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.