Clinical Characteristics and Influenza-Associated Myositis in Children: A Comparative Study of Influenza A and Influenza B in Bahrain.

Background and objective Influenza-associated myositis is an uncommon but increasingly recognized complication of pediatric influenza infection. Although influenza B has traditionally been associated with benign acute childhood myositis (BACM), data comparing the clinical characteristics of influenza A and influenza B in children with influenza-associated myositis remain limited, particularly in the Middle East. Hence, this study aimed to compare the clinical presentation, laboratory findings, outcomes, and prevalence of myositis-related manifestations among children with influenza A and influenza B infections. Methods A retrospective cross-sectional study was conducted at King Hamad University Hospital, Bahrain. Medical records of pediatric patients aged ≤ 14 years with confirmed influenza A or influenza B infection between October 2023 and January 2024 were reviewed. Demographic characteristics, clinical features, laboratory findings, hospitalization data, treatments, and complications were analyzed and compared between influenza A and influenza B groups. Results Ninety-six children with laboratory-confirmed influenza infection were included. Influenza B was associated with a significantly higher frequency of calf pain (n = 8, 25.8% versus n = 5, 7.7% in influenza A cases; p = 0.015) and refusal to walk, reported in seven children (22.6%) compared with five children (7.7%) with influenza A (p = 0.039). No significant differences were identified between the two groups regarding laboratory findings, hospitalization requirements, duration of admission, or complication rates. Markedly elevated creatine kinase (CK) levels were observed in both influenza subtypes, consistent with influenza-associated muscle injury. Conclusions Influenza B infection was associated with a significantly higher frequency of myositis-related manifestations, including calf pain and refusal to walk, compared with influenza A infection. Despite these clinical differences, laboratory findings, hospitalization rates, and clinical outcomes were comparable between the two groups. Recognition of influenza-associated myositis is essential when evaluating children with acute lower-limb pain or gait disturbances during influenza season, as early diagnosis may help avoid unnecessary investigations and support appropriate management.