Abstract
ObjectivesThe aim was to identify and characterize all incident adult cases of idiopathic inflammatory myopathies (IIM) between 1 January 2007 and 31 December 2016 in the City of Salford, UK.MethodsAdults first diagnosed with IIM within the study period were identified by: a Salford Royal NHS Foundation Trust (SRFT) inpatient episode IIM-specific ICD-10 coding search; all new patient appointments to SRFT neuromuscular outpatient clinics; and 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 consensus expert opinion agreed. Cases were excluded if <18 years of age at disease onset, if they did not meet probable criteria or when probable but expert opinion concluded a non-IIM diagnosis.ResultsThe multimodal case ascertainment identified 1156 cases which, after review and application of exclusion criteria, resulted in 32 incident cases during the study period. Twenty-three of 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, and higher for females than for males (25.2 vs 10.0/1 000 000 person years, respectively). A significant incidence increase over time was apparent (13.6 vs 21.4/1 000 000 person years; P = 0.032). Using EULAR/ACR classification criteria, the largest IIM subtype (21/32) was PM, followed by DM (8/32), IBM (2/32) and amyopathic DM (1/32). Expert opinion subtype differed from EULAR/ACR classification criteria in 19/32 cases.ConclusionThe incidence of adult IIM in Salford is 17.6/1 000 000 person years, higher in females, and is increasing over time. Disagreement exists between EULAR/ACR-derived and expert opinion-derived IIM subtype assignments.
Highlights
The idiopathic inflammatory myopathies (IIM) represent a spectrum of rare immune-mediated syndromes usually characterized by inflammation in skeletal muscle [1]
Expert opinion subtype differed from EULAR/ACR classification criteria in 19/32 cases
Disagreement exists between EULAR/ACR-derived and expert opinionderived IIM subtype assignments
Summary
The idiopathic inflammatory myopathies (IIM) represent a spectrum of rare immune-mediated syndromes usually characterized by inflammation in skeletal muscle [1]. Studying the epidemiology of rare conditions can assist in the identification of risk factors, disease associations and temporal trends. Previous international studies have focused on specific IIM subtypes, such as IBM or immune-mediated necrotizing myopathy (IMNM), are historic, were undertaken before recent developments in our understanding of the range of IIM subtypes and used widely varying methodologies and case acquisition strategies [2, 3]. The recently published combined EULAR/ACR classification criteria for adult and juvenile IIM represent potential progress in identifying IIM and various disease subtypes [4]. The gross sum of these scores can be used directly or converted into a probability (as a percentage) of an IIM diagnosis. A second function is in the identification of one of four IIM disease subtypes, namely PM, DM, IBM or amyopathic DM (ADM)
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