From Strength to Strength: Idiopathic Inflammatory Myopathy Diagnosis & Management

Dr. Rohit Aggarwal

ACR Convergence 2021—Idiopathic inflammatory myopathies (IIM) are heterogeneous, systemic diseases with muscle and/or skin as the primary targets, and diagnosing and treating these conditions correctly can be a major challenge for many rheumatologists. During the ACR Convergence 2021 Review Course, Rohit Aggarwal, MD, MS, professor of medicine, rheumatology and clinical immunology and co-director of the University of Pittsburgh Medical Center Myositis Center, provided an overview of this important topic.

Diagnostic Criteria

Many rheumatologists are familiar with the 1975 Bohan and Peter criteria for the diagnosis of dermatomyositis and polymyositis, but Dr. Aggarwal noted these criteria were developed years before the discovery of numerous myositis-associated autoantibodies that have greatly enhanced our understanding of these diseases.^1,2 Given this and several other limitations of the Bohan and Peter criteria, the ACR and the European Alliance of Associations for Rheumatology (EULAR) worked together to publish classification criteria for adult and juvenile IIM in 2017.³

Sixteen variables were weighted and included in the final classification criteria, and a web-based calculator was developed to provide a score that represents the probability of a specific patient having an IIM. This setup allows researchers using the EULAR/ACR classification criteria to have flexibility in tailoring the inclusion criteria in a study to a certain level of sensitivity or specificity depending on the type of study considered.

In studying the performance of these criteria in a large, monocentric cohort of patients with IIM, Barsotti et al. evaluated 439 consecutive patients with a diagnosis of IIM at the Karolinska University Hospital, Sweden.⁴ The authors found the EULAR/ACR criteria had a higher sensitivity (87.7%) than the Bohan and Peter criteria (80.4%) and that the EULAR/ACR criteria demonstrated a high specificity (>98%) for the major IIM subgroups polymyositis, dermatomyositis and inclusion body myositis. Given this information, Dr. Aggarwal explained that these new classification criteria represent a great advancement in the field of myositis.

In discussing the epidemiology of IIM, Dr. Aggarwal noted these diseases are rare, with an incidence of about eight per one million patients. Aside from juvenile dermatomyositis, the typical age of onset of IIM is between the ages of 30 and 60, with specific forms of disease, such as inclusion body myositis (IBM), occurring most specifically in patients 50 and older. On that note, IBM is more common in male patients while other forms of IIM are seen more often in women than men in a ratio of about 2:1. Finally, although all ethnic groups can be affected by IIM, these diseases are most common in Black patients.