Influenza-Associated Benign Acute Childhood Myositis During the 2024-2025 Season: A Retrospective Multicenter Study

Background: The aim of this study was to describe the clinical and laboratory characteristics of hospitalized pediatric influenza cases during the 2024-2025 season in Northwestern Greece, with a focus on influenza-associated benign acute childhood myositis (BACM). Methods: We conducted a retrospective observational study of children aged 0-16 years hospitalized with laboratory-confirmed influenza between October 2024 and May 2025 at two pediatric departments. BACM was diagnosed based on calf pain, difficulty walking, elevated creatine kinase (CK), and symptom resolution without other causes. Results: A total of 113 children (mean age 7.0 ± 4.2 years; 50.4% male) were included; 61.1% had influenza A and 38.9% influenza B. None had received influenza vaccination. BACM was identified in 37 children (32.7%), who were significantly older than patients without myositis (9.3 ± 2.7 vs. 6.0 ± 4.5 years, p < 0.001). Influenza B was strongly associated with BACM (70.3% vs. 29.7%, χ²(1) = 22.7, p < 0.001, Cramer's V = 0.448). Median CK in BACM cases was 2637 IU/L (range: 189-129,390 IU/L); all had preserved renal function. One patient with congenital myopathy developed rhabdomyolysis (peak CK 130,000 IU/L) but had a full recovery. All patients received oseltamivir and supportive care; no intensive care admissions or deaths occurred. Conclusions: In our hospitalized cohort, BACM was observed relatively frequently (32.7%), particularly in children with influenza B; however, this proportion reflects hospitalized cases and does not indicate the true incidence in the general pediatric population. Despite high CK levels, outcomes were favorable with supportive care. These findings underscore the importance of clinician awareness to avoid unnecessary investigations and hospitalizations.