Epidemiology of idiopathic inflammatory myopathies: a population-based cohort in England, 2002-2021

Idiopathic inflammatory myopathies (IIM) are a heterogenous group of conditions with substantial morbidity but population-based epidemiology is lacking. We aimed to provide a contemporary epidemiological assessment of IIM.

Population-based study using the Clinical Practice Research Datalink (CPRD) Aurum primary care database with linkage to hospital episode data. All patients (aged >2 years) from England registered between 1 March 2002 and 31 March 2021 were included. Incidence was estimated for three main IIM subtypes: dermatomyositis (DM), inclusion body myositis (IBM) and other IIM. Incidence rate ratios using Poisson models were compared across subgroups.

We identified n=1590 DM, n=432 IBM and n=2083 other IIM incident cases. Incidence of DM was 0.79 (95% CI 0.75 to 0.83), IBM 0.50 (95% CI 0.46 to 0.55) and other IIM 1.03 (95% CI 0.99 to 1.08) per 100 000 person-years.Females had a higher incidence of DM (adjusted incidence rate ratio (aIRR) 1.84, 95% CI 1.66 to 2.04) and other IIM (aIRR 1.09, 95% CI 1.00 to 1.19), but a lower incidence of IBM (aIRR 0.73, 95% CI 0.60 to 0.89), compared with males. South Asian, black, other and unknown ethnic groups had a higher incidence of DM and other IIM compared with those of white ethnicity, particularly individuals of black ethnicity for other IIM (aIRR 2.80, 95% CI 2.32 to 3.37).Point prevalence at study end was 0.015% for DM, 0.005% for IBM and 0.016% for other IIM.

There are major demographic differences in IIM incidence, notably by sex and ethnicity. Resource planning should account for these marked differences.