Thirty-eight children (32 boys, 6 girls) with 41 episodes of myositis between 1978 and 1997 are reported from the Royal Children’s Hospital, University of Melbourne, Australia. Viral studies during 24 of the episodes were positive in 10 (42%), with influenza B isolated in 5 (50%).
A typical case was a 7-year-old boy admitted with calf pains and difficulty in walking. Fever, cough, and lethargy had preceded the muscle pain by 5 days. He walked on his toes, the calf muscles were tender, and passive ankle dorsiflexion exacerbated the pain. Muscle tone, power, tendon reflexes, and sensation were normal in all extremities. Abnormal laboratory studies included an elevated creatine kinase (CK) of 4762 U/L (normal, 40-240 U/L), sedimentation rate (ESR) of 12 mm/h (normal, <6), and leukopenia (3.0 × 109). Parainfluenza 3 was isolated from a nasopharyngeal aspirate. Pain resolved within 24 hours after admission, and recovery was complete at 6 day-follow-up.
Mean age at onset was 8.1 years. Children were ambulant in 75% of episodes, but gaits were abnormal, on toes or wide-based stiff-legged, with the trunk flexed at the hips and both knees extended. Muscle tenderness was restricted to the gastrocnemius-soleus muscles, and CK was elevated during all episodes. Muscle biopsy in one case revealed active degeneration with necrosis, myophagia, and ghost fiber formation, findings consistent with a viral infection. Symptoms resolved within 1 week, and only 3 had recurrent episodes. Benign acute myositis occurs mainly in boys, almost always in mid-childhood, and in response to a viral infection, especially influenza. 
COMMENT. This acute childhood muscle disorder, first described by Lundberg A , may initially mimic a more serious cause of limb pain and refusal to walk - Guillain-Barre syndrome. Benign acute childhood myositis (BACM) is differentiated by the normal reflexes, normal muscle power, and elevated CK. The disease is epidemiologically associated with viral outbreaks, especially influenza. Could BACM sometimes explain the idiopathic myositis of childhood referred to as “growing pains?“
Myositis, or inflammatory disease of muscle, may be acute, subacute or chronic and occurs in two main forms: 1) caused by an identified virus (eg influenza), parasite (eg trichinosis), or pyogenic bacterium (eg staphylococcus, streptococcus) and 2) idiopathic but presumed to be inflammatory because of histopathologic changes in the muscle (eg polymyositis, dermatomyositis). Polymyositis is closely related to the rheumatic or connective tissue diseases (rheumatic fever, lupus erythematosus, and polyarteritis nodosa). The muscle biopsy in true myositis is characterized by exudation, infiltration by neutrophilic leukocytes, and by degeneration of parenchymal and interstitial cells . The PCR may now be used to define a viral etiology in cases where the diagnosis is in doubt. In children presenting with BACM, a muscle biopsy is rarely required since the symptoms are acute and remission rapid.