The clinical spectrum of muscle syndromes associated with viral infections ranges from benign, commonly experienced myalgias to rhabdomyolysis with myoglobinuric renal failure.
This topic will review the clinical, pathologic, and diagnostic features of these illnesses. The evaluation of adults and children with muscle weakness, as well as the causes, clinical manifestations, and diagnosis of rhabdomyolysis, is discussed separately. (See "Approach to the patient with muscle weakness" and "Etiology and evaluation of the child with muscle weakness" and "Causes of rhabdomyolysis" and "Clinical manifestations and diagnosis of rhabdomyolysis".)
Mild to moderate diffuse myalgias occur frequently during the prodrome or early phase of any acute viral infection. The back and proximal extremities are commonly involved, and mild muscle tenderness may occur without weakness or laboratory abnormalities suggestive of muscle inflammation or necrosis. These self-limited myalgias are probably due to the effect of viral-induced cytokines on muscle tissue, rather than direct viral invasion of muscle .
BENIGN ACUTE CHILDHOOD MYOSITIS
An intermediate muscle syndrome accompanying acute viral infections is primarily seen in children . It consists of marked pain and tenderness, usually localized to the calves. This presentation has been reported most commonly with influenza A or B infections. It is often seen during influenza epidemics and has been described with the 2009 to 2010 pandemic influenza A (H1N1) virus . It occurs as the acute illness is subsiding, usually 24 to 48 hours after the resolution of the presenting symptoms of fever, cough, and coryza.
Patients will often refuse to walk or will have difficulty walking due to pain or true muscle weakness. The ankles are held in a plantar flexed position, and the patient will resist attempts to dorsiflex the ankle because of pain [4,5]. Benign acute childhood myositis should be included in the differential diagnosis of children with sudden difficulty walking .