The influence of BMI on JIA course in children

Introduction: Juvenile Idiopathic Arthritis (JIA) represents the most common chronic rheumatic disease in childhood affecting joints and other structures. According to International League of Association for Rheumatology (ILAR), seven subtypes of arthritis can be defined in relation with the number of joints and the extra-articular involvement occurring in the first six months of disease. In the last years, greater improvements in long-term outcomes have been provided by new therapeutic options. Growing evidence has supported the role of obesity as a risk factor for severity disease both in adults and children, but recent studies have also showed a potential effect of underweight in this context. To date, the influence of Body mass index (BMI) on JIA course is poorly studied, in particular in children. Objectives: To evaluate the influence of BMI on disease course in children with JIA. Methods: We retrospectively examined 113 children with JIA classified according to ILAR criteria attending our Rheumatology Clinic. At the time of the first visit, anthropometric and laboratory data were assessed. Age at disease onset, disease duration, joint involvements, presence of comorbidities, and medications were also collected. Disease activity was assessed by JADAS-10. According to BMI Z-score, our cohort was divided into five groups: underweight, normal weight, overweight (OW), obesity (OB), and severe obesity. Differences for continuous variables were analysed with the independent-sample t test for normally distributed variables and with the Mann-Whitney test in case of non-normality. Qualitative variables were compared using the chi-squared test. Linear regression was used to investigate the association of JADAS-10 with BMI categories. Results: The mean age was 7.43±4.03 years. Forty-one percent were persistent oligoarticular, 9.1% extended oligoarticular, 23.6% RF- polyarticular, 8.2% RF+ polyarticular, 7.3% systemic, 8.2% enthesitis-related arthritis, and 2.7% psoriatic arthritis. The prevalence of underweight, normal weight, overweight, obesity, and severe obesity was 7.2%, 54.1%,10.8%,17.1%, and 10.8%, respectively. Ferritin levels, erythrocyte sedimentation rate values, and disease duration were significantly higher in patients with severe obesity and in those underweight compared to subjects belonging to normal weight, OW, and OB groups (p=0.02, p=0.03, and p=0.04, respectively). Similarly, underweight patients and those with severe obesity showed the greater JADAS-10 than other groups (p=0.013). Obese patients presented with the greater number of affected joints (p=0.04). More, a major involvement of lower limbs was observed in patients with obesity and severe obesity, including sacroiliac and midfoot joints (p=0.004 and p= 0.005, respectively). These patients had also an increased number of relapses compared to other groups (p=0.025). Conclusion: BMI seems to influence JIA course in children through different mechanisms. In addition to the mechanical role of obesity and its potential pro-inflammatory effect, underweight seems also to affect the course of the disease. This could be explained by hypothesising an impairment on weight gain by active disease, but further studies are needed to a better understanding. In addition to the arthritis control, these findings underscore the need to maintain an adequate BMI through dietary and lifestyle intervention in order to avoid a more unfavourable JIA course.