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[A case of isolated focal myositis successfully treated by arterial injection of prednisolone--its findings by CT scan and MRI].

A 25-year-old woman with a 2 years history of painful enlargement of the left calf visited our clinic because of ache in the left calf which became severe enough to interfere her walk. She had no history of trauma to the calf, and family history was negative for muscle disease. Physical examination showed an enlarged left calf muscle with pain which was evoked by stretching of the Achilles tendon. There was no muscle weakness or neurological abnormality. Laboratory tests were all within normal limits, including erythrocyte sedimentation rate and serum enzymes such as creatine kinase and aldolase. The CT scan revealed an isolated round-shaped low density area in the left calf muscle, which showed up as a markedly high signal intensity on T2 weighted images of MRI. The lesion was confirmed to be the lateral head of the gastrocnemius muscle. The specimen obtained from the affected muscle demonstrated degenerative and inflammatory changes together with excess fibrosis. There was no sarcoid lesion or malignancy. From the above results, the present case was thought to have isolated focal myositis. Prednisolone 80 mg (40 mg, twice, at 4-day intervals) was given to the left femoral artery without any trouble. Thereafter the patient became free from pain and difficulty in walking, and the lesion demonstrated by CT scan disappeared in 15 days. However, the high signal intensity on T2 weighted images faintly remained until the 54th day after the treatment. In summary, MRI is more sensitive than CT scan for detection of isolated focal myositis, which might be successfully treated by arterial injection of prednisolone within a short period than any other conventional methods.