Increasing incidence of adult idiopathic inflammatory myopathies in the City of Salford, UK: a ten-year epidemiological study

Objectives: Identify and characterise all incident adult cases of idiopathic inflammatory myopathies (IIM) between Jan 1st 2007 and Dec 31st 2016 in the City of Salford, UK. Methods: Adults first diagnosed with IIM within the study period were identified by: i) a Salford Royal NHS Foundation Trust (SRFT)inpatient episode IIM-specific ICD-10 coding search; ii) all new patient appointments to SRFT neuromuscular outpatient clinics; iii) all Salford residents enrolled within the UKMYONET study. All patients with ‘definite’ IIM by the 2017 EULAR/ACR classification criteria were included, as were ‘probable’ cases if expert opinion agreed. Cases were excluded if <18 years at disease onset, if they did not meet ‘probable’ criteria, or when ‘probable’ but expert opinion concluded a non-IIM diagnosis. Results: The case ascertainment procedures identified 1,156 cases which, after review and application of exclusion criteria, resulted in 32 incident cases during the study period. 23/32 were female with a mean age of 58.1 years. The mean incidence of adult IIM was 17.6/1,000,000 person years (py), higher for females than for males (25.2 versus 10.0/1,000,000py respectively). A significant incidence increase over time was apparent (13.6 versus 21.4/1,000,000py; p=0.032). Using EULAR/ACR classification criteria, the largest IIM subtype (21/32) was polymyositis, followed by dermatomyositis (8/32), inclusion body myositis (2/32) and amyopathic dermatomyositis (1/32). Expert opinion subtype differed from EULAR/ACR Classification criteria in 19/32 cases. Conclusion: The incidence of adult IIM in Salford is 17.6/1,000,000py, higher in females and is increasing over time. Disagreement exists between EULAR/ACR-derived and expert opinion-derived IIM subtype assignments.