Synopsis
Differentiate treatable immune-mediated myopathy from inclusion-body myositis and non-inflammatory weakness, detect swallowing, respiratory, cardiac, lung and malignancy complications, and obtain tissue before prolonged immunosuppression where appropriate.
- Idiopathic inflammatory myopathies include dermatomyositis, antisynthetase-spectrum disease, immune-mediated necrotising myopathy and inclusion-body myositis; the historical label polymyositis is now used far less often after modern classification.
- Most active immune myopathies produce subacute, broadly symmetric proximal weakness affecting chair rise, stair climbing, lifting and neck flexion more than distal movement.
- Dermatomyositis clues include heliotrope eyelid change, Gottron papules over extensor joints, Gottron sign, shawl or V-neck erythema, periungual change and scalp inflammation.
- Antisynthetase syndrome combines myositis variably with interstitial lung disease, inflammatory arthritis, Raynaud phenomenon, fever and hyperkeratotic mechanic's hands.
- Immune-mediated necrotising myopathy often causes severe weakness and very high creatine kinase, sometimes associated with anti-HMGCR antibodies after statin exposure or anti-SRP antibodies.
- Inclusion-body myositis usually progresses slowly after age 50, with disproportionate finger-flexor and knee-extensor weakness, asymmetry, falls and dysphagia; creatine kinase is often only modestly raised.
- A normal or mildly raised creatine kinase does not exclude dermatomyositis, especially amyopathic skin-predominant disease or selected antibody phenotypes.
- MRI can identify oedema and a useful biopsy site, while electromyography supports an irritable myopathy; neither replaces expert clinicopathological correlation.
- Search for interstitial lung disease, cardiac involvement and age-appropriate malignancy according to subtype, antibody, skin signs and risk rather than ordering an unstructured whole-body panel.
- Inflammatory myopathies often need corticosteroid plus an early steroid-sparing agent; inclusion-body myositis usually does not respond durably to immunosuppression and prioritises rehabilitation and swallowing care.
Key red flags
New hypoxia, rapidly falling vital capacity, aspiration, arrhythmia, troponin elevation or severe dysphagia requires urgent multidisciplinary admission.
Investigation priorities
Quantify membrane injury and provide a treatment-response marker.
Management branches
An adult reports progressive difficulty with stairs, rising or overhead tasks.
- Measure neck, scapular, hip, knee and distal power, inspect skin and muscle bulk, and assess reflexes, sensation, bulbar function and gait.
- Review statins, steroids, alcohol, endocrine and electrolyte disease, infection, family history and exercise, obtaining CK and a targeted mimic profile.
- Refer promptly to rheumatology or neuromuscular neurology when objective weakness, rash, CK elevation or organ features support inflammatory muscle disease.
Key medicines
PrednisoloneActive inflammatory myopathy often begins with a specialist weight-based oral regimen followed by a response- and toxicity-led taper; exact dose reflects organ threat and subtype.Screen infection and monitor glucose, blood pressure, mood, bone, eye and gastrointestinal risk; steroid myopathy can paradoxically worsen proximal weakness and IBM usually does not respond.
MethotrexateUse a once-weekly specialist dose with folic acid and gradual titration according to the current BNF, renal function and local shared-care standard.Never prescribe daily; monitor blood count, liver and renal function, avoid pregnancy and review pulmonary toxicity or significant interstitial lung disease with specialists.