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A rare idiopathic inflammatory myopathy characterized by a localized swelling of skeletal muscle that is usually located in the lower extremities.
ORPHA:48918Classification level: Disorder
- Focal nodular myositis
- Inflammatory pseudotumor of skeletal muscle
- Prevalence: <1 / 1 000 000
- Inheritance: Not applicable
- Age of onset: Adult
- ICD-10: M60.8
- OMIM: -
- UMLS: C0751357
- MeSH: -
- GARD: -
- MedDRA: -
Focal myositis prevalence is unknown. There have been approximately 200 cases described to date.
The disease can occur at any age. It presents as a mass (ranging in size from 1.0 to 20.0 cm) localized to a single skeletal muscle that may be painless or tender to the touch and that grows insidiously over several weeks to months. The mass is usually located in the lower extremities and the muscles most commonly affected include the adductor, vastus lateralis and gastrocnemius. It can rarely occur in muscles of the head and neck region. Additional rare systemic manifestations include fever, myalgia, generalized weakness and weight loss. It does not appear to invade the adjacent tendons, fascia or skin and usually regresses spontaneously with time. Recurrence is possible (seen in around 18% of cases) but uncommon. Rarely, classical inflammatory myopathies can present focally, thus mimicking focal myositis.
The etiology is unknown. Possible causes include trauma (20%), genetic factors (that remain unknown), viral infections and autoimmune disease. Some regard focal myositis as a localized form of polymyositis. Chronic nerve irritation caused by compression of a nerve is another suspected cause of muscle fiber hypertrophy that consequently leads to focal myositis.
Clinical and histological findings of an enlarged single muscle are indicative of focal myositis. Magnetic resonance imaging (MRI) is helpful to diagnose focal myositis as it reveals increased signal intensity and hypertrophy of the affected muscle. Electromyography shows spontaneous activity associated with a myopathic pattern. A minor elevation in the level of creatine kinase (CK) is sometimes seen.
Clinically, focal myositis can be mistaken for many other conditions such as soft tissue sarcomas (rhabdomyosarcoma, liposarcoma, leiomyosarcoma), myositis ossificans progressiva, inflammatory myofibroblastic tumor, lymphoma, fibromatosis, rhabdomyoma, intramuscular lipoma, venous thrombosis and other conditions of inflammatory or vascular origin. Pathologically it can resemble an inflammatory myopathy and muscular dystrophy.
Management and treatment
Treatment is often not necessary as most cases resolve spontaneously. Anti-inflammatory drugs (steroid or nonsteroidal) such as oral prednisolone can be given to minimize any pain and inflammation. In cases complicated by limited joint motion, physical and/or surgical therapy may be necessary.
Prognosis is very good as most cases of focal myositis resolve spontaneously.
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