[A case of systemic lupus erythematosus predominantly presenting with myositis at onset].

A 42-year-old woman presented with rapid myalgia of the thigh and muscle weakness in the proximal limbs with markedly elevated serum CK. Despite positive for antibodies to anti-nuclear, anti-ds-DNA, anti-ss-DNA, anti-Sm, anti-SS-A/Ro, C-ANCA, anti-U1-RNP and anti-ribosome and slight lymphocytopenia and hypocomplementemia, there was no symptom associated with systemic lupus erythematosus (SLE). Proteinuria and hematuria were initially considered to be associated with renal damage due to myoglobinuria. Muscle MRI demonstrated high signal intensities in the rectus femoris. Muscle biopsy of the rectus femoris demonstrated a mild variation in fiber size, a few necrotic and several regenerating fibers and minimal lymphocytic infiltration in the endomysium, which suggested myopathic changes with mild necrotic and regenerating processes. Thus she was diagnosed as idiopathic myositis at first, and was treated by corticosteroid therapy. Her myalgia and CK level improved, but the proteinuria and hematuria were persistent. A renal biopsy demonstrated lupus nephritis, and SLE with myositis was confirmed. She was treated with additional tacrolimus administration, and her proteinuria and hematuria also improved. The present case suggests that patients who predominantly present with myositis accompanied by nephritis and autoantibodies should be considered as SLE with myositis.