Background Patients with idiopathic inflammatory myositis (IIM) suffer from comorbidities such as interstitial lung disease (ILD), cancer, and infections related to immunosuppressive agents.
Objectives To evaluate 1) incidence rate ratio (IRRs) of major comorbidities in IIM compared to non-IIM Koreans, and 2) impact of the comorbidities on survival and functional impairment.
Methods A retrospective cohort study was performed using the 2002–2013 National Health Insurance Service (NHIS) database of weighted probability sampled one million cohort. IIM was identified by at least two visits to the tertiary hospitals under ICD9 code of IIM. Age- and sex-matched twenty subjects per patient, who never had ICD9 code at least for 1 year, were included as the unexposed. The occurrence of ILD, cancer, herpes zoster, and tuberculosis, severely disabled status, and death was captured using ICD9 code, disease-specific medication, and/or government-approved qualification for co-payment reduction on these outcomes.
Results Ninety-one patients were newly diagnosed as IIM from 2003 to 2013. Most patients (>90%) defined as such were found to have muscle biopsy, electromyogram, or multiple laboratory examinations on muscle enzymes. Their incidence rate of the above outcomes was significantly elevated (Table); IRRs of ILD 31.8 [95% confidence interval: 13.2 – 76.7], cancer 2.36 [1.61–3.48], herpes zoster 1.75 [1.20–2.54], tuberculosis 2.08 [1.15–3.77], severely disabled status 4.79 [2.55–9.01], and mortality 3.47 [1.96–6.12]. Among IIM patients, the presence of ILD or cancer greatly deteriorated survival; mortality IRR of 73.2 [9.58–559.9] in ILD and 63.2 [17.6–226.7] in cancer. The occurrence of zoster and tuberculosis did not influence on the mortality. ILD and cancer also worsened functional impairment of IIM patients; IRR of 12.0 [2.64–54.9] for severely disabled status in ILD and 13.2 [4.19–41.6] in cancer.
Conclusions This is the first report of the nation-wide population based evaluation of IIM in Korea. The incidence of ILD, cancers, zoster, and tuberculosis was exceptionally higher in IIM than non-IIM. ILD and cancers significantly affected prognosis of IIM. More detailed analysis of comorbidities and their risk factors using the whole national database is underway.
Disclosure of Interest None declared