Infantile polymyositis with normal serum creatine kinase level.

A 30-month-old boy who has been suffering progressive proximal muscle weakness and severe atrophy in shoulder and hip muscles from 11 months of age had prominent perivascular inflammatory cellular infiltration in his muscle biopsy and myopathic EMG changes. But his serum creatine kinase (CK) levels were repeatedly found to be within normal ranges. Oral prednisolone therapy (2 mg/kg/d) brought a complete recovery of muscle power and normalization of EMG and muscle biopsy findings. This report provides an additional support that the cases of infantile polymyositis with normal serum CK levels may respond well to steroid therapy.