acute viral myositis

This patient's presentation (calf pain, abnormal gait, and elevated creatine kinase) is consistent with viral myositis. Viral myositis, also known as benign acute childhood myositis, is most common in young children who are recovering from viral infections. Influenza is the most commonly implicated virus. Viral myositis presents with the acute onset of muscle pain, usually in the calves. The calves are tender to palpation and in severe cases may be visibly swollen. Affected children have abnormal gait, either toe-walking (as in this case) or wide-based; some children may refuse to ambulate altogether.

Laboratory testing shows elevated creatine kinase, but is otherwise normal. Hematuria and renal dysfunction are absent, which differentiates viral myositis from rhabdomyolysis, a more severe muscle breakdown that causes acute kidney injury (Choice C). Viral myositis is a self-limited condition that will resolve spontaneously. Influenza-associated viral myositis is not an indication for antiviral therapy if the child does not meet typical criteria for treatment (eg, hospitalization, age <2).