-up data were available for 350 patients (96%), and 221 (63%) had 2-year follow-up data. Patients with incomplete follow-up were included to avoid bias, as more patients achieving versus not achieving remission tended to be lost to follow-up before 2 years.

Results: Median (25-75 percentiles) duration of joint swelling among the 350 patients with monoarthritis was 23 (7-48) days, mean (SD) age was 46 (14) years, 52.0% were females, 6.6% anti-CCP pos. (14% missing), 8.6% RF pos. (11% missing), 11% anti-CCP and/or RF pos. (14% missing). The most frequently affected joints were the knee (49.1%), ankle (16.9%) and wrist (14.0%) and relatively few patients had small joint involvement (table).

15 patients (4.3%) fulfilled the 2010 ACR/EULAR classification criteria for rheumatoid arthritis (RA) at baseline; these had mostly wrist and small joint involvement. Two patients with clinical synovitis of the knee fulfilled the criteria as a result of additional tender joints. Nine additional patients fulfilled the criteria during follow-up, of whom 8 presented with large joint arthritis (table). No patient with ankle monoarthritis ever fulfilled the criteria, nor progressed to a clinical diagnosis of RA within 2 years of follow-up.

55 patients (16%) were diagnosed with reactive arthritis at baseline, 11 patients (3%) with psoriatic arthritis (PsA) and 3 patients (1%) with other spondyloarthritides (SpA). During follow-up only 5 more patients were classified as PsA and 2 more as other SpA. The patients finally diagnosed with PsA had monoarthritis of the knee (n=8), wrist (n=4), hand (n=2), elbow (n=1) and foot (n=1) at presentation. The corresponding numbers for other SpA were knee (n=2), ankle (n=2) and hip (n=1). In addition 5 (1.4%) patients were diagnosed as Lšfgren’s sarcoidosis. In 261 patients (74.6%) the arthritis had resolved without DMARD treatment at last follow-up. 49 patients (14.0%) were prescribed with DMARDs, 188 (53.7%) received intra-articular corticosteroids, and 80 (22.9%) systemic corticosteroids.

Conclusion: Few patients with acute or recent onset monoarthritis seem to develop chronic inflammatory joint disease over two years. The likelihood of developing RA or other chronic rheumatic diseases was lowest in patients with monoarthritis of the ankle.