This summary discusses polymyositis in children. Polymyositis in adults is addressed separately.

Juvenile polymyositis (JPM) is a diagnosis under a larger umbrella of juvenile idiopathic inflammatory myopathies (JIIMs). JPM involves T-cell invasion of muscle fibers. It has a prevalence of 2%-8% of all JIIMs, and it has a peak onset during adolescence.

It is characterized by proximal and distal muscle weakness, frequent falling episodes, myalgias, arthritis or arthralgias, and increased creatine kinase levels. Other associated symptoms include fevers, muscle atrophy, and dysphagia with palpitation in a minority of cases. Patients who have gastrointestinal involvement, periungal capillary changes, and Raynaud phenomenon likely have some overlap with scleroderma. JPM has clinical characteristics that are similar to juvenile dermatomyositis, but cutaneous findings such as Gottron's papules or heliotrope rash are absent. While muscle weakness is a central characteristic of JPM, up to 5% of patients will not have clinically evident weakness.

Muscle fibers of patients with JPM typically have endomysial lymphocytic infiltration; thus, a muscle biopsy is warranted in all cases in which JPM is suspected. Better delineation of the underlying abnormality in a variety of childhood inflammatory myopathies and improved diagnostic techniques have resulted in a reduction in the incidence of JIIMs that are diagnosed as JPM.