Viral myositis is a condition characterized by muscle inflammation due to a viral infection. Patients present with muscle pain, tenderness, swelling, and often weakness. The most frequent viral causes of myositis are the influenza viruses and enteroviruses, with symptoms of myositis typically starting a few days after the onset of fever. Symptoms are most prominent in the proximal musculature (arms and legs), and pain is often exacerbated by movement.

HIV can occasionally cause myositis. Many other viruses, including Epstein-Barr virus, cytomegalovirus, and adenovirus, have been reported to cause myositis. The exact mechanism by which a virus causes myositis varies and includes direct viral infection of the myocyte, immune-mediated complexes, and through immune dysregulation such as a cytokine release.

The influenza virus may be isolated from muscle tissue on biopsy. For other viruses, the virus may not be found in the muscle tissue, and in those cases, the myositis is felt to be immune-mediated or cytokine-mediated.

Laboratory testing reveals an elevated creatinine kinase (CK) level. Treatment is symptomatic and includes maintaining adequate hydration status. Rhabdomyolysis is a possible complication of viral myositis that can result in severe renal damage and/or disseminated intravascular coagulation (DIC) if not managed properly.