Value of imaging detected joint inflammation in explaining fatigue in RA at diagnosis and during the disease course: a large MRI study

OBJECTIVE: Fatigue in rheumatoid arthritis (RA) is hypothesised to be caused by inflammation. Still ~50% of the variance of fatigue in RA cannot be explained by the Disease Activity Score (DAS), nor by background or psychological factors. Since MRI can detect joint inflammation more sensitively than the clinical joint counts as incorporated in the DAS, we hypothesised that inflammation detected by MRI could aid in explaining fatigue in RA at diagnosis and during the follow-up. METHODS: 526 consecutive patients with RA were followed longitudinally. Fatigue was assessed yearly on a Numerical Rating Scale. Hand and foot MRIs were performed at inclusion, after 12 and 24 months in 199 patients and were scored for inflammation (synovitis, tenosynovitis and osteitis combined). We studied whether patients with RA with more MRI-inflammation were more fatigued at diagnosis (linear regression), whether the 2-year course of MRI-inflammation associated with the course of fatigue (linear mixed models) and whether decrease in MRI-inflammation in year 1 associated with subsequent improvement in fatigue in year 2 (cross-lagged models). Similar analyses were done with DAS as inflammation measure. RESULTS: At diagnosis, higher DAS scores were associated with more severe fatigue (p<0.001). However, patients with more MRI-inflammation were not more fatigued (p=0.94). During 2-year follow-up, DAS decrease associated with improvement in fatigue (p<0.001), but MRI-inflammation decrease did not (p=0.96). DAS decrease in year 1 associated with fatigue improvement in year 2 (p=0.012), as did MRI-inflammation decrease (p=0.039), with similar effect strength. CONCLUSION: Sensitive measurements of joint inflammation did not explain fatigue in RA at diagnosis and follow-up. This supports the concept that fatigue in RA is partly uncoupled from inflammation.