A 15-year-old girl presented with pain in the back of both thighs. On examination there was severe pain but no erythema or palpable tumor in her thighs. Findings from laboratory examinations were normal except for the serum creatine kinase levels, the concentrations of which increased from 3977 U/L to 5877 U/L (to convert to microkatal per liter, multiply by 0.0167) over the course of 3 days of clinical follow-up.
Magnetic resonance imaging of her thighs revealed an increased signal intensity on the T2-weighted image. The abnormal signal was more clearly revealed on the short tau inversion recovery image (Figure 1). Microscopic examination of the biopsied muscle tissue showed myopathic changes with degeneration, necrosis, and myophagia (Figure 2). Findings from immunological investigation for viral infections in her serum samples showed high titers of complement fixation antibody for influenza A virus. Considering the progressive worsening of symptoms and the rapid increase of the creatine kinase levels, this patient was treated with 1000 mg of methylprednisolone sodium succinate intravenously each day for 3 days; this resulted in the resolution of symptoms and normalization of the creatine kinase levels. Magnetic resonance images at 30-day follow-up (Figure 1) showed a normal intensity at the bilateral biceps femoris muscle. Her serum creatine kinase level was 48 U/L. The patient remained symptom free during a 6-month follow-up.
Kawarai T, Nishimura H, Taniguchi K, Saji N, Shimizu H, Tadano M, Shirabe T, Kita Y. Magnetic Resonance Imaging of Biceps Femoris Muscles in Benign Acute Childhood Myositis. Arch Neurol. 2007;64(8):1200-1201. doi:10.1001/archneur.64.8.1200