Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
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.
Inflammatory muscle disease presents at the intersection of neurology, rheumatology, dermatology, respiratory medicine and pathology. The first task is to prove weakness rather than fatigue or pain inhibition, then localise it to muscle. Proximal weakness with preserved sensation and reflexes until late supports myopathy, but endocrine disease, electrolyte disturbance, corticosteroids, alcohol, infection, inherited muscle disease and motor-neurone disorders can mimic it. Creatine kinase magnitude helps but is not a classification system.
Subtype matters because complications and treatment response differ. Dermatomyositis may be skin dominant and carries malignancy risk that varies with age and autoantibody. Antisynthetase and anti-MDA5 phenotypes may be driven by lung disease. Anti-HMGCR or anti-SRP necrotising myopathy can require intensive immunotherapy. Inclusion-body myositis is both inflammatory and degenerative, with endomysial inflammation, rimmed vacuoles and protein accumulation, and usually resists conventional immune treatment despite sometimes modest inflammatory markers.
Dermatomyositis and antisynthetase-spectrum disease reflect immune injury to muscle, skin, lung and other tissues, sometimes with phenotype-defining autoantibodies.
Anti-HMGCR or anti-SRP-associated disease causes severe fibre necrosis, occasionally after statin exposure, but can occur without an identifiable medicine trigger.
Selected dermatomyositis phenotypes accompany malignancy, making age, antibody, skin findings and clinical context important for proportionate cancer assessment.
IBM combines inflammatory and degenerative protein-handling abnormalities, usually progressing slowly with distinctive finger-flexor and knee-extensor weakness.
Humoral or cellular immune responses injure myofibres, capillaries or surrounding connective tissue according to the inflammatory subtype.
Damaged fibres lose contractile capacity and release muscle enzymes, producing proximal weakness, fatigue and sometimes pain or dysphagia.
Repeated injury with incomplete regeneration replaces functional muscle with fat and fibrous tissue, creating fixed weakness despite later suppression of inflammation.
In inclusion-body myositis, rimmed vacuoles and abnormal proteins accompany cytotoxic inflammation, contributing to slow treatment-resistant selective muscle loss.
Difficulty standing without arms, climbing stairs, lifting objects overhead or holding the head up evolves over weeks with sensation preserved.
Violaceous eyelid oedema, Gottron papules, photosensitive upper-trunk erythema and periungual capillary change provide visible evidence of a systemic myositis phenotype.
Breathlessness, inflammatory arthritis, Raynaud phenomenon and mechanic's hands accompanying weakness raises myositis-associated interstitial lung disease.
Rapid profound proximal weakness with markedly elevated creatine kinase after statin exposure or with anti-HMGCR or anti-SRP antibodies suggests immune-mediated necrosis.
Slow asymmetric wasting of quadriceps and deep finger flexors, early falls and impaired grip in an older adult is more typical of inclusion-body myositis than polymyositis.
New hypoxia, rapidly falling vital capacity, aspiration, arrhythmia, troponin elevation or severe dysphagia requires urgent multidisciplinary admission.
Longstanding patterned weakness, contractures, family history and genomic findings favour dystrophy; onset and creatine kinase alone are insufficient discriminators.
Thyroid, adrenal, alcohol, corticosteroid and medicine-related weakness is identified through exposure, systemic findings and targeted laboratory assessment.
Fatigable ocular-bulbar weakness with normal sensation supports junctional disease, while fasciculation and upper motor-neurone signs point towards motor neurone disease.
Generalised low endurance without a coherent proximal pattern, muscle enzyme change or supportive imaging may reflect systemic illness, pain or disuse.
Pharyngeal muscle involvement, particularly in IBM, causes choking, weight loss and aspiration pneumonia even when limb weakness progresses slowly.
Severe myopathy can weaken ventilation and cough, causing nocturnal hypoventilation, infection and perioperative risk during severe inflammatory disease.
Antisynthetase and selected immune phenotypes can involve lungs or myocardium, creating breathlessness, arrhythmia and organ-threatening inflammation.
Knee-extensor, hip and finger-flexor weakness causes falls, loss of grip and dependence, while fibrosis limits recovery from delayed treatment.
Modern antibody and biopsy classification reassigns many historical cases to necrotising myopathy, antisynthetase disease, IBM or inherited muscle disease.
Clinically amyopathic dermatomyositis can carry major lung or malignancy risk despite little weakness and a normal creatine kinase.
Deep finger-flexor and knee-extensor weakness, often asymmetric, is a bedside pattern that should prevent repeated ineffective steroid courses.
Exercise, necrosis and muscle mass influence CK; severe fatty replacement can yield a modest result despite substantial fixed weakness.
Supervised resistance and aerobic rehabilitation generally supports strength and function in stable myositis rather than reigniting inflammation.
Calling pain or fatigue weakness without testing power objectively.
Excluding dermatomyositis because creatine kinase is normal.
Accepting a weak antibody band without phenotype correlation.
Missing rapidly progressive interstitial lung disease in skin-predominant disease.
Biopsying an end-stage or recently needled muscle.
Treating inclusion-body myositis with repeated toxic steroid escalation.
Ignoring dysphagia and malignancy risk while following CK alone.
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A 68-year-old man has slowly progressive falls, asymmetric quadriceps wasting and weakness of deep finger flexion. Creatine kinase is mildly raised and repeated corticosteroids have not helped. What is the most likely diagnosis?
National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.
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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.
Never prescribe daily; monitor blood count, liver and renal function, avoid pregnancy and review pulmonary toxicity or significant interstitial lung disease with specialists.
Assess thrombosis, kidney injury, fluid load, haemolysis and infusion reaction; benefit in sporadic IBM is inconsistent and not a routine long-term strategy.