Monoarticular juvenile idiopathic arthritis as a distinct clinical entity A proof-of-concept study

BACKGROUND: Currently, monoarticular Juvenile Idiopathic Arthritis (monoJIA) is included in the ILAR classification as oligoarticular subtype although various aspects, from clinical practice, suggest it as a separate entity. OBJECTIVES: To describe the clinical characteristics of persistent monoJIA. METHODS: Patients with oligoJIA and with at least two years follow-up entered the study. Those with monoarticular onset and persistent monoarticular course were compared with those with oligoJIA. Variables considered were: sex, age at onset, presence of benign joint hypermobility (BJH), ANA, uveitis, therapy and outcome. Patients who had not undergone clinical follow-up for more than 12 months were contacted by structured telephone interview. RESULTS: Of 347 patients with oligoJIA, 196 with monoarticular onset entered the study and 118 (60.2%), identified as persistent monoJIA, were compared with 229 oligoJIA. The mean follow-up was 11.4 years. The switch from monoarticular onset to oligoarticular course of 78 patients (38.8%) occurred by the first three years from onset. In comparison with oligoJIA, the most significant features of monoJIA were later age at onset (6.1 vs. 4.7 years), lower female prevalence (70.3 vs. 83.4%), higher frequency of BJH (61.9 vs. 46.3%), lower frequency of uveitis (14.4 vs. 34.1%) and ANA+ (68.6 vs. 89.5%) and better long-term outcome. CONCLUSIONS: MonoJIA, defined as persistent arthritis of unknown origin of a single joint for at least three years, seems to be a separate clinical entity from oligoJIA. This evidence may be taken into consideration for its possible inclusion into the new classification criteria for JIA and open new therapeutic perspectives. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12969-023-00869-w.