This retrospective analysis describes the severity, clinical course, and 6-month outcomes of benign acute childhood myositis (BACM) associated with influenza during the 2024–2025 season. The study additionally explored differences between cases associated with influenza A and influenza B.
Researchers retrospectively reviewed children with laboratory-confirmed influenza between October 2024 and March 2025. From 11,528 confirmed influenza cases, 82 children met the diagnostic criteria for BACM and none of the exclusion criteria. Venous laboratory measurements reported are based on the first sample obtained when patients presented for evaluation of leg pain or gait disturbance. Clinical features, initial laboratory findings, and outcomes were compared between influenza A- and influenza B-associated BACM cases.
The 82 patients had a mean age of 6.8 ± 2.1 years (range, 3–14 years) and included 55 boys. Of these, 27 cases were associated with influenza A and 55 with influenza B. Symptoms of leg pain or gait disturbance appeared between 1 and 9 days after onset of influenza symptoms. Reported symptom duration was short: leg pain and gait disturbance resolved within 3–5 days in the cohort.
At first presentation, urinalysis and serum creatinine were normal in all patients. When comparing influenza A- and influenza B-associated cases, the authors report several exploratory laboratory differences:
The report emphasizes these findings as exploratory and explicitly states they do not establish greater overall severity or definitive myocardial injury in influenza B-associated BACM.
None of the 82 patients required admission to an intensive care unit or renal replacement therapy during the acute illness. Initial renal testing was normal for all patients. Management approaches are not detailed beyond these outcomes; the authors recommend individualizing management and using additional cardiac testing when myocardial involvement is suspected.
At 6 months after the acute illness, caregiver reports indicated no patient experienced recurrence of BACM symptoms, required readmission, or had persistent muscular or neurologic symptoms. These follow-up data support a favorable medium-term prognosis in this cohort.
In this series from the 2024–2025 influenza season, influenza-associated BACM presented as a brief, self-limited condition with uniformly favorable 6-month outcomes. Although influenza B-associated cases showed higher myoglobin and a higher frequency of CK-MB ≥50 U/L as well as lower leukocyte and platelet counts, the authors caution that these observations are exploratory and do not prove increased severity or myocardial injury. To contextualize CK-MB elevations when clinicians suspect cardiac involvement, the authors suggest consideration of cardiac troponin I measurement and echocardiography.
Clinicians should note the typically short interval from respiratory symptom onset to myositis symptoms (1–9 days) and the rapid resolution (3–5 days) documented in this cohort. Normal renal function and urinalysis at presentation and the absence of ICU-level care or renal replacement therapy in all cases suggest low risk for severe renal or critical complications in this series.
The analysis is retrospective and limited to cases identified among children with laboratory-confirmed influenza during a single season. Details about specific management strategies, thresholds for cardiac testing, and individual laboratory value distributions beyond the reported group differences were not provided in the abstract. The exploratory nature of comparisons between influenza A and B means the reported laboratory differences should be interpreted cautiously and do not establish causal links or definitive clinical risk differences.
Among 11,528 children with laboratory-confirmed influenza in the 2024–2025 season, 82 met criteria for BACM. The condition occurred most commonly in school-age children, followed a predictable short time course after influenza symptoms, and resolved within days. No patients required intensive care or renal replacement therapy, and all had normal urinalysis and serum creatinine at presentation. Influenza B-associated cases displayed higher myoglobin and more frequent CK-MB ≥50 U/L and had lower white blood cell and platelet counts; however, these were exploratory findings that do not establish greater severity or myocardial injury. At 6 months, caregivers reported no recurrences, readmissions, or persistent muscular or neurologic symptoms. The authors recommend individualized management and consideration of cardiac troponin I and echocardiography when myocardial involvement is suspected.