Refining inflammatory myopathies incidence and characteristics: a quadruple source capture-recapture survey using 2017 ACR/EULAR criteria.

OBJECTIVE: Inflammatory myopathies (IM), characterized by muscle inflammation and weakness, are rare systemic diseases. Our previous study estimated an IM incidence rate of 7.98 cases/million/year (95% CI: [7.38-8.66]) and highlighted important variations that were likely due to methodological issues rather than true epidemiological differences. The aim of this study was to refine the incidence of IM, using the 2017 ACR/EULAR IM classification criteria and a quadruple source capture-recapture method during a 6-year period in Alsace (France), a 2-million-population region, benefiting from good access to healthcare and accredited IM referral centers.

METHODS: Clinical data of potential IM patients were obtained from 4 sources (general practitioners and community specialists, public and private hospital records, public and private laboratories, and archives from the pathology department). Patients residing in Alsace and fulfilling the 2017 ACR/EULAR criteria for IM between 01/01/2006 and 01/01/2013 were included. Potentially incomplete case ascertainment was corrected by capture-recapture analyses. Spatial and temporal distribution of the incidence was studied. Systemic manifestations of the disease were additionally assessed.

RESULTS: The review of 1742 potential cases identified a total of 106 IM patients. No spatial nor temporal heterogeneity was observed. Log-linear modeling determined an estimated 14.9 additional cases were missed. Thus, the incidence rate of IM was 8.22 new cases/million/year (95% CI: [6.76-9.69]). Extramuscular manifestations other than dermatomyositis rash were frequently recorded.

CONCLUSION: The stringent methodology used in this study provides an accurate estimation of the incidence of IM. This study also demonstrates, on a population-based cohort, the systemic nature of IM. This article is protected by copyright. All rights reserved.