Background Undifferentiated arthritis (UA) is defined as cases of inflammatory arthritis where no definitive diagnosis can be made, and that may evolve into a chronic inflammatory disease over time or result in persistent drug-free remission.
Objectives In this study we investigated the 2-year disease course in patients with poly-, oligo- and monoarticular UA, with the main outcomes being 1) fulfillment of the 2010 ACR/EULAR RA classification criteria, 2) absence of clinical synovitis without use of DMARDs and 3) final clinical diagnosis.
Methods Patients (18-75 years old) with ≥1 swollen joint of <16 weeks duration were included in the multi-center observational study NOR-VEAC (Norwegian Very Early Arthritis Cohort) and followed for 2 years. Patients with arthritis due to trauma, septic arthritis, crystal arthritis and osteoarthritis were excluded. The study included 1118 patients in total. Patients with a clinical diagnosis of a rheumatic disease other than RA (N=338) or fulfilling the 2010 ACR/EULAR RA criteria (N=261) at baseline and those without follow-up data (N=24) were excluded from the current analyses. Absence of clinical synovitis was defined as no swollen joints at last visit.
Results Among the 495 patients who were denoted UA, 60 (12.1%) patients had polyarthritis (PA), 179 (36.2%) oligoarthritis (OLA) and 256 (51.7%) monoarthritis (MA) at baseline. Mean (SD) age was 46.6 (14.6) yrs, 54.1% were females and median (25, 75 perc.) duration of joint swelling was 29 (10, 62) days. Only 34 (6.9%) of the 495 UA patients were classified as RA during follow-up, including 8.9% of those presenting as OLA and 13.3% of those presenting as PA. Conversely, among patients later classified as RA, 47.1% were OLA patients and 23.5% PA patients at inclusion. 50.0% of the OLA patients who later fulfilled the RA criteria were seropositive, while none of the PA patients were. The most frequently affected joints in the OLA patients fulfilling the RA criteria were PIP (31.3%), MCP (25.0%), knees (31.3%) and wrists (31.3%). Only 86 of 495 patients used DMARDs during follow-up (38.3% of all PA patients, 20.1% OA patients and 10.4% of MA patients). 41.7%, 63.8% and 76.6% of those presenting as PA, OLA and MA, respectively, achieved absence of clinical synovitis at final follow-up visit without use of DMARDs. The most frequent final clinical diagnosis in PA, OLA and MA patients was UA (40.0%, 50.1% and 68.3%, respectively). Figure 1 shows the distribution of clinical diagnoses at last visit.
Conclusions Less than 1/5 of all the UA patients started DMARDs, and only 6.9% of the UA patients were classified as RA during follow-up, including 9% of those presenting as OLA and 13% of those presenting as PA. Nearly half of patients later classified as RA were patients with OLA at inclusion, however, 64% of OLA cases resolved without ever using DMARDs. The proportion with a final clinical diagnosis of UA was high irrespective of poly-, oligo- or monoarticular presentation.
Disclosure of Interest None declared