Musculoskeletal

Polymyositis and dermatomyositis

Polymyositis and dermatomyositis sit within the idiopathic inflammatory myopathies, a group of multisystem autoimmune diseases causing objective muscle weakness and sometimes characteristic skin, lung, swallowing and cardiac disease. In adults, dermatomyositis and selected antibody phenotypes also signal increased malignancy risk.

In a nutshell

Polymyositis and dermatomyositis are part of the idiopathic inflammatory myopathies. Recognise objective symmetrical proximal weakness and dermatomyositis skin signs, then look actively for dysphagia, ILD and cardiac involvement. Use CK and myositis antibodies with specialist MRI, EMG or biopsy. Treat active muscle inflammation with high-dose glucocorticoid plus early steroid-sparing therapy, escalate severe or refractory disease through specialist pathways, and risk-stratify adult cancer screening.

Classic presentation

A person develops weeks of difficulty standing from a chair and lifting the arms, with a raised CK and Gottron papules or heliotrope rash. Refer promptly, assess swallowing, breathing and cardiac symptoms, and arrange specialist myositis and malignancy-risk assessment.

Key points

The modern umbrella is idiopathic inflammatory myopathy; do not treat polymyositis as a simple diagnosis made by biopsy pattern alone.
Weakness is usually proximal and objective, with sensation preserved; distinguish it from pain, fatigue, neuropathy, neuromuscular-junction disease and steroid myopathy.
Dermatomyositis skin disease may occur with little muscle weakness, and antisynthetase or anti-MDA5 phenotypes may have serious ILD.
Ask every patient about dysphagia and screen for cardiac and respiratory involvement when indicated.
Myositis antibodies support phenotype and risk assessment but antibody titres do not monitor activity.
Use high-dose glucocorticoid for active muscle inflammation with an early steroid-sparing DMARD; exact prescribing follows BNF and specialist protocols.
Adult cancer risk is individualised, with particular concern for rapid onset, dysphagia, cutaneous necrosis, treatment resistance, anti-TIF1-gamma or anti-NXP2; routine cancer screening is not advised for juvenile-onset IIM without another indication.
Specialist physiotherapy, occupational therapy, speech and language therapy, dermatology, respiratory, cardiology and oncology input may all be needed.

First-line investigation

Examine for objective proximal weakness and extra-muscular disease, check CK and related enzymes, obtain myositis antibodies and refer for specialist organ assessment and imaging.

Management

Find organ-threatening disease

Urgently assess respiratory weakness or ILD, dysphagia and aspiration, cardiac symptoms, rapidly progressive weakness and hypoxaemia.^1,2

Confirm the IIM phenotype and risk

Document objective proximal weakness, CK and other enzymes, myositis antibodies, skin disease, lung, heart and swallowing features, then refer to a specialist myositis service.^1,2

Suppress active muscle inflammation

Use specialist-guided high-dose glucocorticoid and add an early steroid-sparing DMARD, with BNF and monitoring guidance used for the regimen.^1,4

Treat refractory muscle, skin or organ disease

Consider IVIG, rituximab, cyclophosphamide or other specialist treatment for severe or refractory IIM, dysphagia or ILD within the relevant UK commissioning pathway.^1,3,4

Protect function and skin

Use individually tailored physiotherapy and occupational therapy, speech and language therapy for dysphagia, sun protection and dermatology treatment for skin disease, and assess bone health and mental wellbeing.^1,2

Monitor all disease domains

Follow strength, function, CK, skin, swallowing, lung, cardiac status, malignancy risk and medicine toxicity; do not use antibody titres alone to judge activity.^1,4

Exam traps

Polymyalgia rheumatica causes pain and stiffness, not true weakness, and usually has a normal CK.
Inclusion body myositis is often slowly progressive, asymmetric and distal, especially affecting finger flexors and quadriceps, and responds poorly to steroids.
A normal CK does not exclude clinically amyopathic dermatomyositis or skin-predominant disease.
Dysphagia can occur without severe limb weakness and can cause fatal aspiration; ask directly and involve speech and language therapy.
Breathlessness may be ILD or respiratory muscle weakness, not deconditioning.
Troponin I is preferred over troponin T for suspected cardiac involvement because regenerating skeletal muscle can confound troponin T.
Do not apply routine adult cancer screening to juvenile-onset IIM; use clinical indication and paediatric specialist guidance.

Illustrations

Gottron's papules in dermatomyositisA clinical photograph showing violaceous scaly papules or plaques over the extensor knuckles, with skin tone and lighting that preserve the diagnostic finding. This image remains subject to the separate image-quality review.Elizabeth M. Dugan, Adam M. Huber, Frederick W. Miller and Lisa G. Rider, Wikimedia Commons · CC-BY-SA-3.0

Muscle biopsy in inflammatory myopathyA muscle biopsy histology image showing an inflammatory myopathy pattern, with the limitation that biopsy appearances vary by subtype and should not be used as a single visual diagnostic rule.Jensflorian, Wikimedia Commons · CC-BY-SA-3.0

Key sources

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.