Influenza-associated Benign Acute Childhood Myositis: Clinical Course, 6-Month Outcomes and an Exploratory Comparison of Influenza A and B (2024-2025 Season).

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.