Inflammatory myopathy and inclusion-body myositis

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

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.

Diagnosis combines clinical pattern, CK and other enzymes, myositis antibodies, MRI, neurophysiology and often biopsy. Antibody results are probabilistic and commercial line-blot low positives can mislead; use a specialist laboratory and phenotype concordance. Treatment is tailored to organ threat and subtype. High-dose corticosteroid is common for active immune myopathy, with methotrexate, azathioprine, mycophenolate, rituximab or immunoglobulin selected by specialists. Exercise is safe and beneficial when supervised; prolonged inactivity compounds disability.

Key points

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.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Systemic autoimmune myositis

Dermatomyositis and antisynthetase-spectrum disease reflect immune injury to muscle, skin, lung and other tissues, sometimes with phenotype-defining autoantibodies.

Immune-mediated necrotising myopathy

Anti-HMGCR or anti-SRP-associated disease causes severe fibre necrosis, occasionally after statin exposure, but can occur without an identifiable medicine trigger.

Cancer-associated inflammation

Selected dermatomyositis phenotypes accompany malignancy, making age, antibody, skin findings and clinical context important for proportionate cancer assessment.

Inclusion-body myositis

IBM combines inflammatory and degenerative protein-handling abnormalities, usually progressing slowly with distinctive finger-flexor and knee-extensor weakness.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Immune targeting of muscle
Humoral or cellular immune responses injure myofibres, capillaries or surrounding connective tissue according to the inflammatory subtype.
2
Fibre necrosis and weakness
Damaged fibres lose contractile capacity and release muscle enzymes, producing proximal weakness, fatigue and sometimes pain or dysphagia.
3
Repair, fibrosis and atrophy
Repeated injury with incomplete regeneration replaces functional muscle with fat and fibrous tissue, creating fixed weakness despite later suppression of inflammation.
4
IBM protein accumulation
In inclusion-body myositis, rimmed vacuoles and abnormal proteins accompany cytotoxic inflammation, contributing to slow treatment-resistant selective muscle loss.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Proximal myopathy

Difficulty standing without arms, climbing stairs, lifting objects overhead or holding the head up evolves over weeks with sensation preserved.

Dermatomyositis skin

Violaceous eyelid oedema, Gottron papules, photosensitive upper-trunk erythema and periungual capillary change provide visible evidence of a systemic myositis phenotype.

Antisynthetase cluster

Breathlessness, inflammatory arthritis, Raynaud phenomenon and mechanic's hands accompanying weakness raises myositis-associated interstitial lung disease.

Necrotising pattern

Rapid profound proximal weakness with markedly elevated creatine kinase after statin exposure or with anti-HMGCR or anti-SRP antibodies suggests immune-mediated necrosis.

IBM distribution

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.

Organ-threatening diseaseRed flag

New hypoxia, rapidly falling vital capacity, aspiration, arrhythmia, troponin elevation or severe dysphagia requires urgent multidisciplinary admission.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Creatine kinase and muscle enzymesFirst step
Why
Quantify membrane injury and provide a treatment-response marker.
Interpretation and limitations
Very high CK supports necrotising disease but is non-specific; IBM can be mildly raised and dermatomyositis can be active with normal CK, so examine strength and organs.
02
Myositis-specific and associated antibodies
Why
Refine subtype, lung, malignancy and treatment risks.
Interpretation and limitations
Interpret anti-Jo-1 and other synthetases, MDA5, TIF1-gamma, NXP2, SRP and HMGCR only with phenotype and assay quality; one weak band is not a diagnosis.
03
MRI of symptomatic muscles
Why
Show oedema, fatty replacement and a viable site for biopsy.
Interpretation and limitations
Oedema supports activity but occurs after exercise, denervation and infection; severe fatty replacement predicts less reversible weakness and poor biopsy yield.
04
Electromyography and nerve conduction
Why
Support irritable myopathy and exclude neuropathic or junctional mimics.
Interpretation and limitations
Short-duration low-amplitude units and spontaneous activity fit myopathy but are not subtype-specific; select a biopsy muscle that was not recently needled.
05
Muscle biopsy
Why
Distinguish perifascicular dermatomyositis, necrotising myopathy, IBM and other pathology.
Interpretation and limitations
Biopsy site and specialist processing matter; IBM diagnosis integrates endomysial invasion, rimmed vacuoles and degenerative markers with the clinical pattern.
06
Pulmonary function and high-resolution CT
Why
Detect myositis-associated interstitial lung disease and respiratory weakness.
Interpretation and limitations
Falling transfer factor, restrictive volumes or exertional desaturation requires respiratory review; forced vital capacity can reflect muscle as well as interstitial disease.
07
ECG, troponin and cardiac imaging
Why
Investigate arrhythmia, myocarditis or cardiomyopathy when symptoms or subtype create risk.
Interpretation and limitations
Use cardiac-specific troponin where possible because regenerating skeletal muscle can confuse some assays; involve cardiology for concordant abnormalities.
08
Risk-adapted malignancy assessment
Why
Detect cancer associated particularly with adult dermatomyositis and selected antibodies.
Interpretation and limitations
Combine standard national screening with targeted imaging and examination based on age, sex, symptoms and antibody risk under specialist guidance.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Inherited muscular dystrophy

Longstanding patterned weakness, contractures, family history and genomic findings favour dystrophy; onset and creatine kinase alone are insufficient discriminators.

Endocrine, toxic or medicine myopathy

Thyroid, adrenal, alcohol, corticosteroid and medicine-related weakness is identified through exposure, systemic findings and targeted laboratory assessment.

Myasthenia or motor neurone disease

Fatigable ocular-bulbar weakness with normal sensation supports junctional disease, while fasciculation and upper motor-neurone signs point towards motor neurone disease.

Deconditioning or pain-limited effort

Generalised low endurance without a coherent proximal pattern, muscle enzyme change or supportive imaging may reflect systemic illness, pain or disuse.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Initial localisationProve and pattern muscle weaknessFirst stepAn adult reports progressive difficulty with stairs, rising or overhead tasks.

1Measure neck, scapular, hip, knee and distal power, inspect skin and muscle bulk, and assess reflexes, sensation, bulbar function and gait.
2Review statins, steroids, alcohol, endocrine and electrolyte disease, infection, family history and exercise, obtaining CK and a targeted mimic profile.
3Refer promptly to rheumatology or neuromuscular neurology when objective weakness, rash, CK elevation or organ features support inflammatory muscle disease.

02Subtype work-upIntegrate antibody, imaging and tissueAn inflammatory myopathy remains likely after first assessment.

1Request specialist myositis antibodies and MRI while screening swallowing, lung and cardiac involvement, rather than waiting for CK to become very high.
2Use MRI and electromyography to select an affected but not end-stage muscle, then arrange biopsy with expert neuromuscular pathology when subtype remains uncertain.
3Classify the clinicopathological syndrome before prolonged immunosuppression, explicitly considering IBM and inherited dystrophy when distribution or tempo is atypical.

03Active immune diseaseSuppress inflammation and protect organsDermatomyositis, antisynthetase or necrotising myopathy is active and clinically significant.

1Admit organ-threatening lung, swallow, respiratory or cardiac disease and start specialist-selected corticosteroid or rapid rescue treatment alongside aspiration and thrombosis prevention.
2Introduce an appropriate steroid-sparing agent early according to subtype, pregnancy, infection, lung disease, malignancy and anticipated treatment duration.
3Add graded physiotherapy and nutritional or swallowing support while monitoring strength, CK and the threatened organ, avoiding both harmful rest and overexertion.

04IBM carePrioritise function over ineffective immunosuppressionClinical pattern and pathology support inclusion-body myositis.

1Explain the typically slow but progressive course and limited durable response to conventional immunotherapy, correcting an earlier polymyositis label where necessary.
2Provide resistance and balance exercise, falls assessment, occupational adaptation and timely orthoses or mobility equipment to preserve participation.
3Assess dysphagia repeatedly with speech and language therapy, using videofluoroscopy, dilation or other specialist options when aspiration or nutrition is threatened.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

Rapidly suppresses immune muscle inflammation in dermatomyositis, antisynthetase and necrotising disease.

Prednisolone

Active 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.

A common steroid-sparing agent for muscle and skin disease when lung phenotype and pregnancy plans permit.

Methotrexate

Use 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.

Provides relatively rapid immunomodulation, with evidence particularly for selected dermatomyositis and severe swallowing disease.

Intravenous immunoglobulin

Severe dysphagia or refractory active myositis may use the current NHS immunoglobulin course divided over several days under specialist commissioning criteria.

Assess thrombosis, kidney injury, fluid load, haemolysis and infusion reaction; benefit in sporadic IBM is inconsistent and not a routine long-term strategy.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Dysphagia and aspiration

Pharyngeal muscle involvement, particularly in IBM, causes choking, weight loss and aspiration pneumonia even when limb weakness progresses slowly.

Respiratory muscle failure

Severe myopathy can weaken ventilation and cough, causing nocturnal hypoventilation, infection and perioperative risk during severe inflammatory disease.

Interstitial lung and cardiac disease

Antisynthetase and selected immune phenotypes can involve lungs or myocardium, creating breathlessness, arrhythmia and organ-threatening inflammation.

Falls and irreversible disability

Knee-extensor, hip and finger-flexor weakness causes falls, loss of grip and dependence, while fibrosis limits recovery from delayed treatment.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Measure the same functional tasks and Medical Research Council muscle groups serially, because CK can normalise before strength and may not reflect skin or lung activity.
Track CK, blood count, renal and liver tests at agent-specific intervals and distinguish steroid toxicity from uncontrolled inflammatory weakness.
Repeat lung symptoms, oxygenation and pulmonary function promptly in antisynthetase or MDA5 phenotypes, escalating any rapid decline to specialist respiratory care.
Review swallowing, weight, meal duration, cough and recurrent chest infection, obtaining instrumental assessment before silent aspiration causes major harm.
Maintain age- and risk-adapted malignancy surveillance after dermatomyositis according to antibody, initial findings and specialist protocol.
In IBM, monitor falls, grip, chair rise, driving, home safety and carer burden even when laboratory markers remain unchanged.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Polymyositis is uncommon

Modern antibody and biopsy classification reassigns many historical cases to necrotising myopathy, antisynthetase disease, IBM or inherited muscle disease.

Skin can lead muscle

Clinically amyopathic dermatomyositis can carry major lung or malignancy risk despite little weakness and a normal creatine kinase.

IBM targets unusual muscles

Deep finger-flexor and knee-extensor weakness, often asymmetric, is a bedside pattern that should prevent repeated ineffective steroid courses.

CK is not activity alone

Exercise, necrosis and muscle mass influence CK; severe fatty replacement can yield a modest result despite substantial fixed weakness.

Exercise is treatment

Supervised resistance and aerobic rehabilitation generally supports strength and function in stable myositis rather than reigniting inflammation.

11Common pitfallsFrequent interpretation and management errors.

01
Calling pain or fatigue weakness without testing power objectively.
02
Excluding dermatomyositis because creatine kinase is normal.
03
Accepting a weak antibody band without phenotype correlation.
04
Missing rapidly progressive interstitial lung disease in skin-predominant disease.
05
Biopsying an end-stage or recently needled muscle.
06
Treating inclusion-body myositis with repeated toxic steroid escalation.
07
Ignoring dysphagia and malignancy risk while following CK alone.