Influenza-associated Benign Acute Childhood Myositis

Background: To characterize the severity, clinical course, and 6-month outcomes of influenza-associated benign acute childhood myositis (BACM) during the 2024–2025 season and to explore differences between cases associated with influenza A and B. Methods: We retrospectively analyzed 82 children who met the BACM diagnostic criteria and none of the exclusion criteria among 11,528 children with laboratory-confirmed influenza between October 2024 and March 2025. Clinical features, first-presentation laboratory findings and outcomes were compared between influenza A– and B–associated cases. Venous measurements were based on the first sample obtained during evaluation for leg pain or gait disturbance. Results: The 82 patients (mean age, 6.8 ± 2.1 years; range, 3–14 years) included 55 boys, 27 influenza A cases and 55 influenza B cases. Leg pain or gait disturbance developed 1–9 days after influenza symptom onset and resolved within 3–5 days. None required intensive care or renal replacement therapy; first-presentation urinalysis and serum creatinine were normal in all patients. Influenza B–associated cases had higher myoglobin, more frequently had creatine kinase (CK)-MB ≥50 U/L, and had lower white blood cell and platelet counts. At 6 months, caregivers reported no recurrence, readmission or persistent muscular or neurologic symptoms. Conclusions: Influenza-associated BACM was brief and self-limited, with favorable 6-month outcomes. Influenza B–associated cases had higher myoglobin and more often had CK-MB ≥50 U/L, but these exploratory findings do not establish greater severity or myocardial injury. Management should be individualized; cardiac troponin I and echocardiography may help contextualize CK-MB elevation when myocardial involvement is suspected.