The longitudinal study of muscle changes with ultrasound: Differential changes in idiopathic inflammatory myopathy subgroups.

We investigated shear wave elastography (SWE), B mode ultrasound (US) and power Doppler (PD) as imaging biomarkers for longitudinal follow-up in idiopathic inflammatory myopathy (IIM), with a particular focus on immune-mediated necrotising myopathy (IMNM) and dermatomyositis (DM). Participants had serial SWE, US and PD on the deltoid (D) and vastus lateralis (VL) muscles on four occasions at intervals of 3-6 months. Clinical assessments included manual muscle testing, and patient and physician reported outcome scales. Thirty three participants were included: IMNM= 17, DM = 12, overlap myositis= 3, polymyositis =1. Twenty were in a prevalent clinic group, and 13 were recently treated cases in an incident group. Differential changes in SWS and US domains occurred with time in both the prevalent and incident groups. In VL-prevalent, echogenicity increased over time (p = 0.040), while in incident cases there was a trend of reduction to normal over time (p = 0.097) with treatment. Muscle bulk reduced in D-prevalent group (p = 0.096) over time, suggesting atrophy. SWS also reduced in the VL-incident (p = 0.096) group over time, suggesting a trend towards improvement in muscle stiffness with treatment. SWE and US appear promising as imaging biomarkers for patient follow-up in IIM and indicate changes over time, especially with echogenicity, muscle bulk and SWS in the VL. Due to the limitations of participant numbers, additional studies with a larger cohort will help to evaluate these US domains further and outline specific characteristics within the IIM subgroups.

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