International consensus outcome measures for patients with idiopathic inflammatory myopathies. Development and initial validation of myositis activity and damage indices in patients with adult onset disease

Objective. To devise new tools to assess activity and damage in patients with idiopathic myopathies (IIM). Methods. An international multidisciplinary consensus effort to standardize the conduct and reporting of the myositis clinical trials has been established. Two tools, known as the myositis intention to treat index (MITAX) and the myositis disease activity assessment visual analogue scale (MYOACT), have been developed to capture activity in patients with IIM. In addition, the myositis damage index (MDI) has been devised to assess the extent and severity of damage developing in different organs and systems. These measures have been reviewed by the myositis experts participating in the International Myositis Assessment and Clinical Studies (IMACS) group and have been found to have good face validity and to be comprehensive. The instruments were assessed in two real patient exercises involving patients with adult dermatomyositis and inclusion body myositis. Results. The reliability of MITAX, MYOACT and MDI, measured by the intraclass correlation coefficient among the physicians, and the inter-rater reliability, as assessed by variation in the physicians' rating of patients, was fair to good for most aspects of the tools. Reliability and inter-rater agreement improved at the second exercise after the participants had completed additional training. Conclusions. The MITAX, MYOACT and MDI tools, which are now undergoing validity testing, should enhance the consistency, comprehensiveness and reliability of disease activity and damage assessment in patients with myositis.