Juvenile dermatomyositis and polymyositis.

Myositis in childhood is characterized by elevated serum levels of muscle-derived enzymes, proximal symmetrical muscle weakness, abnormal EMG and a muscle biopsy which frequently documents an inflammatory process. In the paediatric age group, JDMS is much more common than PM and occurs more frequently among females. Mortality has been reduced from 33 per cent to 7 per cent following the use of steroids. The development of calcifications (33 per cent) can be the most debilitating consequence of JDMS . It is our premise that JDMS is a distinct disease entity and that the increase in HLA-B8 and -DR3 in JDMS places this disease in the company of other immunopathic disorders. There are conflicting data concerning immunological abnormalities in JDMS , but there appears to be impairment of natural killing and evidence of complement activation. The frequent positive ANA in JDMS raises the speculation of its relationship to the antinuclear antibody, Jo-1, found in some adults with PM, which has specificity for tRNAHis. Most newly diagnosed JDMS patients have antibodies to Coxsackie B which may be related to the pathogenesis of this disease. Specific pathological findings of endothelial cells containing reticulotubular inclusions are associated with vessel occlusion, subsequent obliteration and increased Factor VIII levels in clinically active disease. In addition to physical therapy, prednisone is the drug most used, but immunosuppressive agents and plasmapheresis have been tried in severely ill children. Rigorous evaluation of the efficacy of these modalities is needed.