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

Area of Science:

• Pediatrics
• Infectious Diseases
• Rheumatology

Background:

• Childhood viral myositis has been historically linked to influenza, particularly influenza B, in school-aged children.
• The seasonal patterns and specific clinical features of influenza-associated myositis (IAM) require further elucidation.

Purpose of the Study:

• To describe the clinical epidemiology of influenza-associated myositis (IAM) over a five-year period.
• To identify the key epidemiological and clinical characteristics of IAM and compare them with general influenza hospitalizations.

Main Methods:

• Retrospective identification of ICD-10 coded myositis cases from 2011-2015 with subsequent chart reviews.
• Exclusion of non-viral myositis causes and cross-referencing with influenza laboratory test results to confirm IAM.
• Definition of probable IAM for cases occurring during influenza season without alternative causes.

Main Results:

• 283 viral myositis cases were identified, with 85% occurring between May and October; 78% of tested patients were influenza positive.
• An estimated 80% of viral myositis cases are attributable to IAM annually.
• IAM predominantly affected males (82%), children aged 5-9 (73%), and was associated with influenza B (86%); cases were generally benign with elevated creatinine kinase (CK) levels and no renal impairment.

Conclusions:

• Childhood viral myositis exhibits a strong seasonal association with influenza, with up to 80% of cases attributable to IAM.
• IAM presents as a clinically stereotyped, age-restricted, and typically benign condition, strongly linked to influenza B.
• Early clinical recognition of IAM can prevent unnecessary investigations and treatments.