Focal myositis are inflammatory muscle diseases of unknown origin. Contrary to the other idiopathic inflammatory myopathies, they are restricted to a single muscle or a muscle group. They are not associated with extramuscular manifestations, and they have a good prognosis in the absence of treatment. They are characterized by localized swelling affecting mostly lower limbs. The pseudo-tumor can be painful, but is not associated with muscle weakness. Creatine kinase level is normal. Muscle MRI shows an inflammation restricted to a muscle or a muscle group. Muscle biopsy and pathological analyses are necessary for the diagnosis, showing inflammatory infiltrates composed by macrophages and lymphocytes without any specific distribution within the muscle. Focal overexpression of HLA-1 by the muscle fibers is frequently observed. The muscle biopsy allows to rule out differential diagnosis such a malignancy (sarcoma). Spontaneous remission occurs within weeks or months after the first symptoms and relapse is unusual.
