Polymyositis
Polymyositis is an idiopathic inflammatory myopathy causing symmetric, subacute proximal muscle weakness with no rash, driven by endomysial CD8-positive T-cell invasion of non-necrotic, MHC class I-expressing muscle fibers. It is a diagnosis of exclusion confirmed by muscle biopsy, and first-line treatment is high-dose prednisone with a steroid-sparing agent such as methotrexate or azathioprine started concurrently.
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What this article covers
- Overview
Definition: polymyositis is an idiopathic inflammatory myopathy causing symmetric, subacute proximal weakness, defined largely by what it lacks: the pathognomonic rash of dermatomyositis.
- Epidemiology
Uncommon and heterogeneous: the idiopathic inflammatory myopathies as a group have an annual incidence of 2 to 10 cases per million.
- Risk factors
HLA-DRB103:01: the clearest genetic association, a human leukocyte antigen (HLA) allele, though associations vary by ethnicity.
- Pathophysiology
Endomysial CD8+ T-cell invasion: cytotoxic CD8+ T cells localize within the endomysium and surround and invade non-necrotic muscle fibers, the histologic signature of polymyositis.
- Clinical features
Symmetric proximal weakness: shoulder and hip girdle weakness over weeks to months, causing difficulty combing hair, rising from low chairs, and climbing stairs.
- Differential diagnosis
Dermatomyositis: shares proximal weakness and elevated creatine kinase, but is defined by the heliotrope rash, Gottron papules, and Gottron sign polymyositis lacks, with a perifascicular…
- Diagnosis
The diagnosis is established by the 2017 European League Against Rheumatism/American College of Rheumatology classification criteria: objective, usually progressive, symmetric proximal weakness, or…
- Management
First, start high-dose glucocorticoids: prednisone 1 mg/kg/day PO once daily, up to 80 mg/day, maintained 4 to 6 weeks before a taper.
- Complications
Interstitial lung disease: typically nonspecific interstitial pneumonia, the leading driver of morbidity and mortality, particularly severe with concurrent antisynthetase syndrome (interstitial lung disease).
- Prognosis
Highly variable course: outcomes range from full resolution to severe, treatment-resistant disease with functional impairment and early mortality.
- Prevention
Screen for malignancy at diagnosis and for several years after: age- and sex-appropriate cancer screening, often with computed tomography of the chest, abdomen, and pelvis, given the 10% to 15% occult malignancy rate within 1 to 3 years.
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Related articles
- Dermatomyositis
Dermatomyositis is a complement-mediated inflammatory myopathy causing symmetric proximal weakness with pathognomonic skin findings, heliotrope rash and Gottron papules, that separate it from polymyositis. It carries a 3- to 12-fold increased malignancy risk and a distinct interstitial lung disease burden that drive workup and therapy. First-line treatment is high-dose prednisone with an early steroid-sparing agent.
- Inclusion body myositis
Inclusion body myositis is a sporadic inflammatory myopathy of older adults that breaks the pattern of the other myositides: it favors men, causes insidious, often asymmetric weakness of the finger flexors and quadriceps plus dysphagia, and raises creatine kinase only mildly. Biopsy shows endomysial inflammation with rimmed vacuoles and congophilic amyloid inclusions. It does not respond to glucocorticoids or immunosuppression, so treatment centers on structured exercise and physical therapy, dysphagia evaluation and swallowing therapy, and falls prevention.
- Antisynthetase syndrome
Antisynthetase syndrome is an autoimmune overlap disorder defined by an antiaminoacyl-tRNA synthetase antibody, most often anti-Jo-1, together with interstitial lung disease, myositis, non-erosive arthritis, Raynaud phenomenon, mechanic's hands, and fever. Interstitial lung disease, not the myositis, drives morbidity and mortality and can precede or occur without muscle disease. Treatment starts with high-dose glucocorticoids plus a lung-directed steroid-sparing agent such as mycophenolate mofetil or azathioprine, escalating to rituximab or cyclophosphamide for rapidly progressive lung disease.
- Myasthenia gravis
Myasthenia gravis is an autoimmune disorder of the postsynaptic neuromuscular junction, most often caused by acetylcholine receptor antibodies, producing fluctuating, fatigable weakness of ocular, bulbar, and limb muscles. Diagnosis rests on antibody testing and electrodiagnostics; first-line treatment is pyridostigmine, with glucocorticoids and steroid-sparing immunosuppression for generalized disease and thymectomy for eligible patients.
- Interstitial lung disease
Interstitial lung disease (ILD), also called diffuse parenchymal lung disease, is a diverse group of nonmalignant disorders causing inflammation and scarring of the alveolar interstitium, presenting as subacute to chronic exertional dyspnea and nonproductive cough. Diagnosis rests on pulmonary function testing plus high-resolution CT pattern recognition; management is exposure removal, glucocorticoids for inflammatory disease, antifibrotic therapy for progressive fibrotic disease, oxygen for hypoxemia, and early transplant referral.