Background/Purpose:   Obesity is associated with a significantly increased risk of inflammatory arthritis in adult patients with psoriasis.  Obese adults with psoriatic arthritis (PsA) also have more difficult to treat arthritis, however weight loss may improve their response to treatment.  It is not yet known if obesity plays a role in the development of arthritis in children with psoriasis.  We evaluated the association of obesity with pediatric psoriasis and PsA. 

Methods: We conducted a retrospective cohort study of children with psoriasis and PsA enrolled in The Health Improvement Network (THIN) database between 1994 and 2015.  All psoriasis, PsA, and arthritis, cases in the cohort had ≥ 1 READ code for psoriasis, psoriasis and arthritis, or arthritis, respectively.  Controls were matched on age, sex, and practice at a 5:1 ratio.  Age- and sex-specific z-scores for BMI (zBMI) were calculated.  Differences in demographic and clinical characteristics, including overall prevalence of obesity, were assessed using the t-test, test for the equality of proportions, and Wilcoxon rank sum test.  Cox proportional hazard regression was performed to assess risk of developing arthritis in children with psoriasis who were overweight or obese compared to normal or underweight, adjusting for age and sex.

Results: There were 234 children with PsA, 6036 with other types of arthritis, 13162 with psoriasis, and 63262 controls. Median age at PsA diagnosis was 11.3 (IQR 7.8-14.0) years and psoriasis diagnosis was 10.1 (IQR 6.9-13.1) years.  Amongst those children who had both psoriasis and arthritis, approximately 2/3 developed psoriasis first.  Figure 1 shows the probability of remaining arthritis-free in psoriasis patients.  Overall, the risk of arthritis was low among psoriasis patients with a cumulative probability of approximately 2.6% during the follow-up period.  Approximately half of the children with psoriasis who went on to develop arthritis did so within 5.5 years of the diagnosis of psoriasis.  As shown in Table 1, patients with psoriasis had a significantly higher zBMI compared to healthy controls (p <0.001).  Arthritis patients had a significantly lower zBMI than controls (p 0.03).  Although the difference did not reach statistical significance, PsA patients also had a smaller zBMI than controls.  There was no significant association between zBMI at time of psoriasis diagnosis and risk of later developing arthritis (HR 0.87, p 0.10).   

Conclusion: In contrast to adult data, obesity was not associated with development of psoriatic arthritis in children.  Moreover, children with arthritis, including PsA, were leaner than the control population.