Korean J Fam Med 2018; 39(1): 37-41  https://doi.org/10.4082/kjfm.2018.39.1.37
Analysis of Clinical Manifestations and Laboratory Findings in Children with Influenza B-Associated Myositis: A Single Center Study
Jae Woong Yoon1, Du Young Choi1, Seung Hyun Lee1, Sae Ron Sin2, Seung Taek Yu1,*
1Department of Pediatrics, Wonkwang University School of Medicine, Iksan, Korea
2Department of Family Medicine, Wonkwang University School of Medicine, Iksan, Korea
Seung Taek Yu https://orcid.org/0000-0001-9744-5548
Tel: +82-63-859-1510, Fax: +82-63-853-3670, E-mail: yudoc@wonkwang.ac.kr
Received: October 10, 2016; Revised: October 27, 2016; Accepted: October 28, 2016; Published online: January 20, 2018.
© Korean Academy of Family Medicine. All rights reserved.

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Background: Influenza-associated myositis (IAM) is a rare and poorly recognized complication of influenza infection in children, and is characterized by acute onset of severe pain in the lower extremities and a refusal to ambulate walk. We sought to understand the association between IAM and influenza B infection and to investigate its clinical and laboratory characteristics in affected children.
Methods: Influenza B-associated myositis (IBAM) cases diagnosed in the pediatrics department of Wonkwang University Hospital from January 2010 and March 2016 were analyzed retrospectively.
Results: Medical records of affected children were examined, and clinical characteristics and laboratory findings were recorded. Of the 536 children diagnosed with influenza B infection, 47 children complained of bilateral calf pain with or without gait disturbance. All children exhibited elevated serum aspartate aminotransferase (AST) level. The median serum creatine kinase (CK) and lactate dehydrogenase (LDH) levels, reportedly elevated in myositis, were 2,597 IU/L and 678 IU/L, respectively. While the immunofluorescence test results were negative for some patients, the polymerase chain reaction test results indicated influenza B infection in all 47 children. At the time of hospital discharge, the patients’ symptoms had resolved, and their CK levels had improved.
Conclusion: IBAM was generally benign and short, and although the blood AST, CK, and LDH levels were markedly high, the erythrocyte sedimentation rate and C-reactive protein levels were normal. Further, the duration of IBAM symptoms correlated with the duration of fever. The IBAM-associated clinical and laboratory findings are highly characteristic and may allow its rapid diagnosis during the influenza season.
Keywords: Child; Influenza B Virus; Myositis; Myalgia

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