Can we use enthesis ultrasound as an outcome measure of disease activity in spondyloarthritis? A study at the Achilles level

Objective. The aim of this study was to evaluate the construct validity of enthesis US in the assessment of disease activity in SpA. Methods. A longitudinal Achilles enthesis US study in patients with early SpA was undertaken. Achilles US examinations were performed at baseline, 6 and 12 months and compared with clinical outcome measures collected at the baseline visit. Results. Bilateral Achilles enthesis of 146 early SpA patients (68 women) were analysed. Basal mean BASFI, BASRI-spine, BASDAI and Ankylosing Spondylitis Disease Activity Score (ASDAS) were 2.44 (S.D. 2.05, range 0-8), 0.67 (S.D. 0.74, range 0-3), 4.60 (S.D. 2.07, range 0-9.5) and 2.51 (S.D. 1.16, range 0-5), respectively. The mean ESR was 15.0 mm/h (S.D. 16.99, range 0-109) and the mean CRP was 8.67 mg/l (S.D. 16.98, range 1-90). At baseline, the Achilles Doppler signal and US structure alteration were significantly associated with higher CRP and ESR levels. Patients who had very high disease activity at baseline, as assessed by the ASDAS (>3.5), had a significantly higher Achilles total US score at baseline (P = 0.04), and ASDAS <1.3 predicted no Doppler signal at 6 and 12 months. Overall, the Achilles total US score was significantly higher in patients with higher levels of CRP (baseline P = 0.04, 6 months P = 0.006, 12 months P = 0.03) and ESR (baseline P = 0.02, 6 months P = 0.04, 12 months P = 0.005) at baseline. The Doppler signal at the baseline visit predicted a higher total US score at 6 and 12 months. Conclusion. Doppler US has significant associations with other commonly accepted disease activity measures, such as ESR, CRP and ASDAS, and seems to be an objective outcome measure for enthesitis.