Polymyositis and dermatomyositis

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

Idiopathic inflammatory myopathies are a family of autoimmune muscle and systemic diseases, not simply polymyositis versus dermatomyositis. Dermatomyositis has characteristic skin disease with or without weakness; immune-mediated necrotising myopathy often causes severe weakness and high CK; antisynthetase disease is defined by antibody and lung, joint and skin phenotype; inclusion-body myositis follows a different degenerative-inflammatory course. True polymyositis is uncommon and should be diagnosed only after these alternatives are excluded.

Examination should test neck flexion, shoulder abduction, hip flexion and functional tasks, while looking for asymmetry, distal weakness, atrophy, rash, capillary change, arthritis, Raynaud, mechanic hands and calcinosis. Measure dysphagia, cough and respiratory function separately. CK reflects muscle membrane injury but may be normal in amyopathic dermatomyositis, late burnt-out disease or some antibody phenotypes. AST and ALT may derive from muscle; GGT helps identify a liver source.

Diagnosis integrates pattern, enzymes, myositis antibodies, MRI, EMG and tissue. MRI shows active oedema and guides a biopsy away from end-stage fatty replacement. Muscle biopsy separates inflammatory, necrotising, inclusion-body, metabolic and dystrophic patterns; a skin biopsy may support dermatomyositis. Myositis antibodies carry prognostic tasks: anti-MDA5 prompts urgent lung vigilance, antisynthetase antibodies frame ILD and arthritis, and anti-TIF1-gamma or NXP2 can change malignancy assessment.

Glucocorticoid controls active disease quickly, but early methotrexate, azathioprine, mycophenolate or another steroid-sparing treatment reduces cumulative harm. IVIG or rituximab is used for selected severe or refractory disease. Rehabilitation begins early with individually graded resistance and aerobic work, alongside swallowing and respiratory therapy. Response is measured by strength, function, swallowing, skin, lungs and enzymes; a falling CK without recovering function may indicate damage, steroid myopathy or insufficient rehabilitation.

Key points

Suspect inflammatory myopathy with subacute symmetrical proximal weakness affecting stairs, rising, lifting and hair care; distinguish inability from pain, fatigue and breathlessness.
Dermatomyositis signs include heliotrope oedema, Gottron papules or sign, shawl or V-sign erythema, periungual capillary change, scalp inflammation and mechanic hands.
True polymyositis is a diagnosis of exclusion; reconsider inclusion-body myositis, immune-mediated necrotising myopathy, antisynthetase disease, drug, endocrine and inherited causes before applying it.
Check CK, aldolase, AST, ALT and LDH, but normal CK does not exclude dermatomyositis, clinically amyopathic disease or dangerous ILD.
Use myositis-specific antibodies for phenotype and risk, not as stand-alone diagnoses; MDA5 raises rapidly progressive ILD concern and TIF1-gamma increases malignancy vigilance.
MRI identifies oedema and a suitable biopsy site; EMG supports a myopathic process and excludes mimics; muscle or skin biopsy remains important when phenotype and serology do not secure diagnosis.
Screen swallowing and respiratory strength at baseline and whenever voice, cough, aspiration, orthopnoea or morning headache appears; do not wait for profound limb weakness.
Start glucocorticoid promptly for active muscle disease and add a steroid-sparing agent early; combine treatment with physiotherapist-led progressive exercise rather than prolonged rest.
Treat IVIG as an important option for severe dysphagia, refractory muscle or skin disease and selected pregnancy situations; use specialist weight-based dosing and thrombosis or renal safeguards.
Complete routine national cancer screening and add risk-stratified investigation near disease onset based on age, symptoms, examination, antibody phenotype and shared decision-making.

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

Autoimmune genetic susceptibility

HLA and immune-regulatory variants interact with environmental or infectious triggers to generate distinct inflammatory-myopathy phenotypes rather than one uniform disease.

Myositis-specific autoimmunity

Antibodies targeting synthetases, Mi-2, MDA5, TIF1-gamma, NXP2, SRP or HMGCR associate with different skin, lung, cancer and muscle patterns.

Cancer and medicine associations

A tumour can accompany dermatomyositis through shared immune targets, while statins can trigger self-sustaining anti-HMGCR immune-mediated necrotising myopathy in susceptible patients.

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

1
Dermatomyositis microangiopathy
Complement-mediated capillary injury and type-I interferon signalling damage perifascicular muscle and characteristic skin, producing perifascicular atrophy on biopsy.
2
T-cell muscle-fibre injury
Some polymyositis-like phenotypes show endomysial cytotoxic T cells invading non-necrotic fibres, although true polymyositis is much rarer than historically taught.
3
Necrotising myopathy
Anti-SRP or anti-HMGCR disease causes prominent fibre necrosis with relatively sparse inflammation and often very high CK and severe weakness.
4
Systemic organ mechanisms
Shared autoimmune pathways inflame oesophageal and respiratory muscle, lung interstitium, joints and occasionally myocardium, so serum CK alone cannot measure total risk.

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

Proximal myopathic weakness

Difficulty rising without arms, climbing stairs, lifting objects or holding arms overhead with preserved sensation supports proximal muscle disease.

Pathognomonic skin features

Gottron papules over extensor joints and a violaceous heliotrope eruption strongly support dermatomyositis within the appropriate systemic assessment.

Amyopathic dermatomyositis

Characteristic skin disease with little clinical weakness can still associate with severe ILD, especially an MDA5 phenotype, despite normal CK.

Bulbar weaknessRed flag

Wet voice, coughing with meals, nasal regurgitation, weight loss and recurrent chest infection indicates dysphagia and aspiration risk.

Respiratory muscle weaknessRed flag

Orthopnoea, weak cough, reduced chest expansion, low supine FVC or hypercapnia can progress to ventilatory failure.

Inclusion-body pattern

Slow asymmetric quadriceps and deep finger-flexor weakness with falls in an older adult argues against routine polymyositis immunosuppression.

Red flags requiring action

Choking, wet voice, nasal regurgitation, recurrent pneumonia or inability to handle secretions requires urgent speech-and-language and respiratory assessment.
Weak cough, orthopnoea, morning headache, declining FVC or hypercapnia suggests respiratory muscle failure and may need non-invasive or invasive support.
Rapid hypoxia with ground-glass or organising change may represent myositis-associated ILD, infection, aspiration, haemorrhage or oedema.
Chest pain, palpitations, syncope, troponin rise or conduction change requires urgent myocarditis and arrhythmia assessment.
New dermatomyositis, weight loss, lymphadenopathy, dysphagia or treatment-refractory disease requires age-, sex- and phenotype-informed malignancy evaluation.

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
CK, aldolase, AST, ALT, LDH and urinalysisFirst step
Why
Quantify muscle injury, detect rhabdomyolysis complications and create a response baseline.
Interpretation and limitations
CK can be normal despite active dermatomyositis or ILD; AST and ALT may be muscular, while myoglobinuria and AKI require acute renal care.
02
Myositis-specific and associated antibodies
Why
Refine syndrome, lung, cancer and treatment-risk phenotype.
Interpretation and limitations
Interpret the assay and pre-test phenotype; a line-blot positive without matching disease can be false, and antibodies do not replace tissue or organ assessment.
03
MRI of symptomatic muscle
Why
Identify active oedema, fascial involvement, fatty damage and a high-yield biopsy site.
Interpretation and limitations
Oedema is sensitive but not specific and occurs after exercise, denervation and infection; choose an involved but not end-stage muscle.
04
EMG and nerve-conduction studies
Why
Support irritable myopathy and identify neuropathic, motor-neurone or neuromuscular-junction mimics.
Interpretation and limitations
Myopathic units and spontaneous activity support but do not uniquely diagnose inflammation; select the opposite side from a planned biopsy when feasible.
05
Muscle or skin biopsy
Why
Confirm inflammatory, perifascicular, necrotising or inclusion-body pathology when diagnosis remains uncertain.
Interpretation and limitations
Biopsy is the reference tissue test in unresolved cases; site, prior steroid, chronicity and expert neuromuscular pathology influence yield.
06
Swallow, respiratory and ILD assessment
Why
Detect aspiration, ventilatory weakness and myositis-associated interstitial lung disease early.
Interpretation and limitations
Use SALT evaluation, FEES or videofluoroscopy, seated and supine FVC, gas transfer and HRCT according to symptoms and antibody risk.
07
Risk-stratified cancer assessment
Why
Identify malignancy associated with adult-onset myositis without indiscriminate lifelong scanning.
Interpretation and limitations
Complete routine national screening and add CT, pelvic, breast, testicular, endoscopic or other tests from age, symptoms, antibody and examination risk.

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

Inclusion-body myositis

Slow asymmetric finger-flexor and quadriceps weakness, falls and poor immunosuppressive response in an older adult suggests inclusion-body myositis rather than polymyositis.

Endocrine, metabolic or toxic myopathy

Thyroid, adrenal, electrolyte, vitamin-D, alcohol, steroid, statin and other medicine disorders cause weakness with differing pain, enzyme and biopsy patterns.

Neuromuscular disease

Myasthenia, motor-neurone disease, muscular dystrophy, neuropathy and cervical myelopathy produce weakness through nerve, junction or inherited muscle mechanisms.

Pain-limited function

Polymyalgia rheumatica, inflammatory arthritis, fibromyalgia and deconditioning can impair rising or lifting without the objective myopathic pattern of true weakness.

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

01First-line diagnostic sequenceProve weakness and define phenotypeFirst stepFirst lineSubacute proximal weakness, dermatomyositis rash or unexplained CK elevation raises inflammatory-myopathy suspicion.

1Measure functional and manual strength, skin, joints, sensation, reflexes, swallowing, cough and respiratory symptoms and review statin, steroid, endocrine and family history.
2Obtain enzymes, renal and urine, thyroid, inflammatory and phenotype-directed myositis antibodies, then MRI and EMG to select further testing.
3Obtain muscle or skin biopsy when the diagnosis, subtype or mimic remains uncertain, without delaying organ-saving treatment in a severe classic presentation.

02Initial muscle treatmentSuppress inflammation and rehabilitateActive inflammatory muscle disease causes objective weakness without immediate bulbar or respiratory failure.

1Start oral prednisolone at an appropriate weight-based dose and introduce methotrexate, azathioprine, mycophenolate or another phenotype-matched steroid-sparing agent early.
2Provide bone, gastric and infection protection where indicated and a written taper based on strength, function, CK and systemic-organ response.
3Begin supervised resistance and aerobic rehabilitation with falls, occupational and work support, progressing load rather than prescribing prolonged rest.

03Emergency escalationProtect swallowing and ventilationEscalationDysphagia, aspiration, respiratory weakness, myocarditis or rapidly progressive ILD threatens an organ.

1Admit, restrict unsafe oral intake, assess FEES or videofluoroscopy, cough, supine FVC and gas exchange and provide feeding or ventilatory support.
2Give specialist intravenous methylprednisolone and early steroid-sparing combination after cultures and infection assessment; use IVIG promptly for severe dysphagia when selected.
3For rapidly progressive ILD or myocarditis coordinate respiratory, cardiac and intensive-care regimens, monitoring infection and treatment toxicity continuously.

04Cancer and non-responseRevisit diagnosis and riskAdult-onset dermatomyositis presents near onset or strength fails to recover despite biochemical response.

1Complete risk-stratified malignancy screening and repeat targeted assessment for new symptoms rather than using one universal scan schedule.
2Review adherence, drug exposure, active inflammation, irreversible damage, steroid myopathy, inclusion-body myositis, neuropathy and inadequate rehabilitation.
3Use repeat MRI, EMG or biopsy when mechanism remains uncertain, then switch treatment only against a revised objective target.

Key medicines and prescribing safety5 treatments · regimens, roles and cautions

Rapid first anti-inflammatory treatment for clinically significant muscle disease while a slower agent becomes effective.

Prednisolone

For active adult inflammatory myopathy, start about 0.5–1 mg/kg orally once daily, commonly capped near 60 mg/day, then taper against strength, function, enzymes and organ response with early steroid-sparing therapy.

Exclude or treat infection, monitor glucose, pressure, mood, eyes and bone and provide prophylaxis where indicated. Steroid myopathy can mimic persistent disease; never stop prolonged treatment abruptly.

Provides rapid high-intensity control when delayed swallowing, ventilation or organ response carries major risk.

Intravenous methylprednisolone

For severe bulbar, respiratory, cardiac or rapidly progressive disease, specialists commonly give 500–1,000 mg intravenously once daily for three days before a planned oral regimen.

Obtain cultures and infection assessment without delaying organ-saving care; monitor rhythm, pressure, glucose, electrolytes, psychiatric effects and gastrointestinal and infection complications.

Early steroid-sparing treatment for muscle, joint and selected skin disease when lung, liver and reproductive factors permit.

Methotrexate with folic acid

Start 7.5–15 mg orally or subcutaneously once weekly with folic acid at least 5 mg on a different day and titrate, commonly toward 20–25 mg weekly, under specialist monitoring.

Never dose daily. Monitor FBC, liver and renal function, avoid trimethoprim or co-trimoxazole, and review lung symptoms urgently. Stop at least one month before maternal conception and avoid in pregnancy.

Steroid-sparing option for muscle and selected systemic disease, including when reproductive compatibility favours it.

Azathioprine

Start 25–50 mg orally daily after TPMT assessment and increase toward 1–2 mg/kg/day according to response, tolerance and specialist monitoring.

Monitor FBC, liver and renal function and infection; review pancreatitis and hypersensitivity. Allopurinol and febuxostat interactions require expert avoidance or major dose adjustment. BSR supports up to 2 mg/kg/day in pregnancy.

Important adjunct for severe dysphagia, refractory muscle weakness or skin disease and selected patients needing rapid or pregnancy-compatible treatment.

Intravenous immunoglobulin

A common regimen is a total 2 g/kg intravenously divided over two to five days, repeated approximately every four weeks for a defined trial according to response and specialist protocol.

Use the agreed body-weight method and product infusion rate; assess thrombosis, hyperviscosity, renal impairment, fluid overload, haemolysis, headache and aseptic meningitis.

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

Dysphagia and respiratory failure

Bulbar and respiratory muscle weakness causes aspiration, pneumonia, malnutrition, ineffective cough, hypoventilation and potentially fatal ventilatory failure.

Interstitial lung and cardiac disease

ILD may progress rapidly or fibrose, while myocarditis, conduction disease and arrhythmia can cause heart failure or sudden deterioration.

Malignancy

Adult dermatomyositis, particularly selected antibody and clinical phenotypes, carries temporally increased cancer risk requiring structured but proportionate screening.

Disability and treatment toxicity

Persistent weakness, contractures and calcinosis combine with infection, diabetes, osteoporosis, cytopenia, liver injury, gonadal and reproductive toxicity from treatment.

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

Measure manual strength and reproducible functions such as chair rise, timed walk, arm elevation and patient participation alongside CK at each active-disease review.
Repeat swallowing, weight, cough and respiratory function promptly when voice, aspiration, orthopnoea or chest infection changes.
Track PFT, DLCO, HRCT and oxygenation according to ILD phenotype and distinguish infection or aspiration before escalating immunosuppression.
Apply FBC, liver, renal, infection, retinal or immunoglobulin schedules for the chosen medicine and review vaccines and prophylaxis.
Reassess malignancy risk and routine national screening, particularly within three years around adult dermatomyositis onset or when new red flags appear.
Review rehabilitation load, falls, contracture, pain, fatigue, mood, work, contraception and pregnancy at structured multidisciplinary visits.

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

Polymyositis is uncommon

Many historical cases become inclusion-body, necrotising, antisynthetase, overlap, genetic or toxic myopathy after modern antibody and pathology review.

CK is not total disease activity

Skin, lung, swallowing and myocardium can be active with modest enzyme change, while CK may remain raised after function improves.

ALT can be muscle-derived

Parallel CK, AST and LDH rise with normal GGT may prevent an incorrect liver diagnosis and unnecessary discontinuation of treatment.

Exercise is treatment

Supervised progressive resistance and aerobic training improve strength and participation and do not ordinarily worsen controlled inflammatory myopathy.

Biopsy site determines yield

An end-stage fatty muscle has little diagnostic inflammation, while a recently needled EMG site can create misleading local injury.

11Common pitfallsFrequent interpretation and management errors.

01
Calling pain-limited movement weakness without formal power and functional testing.
02
Diagnosing polymyositis before excluding inclusion-body myositis, necrotising disease, antisynthetase syndrome and medicine or endocrine causes.
03
Using a normal CK to dismiss amyopathic dermatomyositis or rapidly progressive ILD.
04
Tapering treatment solely from CK while swallowing, lungs, skin or functional strength worsens.
05
Prescribing prolonged rest and allowing avoidable deconditioning, contracture and falls.
06
Ordering indiscriminate repeated whole-body scans instead of a documented malignancy risk and national-screening plan.