High-frequency ultrasound of the skin in systemic sclerosis: an exploratory study to examine correlation with disease activity and to define the minimally detectable difference

Background: Thickened skin is a major clinical feature in patients with systemic sclerosis (SSc). We investigated changes of skin thickness in patients with SSc using both high frequency ultrasound (HFU) and the modified Rodnan skin score (mRSS) to evaluate the feasibility of application of HFU in skin involvement and the relationship between HFU and clinical profiles.

Methods: We recruited 31 consecutive patients with SSc and 31 age-matched and sex-matched healthy controls in this prospective, cross-sectional study. Skin thickness was measured by an 18-MHz ultrasonic probe at five different skin sites. Total skin thickness (TST) and skin thickness using categorical mRSS scores were recorded and compared to HFU. The European Scleroderma Trial and Research (EUSTAR) group Disease Activity Index (EUSTAR-DAI) and other clinical manifestations were assessed and analyzed.

Results: TST in patients with SSc was thicker than in healthy controls (P < 0.001), and correlated positively with total mRSS and the EUSTAR-DAI and correlated negatively with disease duration (P < 0.05). Patients with higher TST had higher EUSTAR-DAI, mRSS, C-reactive protein (CRP) and lower diffusing capacity of the lung for carbon monoxide (DLCO) (P < 0.05). Even in patients who on clinical assessment were assigned an mRSS that suggested the skin thickness was normal. This was also true to mRSS locally of 1 and 2 (P < 0.01). The area under the receiver operator characteristic (ROC) curve was 0.831 and yielded sensitivity of 77.4% and specificity of 87.1% at the predicted probability of 7.4 mm as the optimal cutoff point to access skin thickness.

Conclusions: In the study, HFU was able to measure skin thickness, it correlated quantitatively with a valid measure of SSc activity, and a minimal detectable difference was identified.