A systematic approach to the consensus definitions and reliability of ultrasound in idiopathic inflammatory myopathies; Preliminary data by the OMERACT ultrasound in myositis working group

Aim Current ultrasound grading tools do not fulfil the criteria of the OMERACT Filter 2.1 Instrument Selection Algorithm (OFISA)and are not idiopathic inflammatory myopathies (IIM)-specific, prompting further work by the OMERACT US Working Group. The aim of this study was, therefore, to develop consensual definitions of elementary US lesions specific to IIM, using a structured methodological approach. Methods A multicentre group of 21 international experts from the OMERACT US in myositis subgroup participated in a multi-round Delphi survey followed by a web-based reliability exercise. Results The group participated in five Delphi rounds, followed by a web-based reliability exercise. Preliminary broad domains of “inflammation” and “damage” resulted in substantial intra-rater reliability testing (kappa: 0.662 and 0.620, respectively) but poor inter-rater reliability testing (kappa: 0.142 and 0.112, respectively). A further two Delphi rounds resulted in a consensus to revise the broad domains to a single domain of “muscle inflammation and damage", with elementary components including echogenicity and fascial thickness. A categorical morphological grading system was then tested in a web-based reliability exercise, which resulted in substantial intra-rater reliability testing (kappa: 0.706, min: 0.051, max: 1.000, SD: 0.295) and an improved moderate inter-rater reliability testing (kappa: 0.506, min: 0.421, max: 1.000, SD: 0.167). Conclusion The systematic approach by the OMERACT US in the myositis subgroup has developed a categorical morphological scoring system, which showed moderate inter-rater reliability using consensus-derived definitions of US domains in IIM.