Influenza-associated myositis: a single-centre, 5-year retrospective study.

We aimed to describe the clinical epidemiology of influenza-associated myositis (IAM) over a 5-year period. We identified ICD-10-coded myositis cases retrospectively 2011-2015 and performed limited chart reviews. We excluded myositis with non-viral causes and cross-referenced with laboratory records of influenza tests to identify confirmed IAM. We defined probable IAM as viral myositis occurring during the influenza season without alternative cause. We described epidemiological and clinical features of IAM and compared IAM with all hospitalised influenza. We identified 283 cases of viral myositis with seasonal peaks (May to October, 85% of cases); 69 were tested for influenza, 52 (78%) were positive. Given the strong seasonality concurrent with the influenza season, we estimated that 80% (95% CI 76-85) of viral myositis is attributable to IAM annually. Of 226 cases of IAM, 21% (n = 49) were confirmed and the remaining probable. IAM was associated with being male (82%), aged 5-9 (73%), and influenza B (86%). The majority had bilateral calf pain; mean creatinine kinase (CK) value was 3579 U/L, and no cases had renal impairment.Conclusion: Childhood viral myositis shows strong association with the influenza season. IAM is clinically stereotyped, age restricted, and benign in most and strongly associated with influenza B. What is Known: • Childhood viral myositis has been reported in association with influenza for decades, more frequently with influenza B and in school-aged children. What is New: • Here, we show over a 5-year period that viral myositis is strongly seasonal with up to 80% of cases attributable to influenza. • Influenza-associated myositis (IAM) typically occurs in boys, aged 5-9 years, with influenza B and is most often benign. Early clinical recognition may avoid unnecessary treatment and testing.