Severity and Diurnal Improvement of Morning Stiffness Independently Associate with Tenosynovitis in Patients with Rheumatoid Arthritis

Although morning stiffness has long been recognized as a characteristic feature of rheumatoid arthritis (RA), it is no more included in the 2010 ACR/EULAR Classification Criteria or in the current major instruments for evaluating disease activity of RA. In this cross-sectional study, we aimed to determine the independent value and the optimal measurement of morning stiffness by clarifying the associations between morning stiffness and synovial inflammation. We enrolled 76 consecutive RA patients who underwent musculoskeletal ultrasound examination and agreed to participate in the study. In addition to asking the duration of morning stiffness, we asked patients to complete a diagram which represents the time course of their morning stiffness in the dominant hand. Based on this diagram, we calculated the severity and the diurnal improvement of morning stiffness. We also determined the activity of intra-articular synovitis in 11 joints and tenosynovitis in 8 tendons/tendon compartments in the same hand by using power Doppler (PD) ultrasound with a semiquantitative score (0-3). For intra-articular synovitis, swollen/tender joint counts more strongly correlated with total PD scores (ρ = 0.379-0.561, p ≤ 0.001) than did any parameters of morning stiffness (ρ = 0.217-0.314, p = 0.006-0.021). For tenosynovitis, however, the severity on awakening and the improvement of morning stiffness more strongly correlated with total PD scores (ρ = 0.503-0.561, p < 0.001) than did swollen/tender joint counts (ρ = 0.276-0.388, p = 0.001-0.016). Multivariate analyses identified the severity on awakening and the improvement but not the duration of morning stiffness as factors that independently associate with the total tenosynovial PD score. Our data demonstrate a pathophysiological link between morning stiffness and tenosynovitis and also give an insight into the optimal measurement of morning stiffness. Our data support an independent value of evaluating morning stiffness in the management of RA.