Idiopathic inflammatory myopathies are a heterogeneous group of connective tissue disorders characterised by progressive muscle weakness. Other organs - including the lungs, skin, joints, and heart - are often also involved.
Presentation is subacute or chronic and rarely acute.
May be associated with dysphagia, fatigue, difficulties with breathing, and skin lesions.
Diagnosis is confirmed by elevated serum muscle-derived enzymes, typical electromyography findings, and inflammation on muscle biopsy. Autoantibody testing is increasingly important in diagnosis and in defining the disease subtype.
For induction of remission, corticosteroids are usually first-line therapy. Immunosuppressants and intravenous immunoglobulin should be considered for patients who are refractory to, or intolerant of, corticosteroids.
Maintenance of remission is achieved with immunosuppressants. As corticosteroid-sparing agents, they allow the dose reduction or withdrawal of corticosteroid treatment.
Specialist multidisciplinary team involvement is essential for optimal patient outcomes.
Idiopathic inflammatory myopathies (IIMs) constitute a heterogeneous group of subacute, chronic, and, rarely, acute diseases of skeletal muscle that have in common the presence of moderate to severe proximal muscle weakness and inflammation on muscle biopsy.
Based on distinct features, they are broadly divided into 3 groups: polymyositis, dermatomyositis, and inclusion body myositis.
History and exam
Key diagnostic factors
- presence of risk factors
- difficulty with motor tasks
- muscle weakness
- muscle atrophy
- heliotrope rash with eyelid oedema
- Gottron's papules
Other diagnostic factors
- frequent falls
- fatigue and generalised malaise
- weight loss
- shortness of breath
- mild fever
- abnormal breath sounds
- facial rash
- erythematous rash
- nail fold changes
- facial muscle weakness
- skin calcinosis
- joint swelling
- signs of heart failure and/or myocardial infarction
- physical findings of malignancy
- systemic signs of autoimmune disease
- peripheral neuropathy
- age >40 years
- exposure to high intensity of global UV radiation
- genetic predisposition
- female sex and/or black ethnicity (polymyositis and dermatomyositis)
- male sex and/or white ethnicity (inclusion body myositis)
- lipid-lowering agents
- viral infections (excluding HIV)
- non-viral infection
- other drugs or toxins
1st investigations to order
- creatine kinase
- muscle biopsy
- myositis-specific and associated autoantibodies
- magnetic resonance imaging
- lactate dehydrogenase
- alanine aminotransferase
- aspartate aminotransferase
Investigations to consider
- erythrocyte sedimentation rate
- C-reactive protein
- antinuclear antibodies
- serum creatinine
- serum ferritin
- fluorodeoxyglucose positron emission tomography (FDG-PET)/computed tomography
- ultrasound scan
induction of remission: with severe muscle weakness or life-threatening complications
induction of remission: without severe muscle weakness or life-threatening complications
maintenance of remission: any severity
- Hereditary inclusion body myositis
- Oculopharyngeal muscular dystrophy
- Late-onset distal myopathy
- EFNS guidelines for the use of intravenous immunoglobulin in treatment of neurological diseases
- Guidelines on the use of intravenous immune globulin for neurologic conditions
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