Thermographic and ultrasound assessment in patients with rheumatoid arthritis: can thermography detect subclinical synovitis at the wrist?

Background: Thermography is an emerging imaging modality which allows for a quick and objective measure of joint surface temperature in patients with rheumatoid arthritis (RA). To date, there are no published studies comparing thermography with ultrasonography in the subclinical assessment of joint inflammation at the wrist of patients with RA, and no published data on inter-rater reliability for multiple raters for thermographic assessment at the RA wrist. In our study comparing thermography and ultrasonography at the RA wrist, we aim to determine if thermography can detect subclinical synovitis. Additionally, we performed inter-reliability testing (multiple raters) for both thermography and ultrasonography.

Methods: Thermographic (average (Tavg), maximum (Tmax) and minimum (Tmin) temperatures) and ultrasound (total grey-scale (TGS) score and total power Doppler (TPD) scores) parameters were compared between two patient groups: Asymptomatic Group (with non-swollen and non-tender wrists) and Symptomatic Group (with swollen and/or tender wrists). Among Asymptomatic Group patients, thermographic parameters were compared between those with and without wrist joint recess(es) having ultrasound synovitis (PD ≥ 1 or GS ≥ 2); Spearman's correlation and simple linear regression were used to study the relationship between thermographic and ultrasound parameters. Intra-class correlation coefficient (ICC) was used for inter-rater reliability calculation.

Results: Eighty-seven RA patients' right wrists were imaged in this cross-sectional study. Thermographic temperatures, TPD and TGS scores were all significantly (p < 0.05) greater among Symptomatic Group versus Asymptomatic Group patients. Among Asymptomatic Group patients, thermographic temperatures were all significantly higher (P < 0.01) in wrists having joint recess(es) with ultrasound PD ≥ 1 or GS ≥ 2, while all thermographic parameters correlated significantly with TPD (correlation coefficient ranging from 0.43 to 0.48, p < 0.001) and TGS (correlation coefficient ranging from 0.33 to 0.37, p < 0.01). The ICC values based on a subset of images obtained for inter-reliability testing were high for thermography (0.994 to 0.998) and ultrasonography (0.933 to 0.952).

Conclusions: Swollen and/or tender RA wrists displayed greater thermographic and ultrasound-detected joint inflammation. At clinically quiescent (non-swollen; non-tender) wrists, thermographic temperatures significantly correlated with ultrasound-detected joint inflammation.

Clinical trial number: Not applicable.