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Dermatomyositis and polymyositis

Definition
DM/PM are autoimmune myopathies characterized by subacute or chronic inflammation and weakness of proximal muscles with extra muscular manifestations.
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Pathophysiology
Dermatomyositis is mostly caused by humoral immune response whereas polymyositis is caused by T cell-mediated immune response.
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Disease course
Autoimmunity results in polymyositis (PM) and dermatomyositis (DM), which cause clinical manifestations of symmetrical proximal muscle weakness, pain, inflammation, typical rash in DM, fever, non-erosive arthritis, hyperkeratotic lesions along the radial and palmar aspects of fingers. Disease progression may lead to ILD and increased risk of malignancy.
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Prognosis and risk of recurrence
The in-hospital mortality rate of DM/PM is 4.58%.
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Key sources
The following summarized guidelines for the evaluation and management of dermatomyositis and polymyositis are prepared by our editorial team based on guidelines from the American College of Rheumatology (ACR 2023), the British Society for Rheumatology (BSR 2022), the European League Against Rheumatism (EULAR 2022), and the Japan College of Rheumatology (JCR 2019).
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Guidelines

1.Classification and risk stratification

Prognostic features: some clinical symptoms/signs and laboratory tests can predict life prognosis and responsiveness to treatment in patients with DM/PM.
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2.Diagnostic investigations

Initial evaluation: measure anti-aminoacyl-tRNA synthetases (ARS) antibodies - including anti‐Jo‐1 and other specific autoantibodies - in patients with DM/PM, as they are closely associated with clinical subsets, pathogenesis, clinical course and therapeutic responsiveness of patients with myositis.
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  • Antibody testing

  • Assessment of disease activity

  • Evaluation for cardiac involvement

  • Evaluation for dysphagia

  • Bone health assessment

  • Mental health assessment

  • Screening for chronic infections

3.Medical management

General principles: consider treating malignancy, if present concomitantly with DM/PM, as the first step in management, unless the symptoms of DM/PM require urgent treatment.

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  • Corticosteroids

  • Immunosuppressants

  • Biologics

  • IVIG

  • Management of cutaneous manifestations

  • Management of dysphagia

  • Management of ILD

  • Management of cardiac myositis

  • Management of relapse

4.Nonpharmacologic interventions

Sun protection: consider advising sun avoidance and regular use of high factor broad spectrum sun cream to reduce likelihood of a disease flare affecting skin or muscle.
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  • Physiotherapy

5.Specific circumstances

Pediatric patients, evaluation: take age-specific considerations into account when using tools measuring muscle strength, function, and QoL.
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  • Pregnant patients

  • Pediatric patients (specialist referral)

  • Pediatric patients (corticosteroids and immunosuppressants)

  • Pediatric patients (management of cutaneous manifestations)

  • Pediatric patients (physiotherapy)

6.Preventative measures

Routine immunizations: consider offering high-dose or adjuvanted influenza vaccination, rather than regular-dose influenza vaccination, in ≥ 65 years old patients with rheumatic or musculoskeletal diseases and in 18-65 years old patients with rheumatic or musculoskeletal diseases on immunosuppressive medications.
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  • Prophylaxis for P. jirovecii pneumonia

7.Follow-up and surveillance

Surveillance for malignancy: take into account the risk of cancer in all patients and consider screening particularly in patients with the following risk factors:
older age at onset
male gender
dysphagia
cutaneous necrosis
resistance to immunosuppressive therapy
rapid disease onset
positive anti-TIF-1-gamma autoantibodies
positive anti-nuclear matrix protein-2 autoantibodies
negative for known myositis-specific autoantibodies
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8.Quality improvement

Ethnicity considerations: take into account ethnicity when assessing patients as clinical manifestations, associated autoantibodies and underlying risk factors may vary according to ethnicity.
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