Background Generally, an increased body mass index (BMI) is associated with higher inflammatory markers. Within rheumatoid arthritis (RA) it has been observed that a high BMI is associated with less severe radiographic joint damage.[1–4] The biologic mechanism underlying this association is unidentified.

Objectives As joint damage is the consequence of inflammation, and because MRI is sensitive in detecting local inflammation, this study aimed to explore the association between BMI and MRI-detected inflammation in patients with RA, other arthritides and symptom-free persons recruited from the general population.

Methods We studied 202 RA patients, 170 patients with other inflammatory arthritides from the Leiden Early Arthritis Clinic cohort and 193 symptom-free persons from the general population.[5] At baseline, all participants underwent a contrast-enhanced 1.5T MRI of the dominant or most painful MCP, wrist and MTP joints. Each MRI-scan was scored by two readers on synovitis, BME and tenosynovitis. The sum of the scores yielded the total MRI- inflammation score. Linear regression models (on log transformed MRI-data) were used for associations with the total MRI-inflammation score and Spearman's correlations for correlations with separate inflammatory features as they were non-normally distributed.

Results A higher BMI was associated with higher MRI-inflammation scores in symptom-free persons (β=1.029, p=0.040) and arthritides other than RA (β=1.082, p<0.001). These β's indicate an 1.029 fold and an 1.082 fold increase in MRI-inflammation with every point increase in BMI (Figure). However, in RA patients a higher BMI was associated with lower MRI-inflammation score (β=0.969, p=0.005). Similar results were seen when correcting for age and gender. Evaluating the different inflammatory features separately showed that BMI was correlated with higher synovitis and tenosynovitis scores in symptom-free persons (respectively ρ=0.17, p=0.016 and ρ=0.16, p=0.027) and in arthritides other than RA (respectively ρ=0.40, p<0.001 and ρ=0.26, p<0.001). This is in contrast to RA where a higher BMI was correlated with lower synovitis scores (ρ=-0.18, p=0.010) and was not correlated with tenosynovitis (ρ=-0.10, p=0.15). Similarly, BMI was positively correlated with BME in other arthritides (ρ=0.16, p=0.048)) and negatively correlated with BME in RA (ρ=-0.23, p=0.001).

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Conclusions The association between BMI and MRI-detected inflammation differs in patients with RA compared to patients with other arthritides and symptom-free controls. Within RA a higher BMI is associated with less MRI-detected synovitis and BME; this might explain the previously observed association of BMI with less radiographic damage.

1. Kaufmann J, et al. J Rheumatol 2003;30:2350–5.

2. Mil AHM van der H, et al. Ann Rheum Dis 2008;67:769–74.

3. Westhoff G, et al. Arthritis Rheum 2007;56:3575–82.

4. Baker JF, et al. Ann Rheum Dis 2014;73:1923–8.

5. Mangnus L, et al. Ann Rheum Dis 2015;74(Suppl2):153–153.

Disclosure of Interest None declared