{ "id": "ep8bzw1gqm", "slug": "focal-myositis", "title": "Focal myositis", "updated": "2026-10-21", "topic_path": [ { "id": "life", "label": "Life and health", "api_url": "https://www.edgepedia.org/api/v1/topics/life" }, { "id": "life.health", "label": "Human health and medicine", "api_url": "https://www.edgepedia.org/api/v1/topics/life.health" }, { "id": "life.health.conditions", "label": "Diseases and injuries", "api_url": "https://www.edgepedia.org/api/v1/topics/life.health.conditions" }, { "id": "life.health.conditions.structural", "label": "Skin and musculoskeletal conditions", "api_url": "https://www.edgepedia.org/api/v1/topics/life.health.conditions.structural" }, { "id": "life.health.conditions.structural.musculoskeletal_conditions", "label": "Musculoskeletal conditions", "api_url": "https://www.edgepedia.org/api/v1/topics/life.health.conditions.structural.musculoskeletal_conditions" }, { "id": "life.health.conditions.structural.muscle_disease", "label": "Muscle disease", "api_url": "https://www.edgepedia.org/api/v1/topics/life.health.conditions.structural.muscle_disease" }, { "id": "life.health.conditions.structural.myopathy", "label": "Myopathy", "api_url": "https://www.edgepedia.org/api/v1/topics/life.health.conditions.structural.myopathy" } ], "geo": [ { "id": "geo.us.t1946.life.health.conditions.structural.muscle_disease", "label": "United States · 1946 to 2000: Muscle disease", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t1946.life.health.conditions.structural.muscle_disease", "path": [ { "id": "geo.us", "label": "United States", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us" }, { "id": "geo.us.t1946", "label": "United States · 1946 to 2000", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t1946" }, { "id": "geo.us.t1946.life", "label": "Life and health", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t1946.life" }, { "id": "geo.us.t1946.life.health", "label": "Human health and medicine", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t1946.life.health" }, { "id": "geo.us.t1946.life.health.conditions", "label": "Diseases and injuries", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t1946.life.health.conditions" }, { "id": "geo.us.t1946.life.health.conditions.structural", "label": "Skin and musculoskeletal conditions", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t1946.life.health.conditions.structural" }, { "id": "geo.us.t1946.life.health.conditions.structural.musculoskeletal_conditions", "label": "Musculoskeletal conditions", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t1946.life.health.conditions.structural.musculoskeletal_conditions" }, { "id": "geo.us.t1946.life.health.conditions.structural.muscle_disease", "label": "Muscle disease", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.us.t1946.life.health.conditions.structural.muscle_disease" } ] } ], "excerpt": "Focal myositis (focal nodular myositis, inflammatory pseudotumor of skeletal muscle) is a rare, benign, localized inflammation of a single skeletal muscle, usually in the lower limb, presenting as a mass that typically regresses spontaneously; biopsy confirms diagnosis.", "snippet": "Focal myositis (focal nodular myositis, inflammatory pseudotumor of skeletal muscle) is a rare, benign, localized inflammation of a single skeletal muscle, usually in the lower limb, presenting as a mass that typically regresses spontaneously; biopsy confirms diagnosis.", "node": "life.health.conditions.structural.myopathy", "markdown": "# Focal myositis\n\n**Focal myositis** is a rare inflammatory disease of skeletal muscle in which inflammation is confined to a single muscle, or a single muscle group, and presents as a benign tumor-like mass (an inflammatory pseudotumor) that usually regresses on its own[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11537714/) • [2](https://www.orpha.net/en/disease/detail/48918). It differs from the other idiopathic inflammatory myopathies, such as polymyositis, by its strictly localized nature, the absence of extramuscular manifestations, and a good prognosis without treatment[3](https://pubmed.ncbi.nlm.nih.gov/28947258/). The condition was first identified as a distinct clinicopathologic entity in 1977 by Heffner and colleagues, who separated it from other inflammatory pseudotumors of skeletal muscle such as proliferative myositis and nodular pseudosarcomatous fasciitis[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11537714/).\n\n| Key fact | Detail |\n|---|---|\n| Definition | A rare idiopathic inflammatory myopathy with localized swelling of skeletal muscle, usually in the lower extremities (ORPHA:48918; ICD-10 M60.8)[2](https://www.orpha.net/en/disease/detail/48918) |\n| Frequency | Prevalence unknown; approximately 200 cases described to date[2](https://www.orpha.net/en/disease/detail/48918) |\n| Typical presentation | A rapidly growing solitary mass within one muscle, 1.0 to 20.0 cm in size, growing over weeks to months[2](https://www.orpha.net/en/disease/detail/48918) • [4](https://pubmed.ncbi.nlm.nih.gov/27726926/) |\n| Common sites | Lower limb muscles, especially the adductors, vastus lateralis, and gastrocnemius; rarely head and neck[2](https://www.orpha.net/en/disease/detail/48918) |\n| Laboratory findings | Creatine kinase normal in most patients (81% in a Lyon cohort); systemic inflammation markers usually absent[5](https://www.neurology.org/doi/10.1212/WNL.0000000000005160) • [4](https://pubmed.ncbi.nlm.nih.gov/27726926/) |\n| Course | Spontaneous regression within weeks or months is usual; recurrence reported in around 18% of cases[2](https://www.orpha.net/en/disease/detail/48918) • [3](https://pubmed.ncbi.nlm.nih.gov/28947258/) |\n| Definitive diagnosis | Muscle biopsy, which also rules out malignancy such as sarcoma[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11537714/) • [3](https://pubmed.ncbi.nlm.nih.gov/28947258/) |\n\n## Clinical features\n\nThe disease can occur at any age and in both sexes. It presents as a mass, ranging from 1.0 to 20.0 cm, localized to a single skeletal muscle that may be painless or tender and grows insidiously over several weeks to months[2](https://www.orpha.net/en/disease/detail/48918). The muscles most commonly affected are the adductor, vastus lateralis, and gastrocnemius, and the mass is usually in the lower extremities; involvement of head and neck muscles is rare[2](https://www.orpha.net/en/disease/detail/48918). The inflammation can involve part of a muscle, an entire muscle, or two or more muscles not necessarily in the same area[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11537714/). Pain, erythema, and fever can accompany the mass, usually without muscle weakness[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11537714/). The lesion does not appear to invade adjacent tendons, fascia, or skin[2](https://www.orpha.net/en/disease/detail/48918).\n\n**Cohort data.** In a cohort study of the Lyon University Hospital myopathology database (2000 to 2016), 37 of 924 patients (4%) had confirmed focal myositis. The main symptoms were pain (81%), a muscular mass (43%), erythema at the site (32%), and fever (24%)[5](https://www.neurology.org/doi/10.1212/WNL.0000000000005160). Serum creatine kinase was normal in 81%, while serum immune abnormalities were frequent: an inflammatory syndrome in 39%, dysglobulinemia in 52%, and antinuclear antibody positivity in 29%[5](https://www.neurology.org/doi/10.1212/WNL.0000000000005160).\n\n**Childhood disease.** In children, focal myositis is commonly painless and localized to the calf, with no correlation between recurrence and creatine kinase levels[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11537714/). A systematic review of recurrent focal myositis in childhood identified seven pediatric patients in addition to the index case, with a median age of ten years; in children the calf was the most frequently involved muscle and, unlike in adults, the myositis was usually painful[6](https://pubmed.ncbi.nlm.nih.gov/28476523/).\n\n## Cause and mechanisms\n\nThe cause is unknown[2](https://www.orpha.net/en/disease/detail/48918). Possible causes include trauma (reported in about 20% of cases), unknown genetic factors, viral infections, and autoimmune disease, and some authors regard focal myositis as a localized form of polymyositis[2](https://www.orpha.net/en/disease/detail/48918). Several cases have been described in association with chronic radiculopathy, and chronic nerve irritation from nerve compression is a suspected cause of muscle fiber hypertrophy leading to the condition; it remains unclear whether denervation causes the hypertrophy or is secondary to the inflammation[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11537714/) • [2](https://www.orpha.net/en/disease/detail/48918).\n\nThe Lyon cohort found focal myositis frequently associated with other diseases: immune-mediated inflammatory disease in 32%, neoplasia in 24%, and radiculopathy in 11%[5](https://www.neurology.org/doi/10.1212/WNL.0000000000005160). The authors concluded that the condition is not as innocuous as previously believed and that patients should receive careful screening for immune-mediated inflammatory disease and neoplasia[5](https://www.neurology.org/doi/10.1212/WNL.0000000000005160).\n\n## Diagnosis\n\nClinical, laboratory, neurophysiological, and imaging findings can support the diagnosis, but muscle biopsy appears to be the only diagnostic tool that differentiates focal myositis from other localized soft tissue masses[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11537714/).\n\n- **Electromyography** shows spontaneous activity associated with a myopathic pattern[4](https://pubmed.ncbi.nlm.nih.gov/27726926/).\n- **MRI** reveals a contrast-enhanced enlarged muscle appearing hyperintense on fat-suppressed T2-weighted images, with adjacent structures spared and no calcifications[4](https://pubmed.ncbi.nlm.nih.gov/27726926/).\n- **Biopsy** shows marked variation in fiber size, inflammatory infiltrates composed mostly of CD4+ T lymphocytes and macrophages, degenerating and regenerating fibers, and interstitial fibrosis[4](https://pubmed.ncbi.nlm.nih.gov/27726926/). Focal overexpression of HLA class I by muscle fibers is frequently observed[3](https://pubmed.ncbi.nlm.nih.gov/28947258/).\n\nA large clinicopathologic study of 115 cases from the Armed Forces Institute of Pathology found myopathic changes in 93%, focal neurogenic changes in 89%, and inflammation in 97%; the infiltrate was rich in macrophages and T cells, with CD20-positive B cells increasing in severely inflamed cases, and no B-cell or [T-cell receptor](https://www.edgepedia.org/t-cell-receptor) gene rearrangement on polymerase chain reaction[7](https://pubmed.ncbi.nlm.nih.gov/19363438/).\n\n**Differential diagnosis** is broad because focal myositis can mimic any disease presenting as a solitary inflammatory intramuscular mass. Clinically it can be mistaken for soft tissue sarcomas (rhabdomyosarcoma, liposarcoma, leiomyosarcoma), myositis ossificans progressiva, inflammatory myofibroblastic tumor, lymphoma, fibromatosis, [rhabdomyoma](https://www.edgepedia.org/rhabdomyoma), intramuscular lipoma, venous thrombosis, and other inflammatory or vascular conditions; pathologically it can resemble an inflammatory myopathy or muscular dystrophy[2](https://www.orpha.net/en/disease/detail/48918) • [4](https://pubmed.ncbi.nlm.nih.gov/27726926/). Nodular or granulomatous myositis and soft tissue tumors are the most challenging differentials[4](https://pubmed.ncbi.nlm.nih.gov/27726926/).\n\n## Treatment and prognosis\n\nTreatment is often unnecessary because most cases resolve spontaneously[2](https://www.orpha.net/en/disease/detail/48918). Anti-inflammatory drugs, steroid or nonsteroidal, such as oral prednisolone, can be given to minimize pain and inflammation, and physical or surgical therapy may be needed when joint motion is limited[2](https://www.orpha.net/en/disease/detail/48918). The 2016 review recommends reserving steroids for patients with major pain, nerve lesions, associated autoimmune disease, or elevated [C-reactive protein](https://www.edgepedia.org/c-reactive-protein) or creatine kinase[4](https://pubmed.ncbi.nlm.nih.gov/27726926/).\n\n[Spontaneous remission](https://www.edgepedia.org/spontaneous-remission) occurs within weeks or months after the first symptoms, and relapse is unusual[3](https://pubmed.ncbi.nlm.nih.gov/28947258/). Orphanet reports recurrence in around 18% of cases[2](https://www.orpha.net/en/disease/detail/48918), while the Lyon cohort recorded a relapse rate of 41%, with 64% of cases having received immunosuppressive drugs[5](https://www.neurology.org/doi/10.1212/WNL.0000000000005160). In most cases the process resolves spontaneously or after immunosuppressant therapy, but it sometimes recurs or progresses to systemic inflammation[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC11537714/). Prognosis is very good[2](https://www.orpha.net/en/disease/detail/48918).\n\n## References\n\n1. [Focal myositis: a literature review of clinical and immunopathological aspects, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC11537714/)\n2. [Focal myositis, Orphanet ORPHA:48918](https://www.orpha.net/en/disease/detail/48918)\n3. [Gallay L, et al. [Focal myositis: An unknown disease]. Rev Med Interne, 2017](https://pubmed.ncbi.nlm.nih.gov/28947258/)\n4. [Devic P, et al. Focal myositis: A review. Neuromuscular Disorders, 2016](https://pubmed.ncbi.nlm.nih.gov/27726926/)\n5. [Gallay L, et al. Focal myositis: New insights on diagnosis and pathology. Neurology, 2018](https://www.neurology.org/doi/10.1212/WNL.0000000000005160)\n6. [Recurrent Focal Myositis in Childhood: A Case Report and Systematic Review of the Literature, PubMed](https://pubmed.ncbi.nlm.nih.gov/28476523/)\n7. [Auerbach A, et al. Focal myositis: a clinicopathologic study of 115 cases. Am J Surg Pathol, 2009](https://pubmed.ncbi.nlm.nih.gov/19363438/)\n\n---\n*Topic: Encyclopedia › Life and health › Human health and medicine › Diseases and injuries › Skin and musculoskeletal conditions › Musculoskeletal conditions › Muscle disease › Myopathy*\n\n*Initially written Oct 21, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n", "same_as": [], "url": "https://www.edgepedia.org/focal-myositis", "markdown_url": "https://www.edgepedia.org/focal-myositis.md", "license": { "name": "Edgepedia Community License 1.0", "url": "https://www.edgepedia.org/edgepedia/license", "summary": "Free with credit, commercial use included. AI training is open to everyone. For other uses, organizations over USD 100M in revenue or 100M monthly users license separately.", "spdx": "LicenseRef-Edgepedia-Community-1.0" }, "credit": "\"Focal myositis\", Edgepedia (EdgeChat), https://www.edgepedia.org/focal-myositis. Edgepedia Community License 1.0.", "credit_md": "\"[Focal myositis](https://www.edgepedia.org/focal-myositis)\", Edgepedia (EdgeChat), [https://www.edgepedia.org/focal-myositis](https://www.edgepedia.org/focal-myositis). [Edgepedia Community License 1.0](https://www.edgepedia.org/edgepedia/license).", "credit_html": "\"Focal myositis\", Edgepedia (EdgeChat), https://www.edgepedia.org/focal-myositis. Edgepedia Community License 1.0.", "speakable": "Focal myositis is a rare, benign, localized inflammation of a single skeletal muscle, usually in the lower limb, presenting as a mass that typically regresses spontaneously; biopsy confirms diagnosis." }