Benign acute childhood myositis complicating influenza B infection in a boy with idiopathic nephrotic syndrome

INTRODUCTION: Benign acute childhood myositis (BACM) is an acute complication of an infection characterized by calf pain, limitation of lower limb mobility, an increase in serum creatine kinase, and a self-limiting course. No reports of BACM in children with idiopathic nephrotic syndrome (INS) can be found in the literature. CASE REPORT: A 5-year-old boy with steroid-sensitive INS presented with fever, leg pain, and problems with walking. Physical examination showed pharyngeal erythema, preserved movements in all joints, and weakness of leg muscles. Laboratory tests showed white blood cell count 3900/µl, albumin 2.3 g/dl, urea 25 mg/dl, creatinine 0.3 mg/dl, increased transaminases (AspAT 440 U/l, AlAT 100 U/l) and creatine kinase (10 817 U/l), and proteinuria 3500 mg/dl. The boy was diagnosed with an INS bout and BACM. Testing for infective causes of myositis showed evidence of an influenza B virus infection. Treatment included prednisone and oseltamivir. A rapid improvement of motor function was observed, with normalization of serum creatine kinase and transaminases, and resolution of proteinuria. CONCLUSIONS: 1. As influenza virus infection in a child with INS is a risk factor for complications and a disease bout, these patients should be vaccinated against influenza. 2. Differential diagnosis of leg pain and mobility limitation in a child with INS should include lower limb deep venous thrombosis, arthritis, post-infectious neurological complications (including Guillain-Barré syndrome), and BACM. 3. Serum creatine kinase level should be measured in all cases of motor disturbances in a child with symptoms of respiratory tract infection.