Background/Purpose:

Microwave Radiometry (MR) is an easy-to-perform, rapid, non-invasive, objective method, able to measure in-depth tissue temperature. In a proof-of-concept study using joint ultrasound as reference method, we showed that an increased in-depth knee joint temperature detected by MR in the absence of relevant clinical signs reflects the presence of subclinical synovial inflammation in rheumatoid arthritis (RA) (1).

We aimed to test the hypothesis that a ‘thermo-score’ constructed as the sum of MR-derived temperatures of small and large joints reflects global disease activity levels in RA.

Methods:

Consecutive RA patients (N=61) underwent clinical, laboratory and MR (RTM 01 RES microwave computer based system, Bolton, UK) assessments, as well as joint ultrasound. Unselected patients with active disease (N=19) were re-examined 2 months after treatment initiation. Age- and sex-matched healthy individuals (N=23) and patients with osteoarthritis (N=23) served as controls. We created a ‘thermo-score’ by summing the relative temperatures (Dt) of 7 small joints (wrist, 2^nd-3^rd metacarpophalangeal, 2^nd-3^rd proximal interphalangeal, 2^nd and 5^th metatarsophalangeal) of the clinically dominant hand and foot, based on the German US7 ultrasound score (2), as well as of 3 large joints (elbow, knee, lower leg) of the clinically dominant arm and leg. The relative temperature (Dt) was calculated as the difference between the reference and a control point for each joint, thus, resulting in lower Dt values in inflamed versus healthy joints. Among healthy individuals examined 2 months apart ‘thermo-score’ values were repeatable (ICC=0.714).

Results:

The baseline ‘thermo-score’ could discriminate RA patients in high [mean (SD)=7.36 (1.68)], moderate [mean (SD)=9.71 (1.70)], low disease activity [mean (SD)=10.74 (1.37)] and remission [mean (SD)=11.63 (1.71)] (p<0.001 by Kruskal-Wallis test). Healthy subjects [mean (SD)=11.84 (2.42)] and patients with osteoarthritis [mean (SD)=10.99 (3.05)] had comparable values with RA patients in remission and low activity, respectively. Among RA patients, the ‘thermo-score’ correlated inversely to the DAS28 disease activity score (p<0.001), tender joint count (p<0.001), swollen joint count (p=0.028), patient’s visual analogue scale (p<0.001), CRP (p=0.014), ESR (p=0.013), as well as the standard Ultrasound Scores of 7 joints (2) (all p<0.05). Notably, individual ‘thermo-score’ changes from baseline to follow-up mirrored the corresponding DAS28 changes in 15/19 patients.

Conclusion:

In-depth temperatures of small and large joints detected by MR allow the construction of a ‘thermo-score’ that reflects the global disease activity levels in RA and may serve as an additional biomarker. Optimization of MR equipment and technique may result in an objective measurement of RA disease activity in clinical practice.

References

1. Zampeli E, et al. Detection of subclinical synovial inflammation by microwave radiometry. PLoS ONE. 2013;8(5):e64606.
2. Backhaus M, et al. Evaluation of a novel 7-joint ultrasound score in daily rheumatologic practice: a pilot project. Arthritis Rheum. 2009;61(9):1194-201.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/in-depth-temperature-of-small-and-large-joints-assessed-by-microwave-radiometry-as-an-additional-biomarker-in-rheumatoid-arthritis/