Benign acute childhood myositis in the COVID-19 Era: how does it compare to influenza?

Benign acute childhood myositis (BACM) is a self-limiting muscle disorder affecting school-aged children, often following viral infections like influenza. Severe cases may require hospitalization, yet research in Asian populations, particularly in Taiwan, is limited. This study investigates the clinical characteristics, laboratory findings, and predictors of severity in BACM, focusing on differences between influenza and COVID-19. A retrospective study of pediatric BACM cases at Fu Jen Catholic University Hospital, Taiwan, from February 2018 to April 2024 was conducted. Patients with elevated creatine phosphokinase (CPK) levels (> 223 IU/L) and muscle symptoms following viral infections were included. Clinical data, laboratory findings, and hospitalization rates were analyzed. Of 193 children with elevated CPK, 89 met BACM criteria (median age 7 years; 76.4% male). Bilateral calf pain (94.4%), abnormal gait (36.0%), and inability to bear weight (27.0%) were common. Influenza was identified in 42.7% of cases, while SARS-CoV-2 accounted for 16.9%. A longer fever-to-symptom interval was significantly associated with severe motor symptoms. Elevated CPK was correlated with hospitalization (p = 0.008), but not with symptom severity. Influenza often led to more pronounced motor impairments, whereas SARS-CoV-2-associated cases showed a trend toward higher admission rates. CONCLUSION: This first Taiwanese BACM analysis reveals influenza cases are characterized by rapid progression and motor impairment, while COVID-19 cases show biochemical abnormalities and higher hospitalization rates. Tailored clinical approaches are crucial during seasonal outbreaks to optimize care and resource allocation. WHAT IS KNOWN: • BACM is a self-limiting muscle disorder in school-aged children, which is most often linked to influenza. • Typical features include bilateral calf pain, abnormal gait, and elevated CPK levels; in some cases, the severity of symptoms may necessitate hospitalization. WHAT IS NEW: • Influenza remains the primary etiology, often causing more severe motor impairment, whereas SARS-CoV-2 has newly emerged with a tendency for higher hospitalization rates. • A novel finding is that a longer interval between fever onset and BACM symptoms predicts greater symptom severity.