Dermatomyositis & Polymyositis (Idiopathic Inflammatory Myopathies)

Meet the patient

A 52-year-old woman cannot lift her arms above her head to comb her hair, and cannot rise from the lavatory without hauling herself up on the basin. Six weeks it has taken — subacute, painless, symmetric. Her general practitioner checked her liver because the AST and ALT were both raised, and referred her to hepatology. She never got there.[1][2]

On the ward round you find a violaceous lilac discolouration over her upper eyelids with mild periorbital oedema, and scaly flat-topped papules strung across her knuckles. Her creatine kinase is around 3500 units per litre. Rash plus weakness plus CK — the whole diagnosis in three signs, and the AST and ALT were never her liver.[1][2]

Two questions decide her pathway, and this topic: weakness or pain? (which separates myositis from polymyalgia), and which subtype? (which decides whether you reach for steroids, screen for cancer, screen for lung disease, or stop).[1]

Weakness, not pain — the two questions that anchor the diagnosis

Myositis is weakness, not pain. That single line stops you mislabelling IIM as polymyalgia rheumatica. PMR hurts and stiffens with normal strength and a normal CK; myositis weakens with a markedly raised CK and little or no pain. Confuse the two and you either under-dose myositis with PMR-strength steroids, or dismiss real myositis as "just aches".[1][2]

Myositis is myopathic, not neuropathic. The weakness is proximal and symmetric, the EMG is myopathic, the muscle enzymes are raised. That tripod rules out myasthenia (fatigable, ocular, normal CK), motor neuron disease (mixed upper and lower motor neurone signs), and the muscular dystrophies (chronic, familial, genetic).[2]

The three clinical anchors that define the group are subacute onset (weeks to months, not days and not lifelong), symmetric proximal weakness, and muscle inflammation with a raised CK. Miss any one and you are in the wrong chapter.[1][2]

Five subtypes — and the one that is the trick question

The subtype is the answer the examiner is hunting for. Five subtypes behave so differently that lumping them as "polymyositis" is a diagnostic crime. Learn the discriminator for each:[1][2]

• Dermatomyositis (DM) — muscle plus the characteristic rash. Cancer-prone and lung-prone. Driven by a type-I-interferon, complement-mediated microangiopathy.[2]
• Polymyositis (PM) — muscle only, no rash, driven by CD8+ T-cell cytotoxicity. Rarer than the textbooks pretend; most old "PM" was undiagnosed IBM.[2]
• Inclusion body myositis (IBM) — older men, asymmetric quadriceps and finger-flexor weakness, steroid-refractory, rimmed vacuoles on biopsy.[5]
• Immune-mediated necrotising myopathy (IMNM) — anti-SRP or anti-HMGCR, very high CK, necrosis with little inflammation, often statin-exposed.[8][9]
• Antisynthetase syndrome — anti-Jo-1 (and PL-7, PL-12, EJ, OJ), lung-predominant, with fever, mechanic's hands, and non-erosive arthritis.[1]

Juvenile DM (onset under 18) is the sixth group, with prominent vasculopathy and calcinosis.[1]

Three waves of criteria — Bohan, then ENMC, then EULAR/ACR

Classification evolved in three waves, and examiners ask for all three. The 1975 Bohan and Peter criteria are the historic, clinically intuitive scheme every final-prof student still learns. The 2004 ENMC 119th workshop added antibody- and biopsy-defined subgroups and codified IBM. The current standard is the 2017 EULAR/ACR criteria of Lundberg and colleagues — a data-driven, probability-scored instrument that folds in clinical, enzymatic, antibody, and biopsy variables.[3][4][6][1]

Bohan and Peter (1975) — the five classic criteria

Confidence is just a count. For PM, definite needs four of the first four criteria, probable three, possible two. For DM the rash is added — definite DM is the rash plus any three of the first four, probable the rash plus any two, possible the rash plus any one.[3][4] The scheme's fatal flaw: it over-diagnoses PM by swallowing IBM and IMNM cases. That is exactly why the antibody- and biopsy-driven 2017 criteria supersede it for research and subclassification.[1]

2017 EULAR/ACR — the current standard

The 2017 criteria combine sixteen weighted variables across age at onset, clinical features (symmetric proximal weakness, distal weakness, neck weakness, dysphagia, Gottron papules or sign, heliotrope rash, shawl or V sign, mechanic's hands, Raynaud), laboratory features (raised CK, ANA, anti-Jo-1), and muscle biopsy (perifascicular, endomysial, perivascular or perimysial inflammation, rimmed vacuoles, necrosis or regeneration).[1]

A patient is classified as IIM when the probability score is at least 55 per cent, with a tree that first splits juvenile-onset from adult-onset. A subclassifying algorithm then assigns one of the subgroups using biopsy pattern, anti-Jo-1, and clinical phenotype.[1]

IBM — the dedicated criteria

IBM is never diagnosed well by Bohan and Peter. Use the Griggs 1995 research criteria and the ENMC 119th workshop framework, which demand a constellation that is quite unlike PM or DM:[5][6]

• onset over age 30 (usually over 50) with male predominance[5]
• knee extensor (quadriceps) weakness equal to or greater than hip flexor weakness
• finger-flexor weakness (especially the deep forearm flexors) greater than shoulder abductor weakness
• biopsy with endomysial inflammation and rimmed vacuoles
• slow progression and a poor response to glucocorticoids

Recognising this pattern matters because IBM is mislabelled as refractory PM more often than any other myopathy.[5]

Who gets it — and who to worry about

Uncommon, not rare. Annual incidence runs at roughly 5 to 11 cases per million, with a prevalence of around 14 to 18 per 100,000. The patterns that matter at the bedside are these:[2]

• DM is bimodal — a juvenile peak (ages 5 to 15) and an adult peak (ages 40 to 60). DM outnumbers PM. IBM is the commonest inflammatory myopathy over age 50, and is systematically under-recognised.[5]
• Sex: DM and PM are female-predominant (about 2 to 1); IBM is male-predominant.
• Cancer-associated myositis: roughly 15 to 25 per cent of adults with DM carry an associated malignancy — far more than in PM. Risk peaks in the first year around diagnosis and is most enriched with anti-TIF1-gamma and anti-NXP2 antibodies. The 2023 International Myositis Society guideline wants intensified screening and rescreening for up to three years.[13]
• Antisynthetase syndrome: anti-Jo-1 is the commonest antisynthetase antibody (about a fifth of IIM). PL-7, PL-12, EJ, and OJ are rarer but carry a higher ILD burden.[1]
• Anti-MDA5 disease: amyopathic or mildly myopathic DM with rapidly progressive ILD and cutaneous ulcers — high early mortality, especially aggressive in East Asian cohorts.[7]
• IMNM: anti-HMGCR disease often follows statin exposure and may persist and progress despite statin withdrawal.[8]
• Environmental triggers: viral infections (coxsackievirus, parvovirus, retroviruses), ultraviolet light (a documented latitude gradient for DM), drugs (statins for anti-HMGCR IMNM), and malignancy.[2]

[2] [5] [13] [7]

Two mechanisms — capillaries versus T cells

Two fundamentally different immune mechanisms underlie the myopathy, and they explain why the biopsies — and the steroid responses — diverge so completely.[1][2]

In dermatomyositis, the capillary is the target. A type-I-interferon-driven, complement-mediated microangiopathy is central. Immune complexes activate complement, generating the membrane attack complex (MAC, C5b-9) on endomysial capillaries; capillaries die, the muscle ischaemias, and the watershed perifascicular region atrophies first — the hallmark perifascicular atrophy. Plasmacytoid dendritic cells drive the interferon-alpha and beta excess, which is why the skin and microvasculature are involved and why JAK inhibitors are mechanistically rational. MAC deposition on capillaries is the earliest biopsy change and precedes inflammation.[2]

In polymyositis and IBM, the muscle fibre is the target. CD8+ cytotoxic T cells and macrophages surround and invade MHC-class-I-upregulated, otherwise healthy fibres through the endomysium, releasing perforin and granzyme. MHC class I is aberrantly upregulated on the surface of muscle fibres in PM and IBM (but not in normal muscle), handing the T cells their target.[2]

IBM adds a degenerative layer on top of the T-cell attack — rimmed vacuoles packed with protein aggregates (TDP-43, p62, beta-amyloid, tau) and ragged-red or cytochrome-oxidase-negative fibres. This mixed autoimmune-degenerative biology is the principal reason IBM is steroid-refractory: immunosuppression cannot reverse a degenerative process.[5]

IMNM (anti-SRP, anti-HMGCR) is necrosis with minimal inflammation and MAC on non-necrotic fibres. The antibody targets are the signal recognition particle (a ribonucleoprotein complex essential for protein translocation) and HMG-CoA reductase (the rate-limiting enzyme of cholesterol synthesis and the pharmacological target of statins — which is why statin exposure links in).[8][9]

[1] [2] [5]

At the bedside — combing hair, rising from a chair

The tempo is the tell. Weakness develops subacutely over weeks to months — slower than acute rhabdomyolysis (days), faster than the muscular dystrophies (years). The symptom complex splits into muscle, skin, systemic, and subtype-specific clusters.[1][2]

Muscle — symmetric, proximal, weakness not pain

The classic pattern is symmetric proximal weakness of the shoulder and hip girdles. Patients describe difficulty lifting the arms overhead (combing hair, reaching a high shelf, hanging washing), rising from a low chair or the floor, climbing stairs, and stepping onto a bus.[1][2]

• Neck-flexor weakness produces a head drop and fatigue holding the head up.
• Dysphagia (solids more than liquids) and dysphonia (nasal voice) reflect pharyngeal and proximal oesophageal striated-muscle involvement — and predict aspiration.
• Respiratory muscle weakness (rare but dangerous) presents with morning headache, orthopnoea, and a falling vital capacity.
• Pain is usually mild or absent. Weakness is the cardinal feature.[1]

Dermatomyositis skin — pathognomonic clues

The DM rash is photosensitive, often precedes the myopathy by weeks or months, and several features are pathognomonic:[1][2]

• Heliotrope rash — symmetric violaceous (lilac) erythema with oedema of the periorbital skin, especially the upper eyelid; the most specific DM sign. Etymology for viva gold: helios (sun) plus trope (turn) — the heliotrope flower turns to follow the sun, and its lilac is the colour on the eyelids.
• Gottron papules — violaceous, scaly, flat-topped papules over the metacarpophalangeal and interphalangeal joints; pathognomonic. Named for Heinrich Gottron, German dermatologist — the knuckle papules are his sign.
• Gottron sign — the same violaceous macular eruption over extensor surfaces of elbows, knees, and malleoli.
• Shawl sign (posterior shoulders and neck) and V-sign (anterior chest in the V of the neck) — photosensitive distributions.
• Holster sign — lateral thighs.
• Mechanic's hands — hyperkeratotic, fissured, dirty-appearing lateral fingers and palms; characteristic of antisynthetase syndrome but also seen in overlap.
• Nailfold changes — dilated and tortuous capillary loops, capillary dropout, dystrophic or ragged cuticles (Samitz sign), and periungual erythema — shared with systemic sclerosis.
• Poikiloderma (salt-and-pepper atrophy with telangiectasia), flagellate erythema (linear trunk streaks), calcinosis cutis (especially juvenile DM), and cutaneous ulceration with palmar papules (a hallmark of anti-MDA5 disease).[1]

Antisynthetase syndrome — the lungs decide prognosis

The classic tetrad is myositis, interstitial lung disease, non-erosive inflammatory arthritis, and fever, often joined by mechanic's hands and Raynaud phenomenon.[1]

ILD dominates prognosis. A dry cough and exertional dyspnoea may be the presenting complaint, and the myositis itself may be mild or amyopathic. Anti-Jo-1 is the prototype; PL-7, PL-12, EJ, and OJ are rarer but carry a higher ILD burden.[1]

Amyopathic and hypomyopathic DM — the rash without the weakness

About one in five DM patients carry the characteristic rash for at least six months with no or minimal weakness and a near-normal CK — amyopathic DM. Hypomyopathic DM has subclinical myopathy on investigation.[2]

The trap: amyopathic does not mean benign. Malignancy and ILD risk persist unchanged, so the screens are identical to classic DM.[2]

Inclusion body myositis — the pattern that gets missed

Actively seek IBM in any older patient labelled "refractory PM". The pattern is asymmetric, mixed distal and proximal: early quadriceps involvement (knee buckling, frequent falls, difficulty rising from a chair out of proportion to hip weakness) and deep finger-flexor involvement of the forearms (difficulty gripping, buttoning). Dysphagia is common and may dominate.[5]

Onset is over age 50 with male predominance, progression is slow (years), and the response to corticosteroids is poor. The CK is often only mildly to moderately raised, which feeds the delay.[5]

The differential — what looks like myositis but is not

Separate true inflammatory myopathy from its mimics. The combination of proximal weakness plus high CK plus myopathic EMG plus muscle biopsy narrows the field; the DM rash and antibody profile then define the subtype.[2]

[1] [2]

Also keep infectious myositis (viral — influenza, coxsackie, HIV; pyomyositis; trichinosis), overlap connective tissue disease with myositis (systemic sclerosis, SLE, mixed connective tissue disease, Sjogren syndrome), and rhabdomyolysis with acute kidney injury on the list.[1]

The focused exam — three places to look

Pattern, rash, system. Three things decide the diagnosis: the pattern of weakness (proximal versus distal, symmetric versus asymmetric), the presence and type of rash, and systemic screening for ILD and malignancy.[1][2]

• History: onset and tempo; the specific failed tasks (combing hair, rising from a chair, climbing stairs); dysphagia or dysphonia; skin changes and photosensitivity; dry cough and exertional dyspnoea (ILD); fever, arthritis, Raynaud (antisynthetase); weight loss or constitutional symptoms (malignancy); drug exposure (statins); family history; sun exposure.
• Muscle examination: test shoulder abduction, elbow flexion, hip flexion, knee extension, neck flexion; grade with the Medical Research Council 0 to 5 scale. In suspected IBM, specifically test the distal finger flexors (deep forearm flexors) and quadriceps — easily missed, and the IBM signature.
• Skin examination: heliotrope rash, Gottron papules or sign, shawl or V sign, mechanic's hands, nailfold (dilated loops, dropout, ragged cuticles), poikiloderma, calcinosis, ulceration and palmar papules (anti-MDA5). Nailfold capillaroscopy with a dermatoscope or ophthalmoscope plus immersion oil reveals dilated, tortuous loops and dropout.
• Systemic: lung auscultation (basal Velcro crackles of ILD), joints (non-erosive arthritis), Raynaud, a bedside swallow screen, cardiac examination (myocarditis, conduction disease), and a full malignancy review by system.[1]

CK, antibodies, biopsy — and the two screens

Diagnosis is five tests plus two screens. Combine muscle enzymes, autoantibodies, EMG, MRI, and muscle biopsy, then add targeted screening for ILD and malignancy.[1][2]

• Muscle enzymes. CK is the key marker — markedly raised in IMNM (often 10 to 50 times the upper limit of normal), moderately raised in DM and PM, and normal or near-normal in IBM and amyopathic DM. AST, ALT, LDH, and aldolase are also of muscle origin — a raised AST/ALT with proximal weakness is muscle-derived, not liver disease, the classic examination pitfall. Aldolase earns its place when CK is normal (some IBM and amyopathic DM cases).
• Autoantibodies. ANA is often positive in DM. Myositis-specific antibodies (MSA) define phenotype and prognosis and should be requested in every suspected IIM. Myositis-associated antibodies (anti-Ro52, anti-La, anti-U1-RNP, anti-Ku, anti-PM-Scl) suggest overlap syndromes.
• EMG. Myopathic motor unit potentials (small, short, polyphasic) with spontaneous activity (fibrillations, positive sharp waves, bizarre high-frequency repetitive discharges). EMG excludes a neurogenic process and helps pick the biopsy site — biopsy the contralateral paired muscle to avoid end-stage artefact.
• MRI (STIR sequences). Demonstrates oedema in inflamed muscle and guides the biopsy site. Increasingly first-line, and may spare biopsy in classic DM with a pathognomonic rash and antibody.
• Muscle biopsy (the gold standard). DM shows perifascicular atrophy with perifascicular MAC deposition and perivascular or perimysial inflammation. PM shows endomysial CD8+ T-cell infiltration with MHC-class-I upregulation. IBM shows endomysial inflammation with rimmed vacuoles and protein aggregates (TDP-43, p62). IMNM shows necrosis with minimal inflammation and MAC on non-necrotic fibres.[2]
• The ILD screen (mandatory in every patient). High-resolution CT chest (ground-glass, reticulation, honeycombing — typically a non-specific interstitial pneumonia pattern) and pulmonary function tests (reduced FVC and DLCO, restrictive). A falling FVC or DLCO on serial testing signals progression.
• The malignancy screen (mandatory in adult DM). CT chest, abdomen, and pelvis plus age-appropriate tests (mammography, colonoscopy, cervical screening, prostate-specific antigen, pelvic ultrasound). Screening is intensified and extended for up to three years with anti-TIF1-gamma or anti-NXP2 antibodies.[13]
• Other. TSH (exclude thyroid myopathy), HIV and hepatitis serology, echocardiogram and ECG (myocarditis, conduction disease), and skin biopsy (interface dermatitis with basement membrane thickening in DM).

Resuscitation — the four time-critical scenarios

[1]

Most IIM is subacute, not an emergency. But four scenarios are time-critical and demand same-day escalation:[2]

• Rapidly progressive ILD (anti-MDA5) — hypoxaemia can evolve over days. Urgent HRCT and aggressive combination immunosuppression (high-dose corticosteroids plus a calcineurin inhibitor or cyclophosphamide, with rituximab or a JAK inhibitor). Early mortality is high; early combination therapy is associated with improved survival.[7]
• Severe dysphagia with aspiration risk, or respiratory muscle weakness — assess swallow (speech and language therapy), arrange nasogastric feeding, and monitor forced vital capacity. A falling FVC toward 1.5 L or 50 per cent predicted, or a negative inspiratory force worse than minus 60 cm water, signals the need for non-invasive ventilation, as in any neuromuscular respiratory failure.
• Myocarditis or arrhythmia — cardiology input, troponin, echocardiogram, and a rhythm monitor.
• Very high CK with dark urine — rhabdomyolysis and acute kidney injury (more typical of IMNM or statin toxicity): intravenous fluids, urine alkalinisation, and renal protection.[1]

Definitive treatment — steroids, steroid-sparing, then the right phenotype

The mantra for this topic: weakness not pain, biopsy the subtype, screen DM for cancer and lung — and leave IBM alone. Treatment is phenotype-stratified: induce with high-dose corticosteroids, transition early to a steroid-sparing agent (steroid monotherapy is inadequate and toxic), treat the phenotype (the lung in antisynthetase or MDA5, the cancer in cancer-associated DM, the skin in DM), and avoid futile immunosuppression in IBM.[1][2]

Induction — the corticosteroid backbone

• Oral prednisolone 0.5 to 1 mg/kg/day (typically 60 to 80 mg once daily) for 4 to 8 weeks, then a slow taper (about 10 per cent every 1 to 2 weeks) over 6 to 12 months, guided by CK trend and muscle strength.[2]
• Intravenous methylprednisolone (500 to 1000 mg daily for 3 to 5 days) for severe disease — dysphagia, ILD, respiratory involvement, or a markedly raised CK — followed by oral prednisolone.
• Co-prescribe protection: gastric protection (proton pump inhibitor); bone protection (calcium 1000 to 1200 mg and vitamin D 800 to 1000 IU daily, with a bisphosphonate if osteoporotic); and Pneumocystis jirovecii prophylaxis (co-trimoxazole 480 mg once daily or 960 mg three times weekly) once prednisolone exceeds 20 mg daily for more than four weeks.

Steroid-sparing agents — start them early

Steroid monotherapy is inadequate for most IIM. Start a steroid-sparing agent early, in parallel with the taper:[2]

• Methotrexate 10 to 25 mg once weekly (oral or subcutaneous, with folic acid 5 mg the day after). First-line steroid-sparing for DM and PM without ILD. Monitor liver function and consider a chest CT first (rare methotrexate pneumonitis).
• Azathioprine 2 to 3 mg/kg/day (typically 150 to 200 mg daily). Check thiopurine methyltransferase (TPMT) activity before starting to avoid fatal myelosuppression. First-line in pregnancy.
• Mycophenolate mofetil 2 to 3 g/day in two divided doses (or mycophenolate sodium 1440 to 2160 mg daily). Favoured when ILD coexists.
• Tacrolimus (trough 4 to 12 ng per mL) and ciclosporin (2.5 to 5 mg/kg/day) — calcineurin inhibitors, favoured for ILD and overlap disease.[1]

Biological and advanced therapy

• Intravenous immunoglobulin (IVIG) 2 g/kg per month (often split across two to five days; for example, 0.4 to 1 g/kg daily for two to five days), titrated to response. Particularly effective in DM refractory to first-line therapy and in severe dysphagia, with onset within one to two weeks. The 1993 Dalakas randomised controlled trial established IVIG in steroid-resistant DM.[11]
• Rituximab — either two 1 g intravenous infusions two weeks apart, or 375 mg per square metre weekly for four weeks, repeated at six months if needed. For refractory DM and PM. The RITUXIMAB in Myositis (RIM) trial was negative on its primary combined endpoint, but a post-hoc analysis and a subsequent predictors analysis favoured dermatomyositis.[10]
• Cyclophosphamide (intravenous pulse 500 to 1000 mg per square metre monthly, or 1 to 2 mg/kg/day oral) — reserved for severe or progressive ILD and overlap disease with organ threat.
• JAK inhibitors (tofacitinib 5 to 11 mg daily; baricitinib 2 to 4 mg daily) — increasingly used for refractory DM and anti-MDA5 ILD on the type-I-interferon rationale; an active area of investigation.

Skin

Rigorous photoprotection (broad-spectrum SPF 50-plus sunscreen, protective clothing, sun avoidance), hydroxychloroquine 200 to 400 mg daily, topical corticosteroids or calcineurin inhibitors, and IVIG for refractory rash. Hydroxychloroquine is less effective than in cutaneous lupus and can rarely cause a drug-induced myopathy that mimics active DM — watch the CK trend.[1]

IBM — the disease you must not over-treat

IBM is largely refractory to immunosuppression. The harm of escalating steroids and immunosuppressants exceeds the benefit. The RESILIENT trial of bimagrumab (an activin-receptor type IIA antagonist targeting muscle growth) in IBM was negative for its primary endpoint.[12]

Management is supportive: physiotherapy with quadriceps strengthening and fall prevention, swallowing therapy (and, where needed, cricopharyngeal myotomy or dilatation for severe dysphagia), and assistive devices (rolling walkers, knee braces, ankle-foot orthoses). A time-limited trial of low-dose steroids or IVIG for dysphagia is reasonable, with a clear stop date.[5][12]

Cancer-associated myositis — find and treat the malignancy

Search for and treat the underlying malignancy — resection may improve the myositis. Follow the 2023 International Myositis Society cancer-screening guideline: age-appropriate screening plus CT chest, abdomen, and pelvis, intensified and extended for up to three years in patients with anti-TIF1-gamma or anti-NXP2 antibodies.[13]

Consultant confession: I have rewritten "refractory polymyositis" as "inclusion body myositis" on more referrals than I can count. The biopsy is always the kindest test — it saves the patient from a third immunosuppressant and saves you from the conversation about why they are not getting better.[5]

Subtypes and scenarios — a quick field guide

• Juvenile DM — the commonest IIM of childhood. Calcinosis cutis, vasculopathy (including intestinal vasculitis), and lipodystrophy often overshadow the myopathy. Treat with high-dose corticosteroids (often intravenous methylprednisolone pulses) plus methotrexate, with aggressive early therapy to prevent calcinosis and contractures. IVIG is used for refractory disease and to spare steroids.[1]
• Amyopathic DM — the classic rash for at least six months with no or minimal weakness and a near-normal CK. Malignancy and ILD risk are unchanged, so a full malignancy screen and HRCT plus PFTs are mandatory. Anti-MDA5 is over-represented and demands close respiratory surveillance.[2]
• Antisynthetase syndrome (anti-Jo-1, PL-7, PL-12, EJ, OJ) — interstitial lung disease dominates prognosis. Treat with aggressive immunosuppression (often a steroid plus mycophenolate, tacrolimus, or cyclophosphamide, with rituximab for refractory ILD) and PFT and HRCT surveillance every three to six months.[1]
• Immune-mediated necrotising myopathy (anti-SRP, anti-HMGCR) — very high CK, necrosis with minimal inflammation. Statin withdrawal alone is insufficient for anti-HMGCR disease; aggressive immunosuppression (high-dose steroids plus methotrexate, azathioprine, or rituximab, often IVIG) is required. Anti-SRP disease tends to be severe, refractory, and relapsing.[8][9]
• Inclusion body myositis — older men; steroid-refractory. Supportive care, physiotherapy, fall prevention, swallowing therapy, and assistive devices; avoid immunosuppression harm.[5][12]
• Overlap myositis — with systemic sclerosis, SLE, mixed connective tissue disease, or Sjogren; treat the dominant connective tissue disease. Anti-PM-Scl, anti-Ku, and anti-U1-RNP antibodies suggest overlap.
• Cancer-associated myositis — strongest with anti-TIF1-gamma and anti-NXP2 in adults; find and treat the malignancy, rescreen for up to three years.[13]

Complications, pitfalls and the preventable-harm list

ILD is the leading cause of IIM morbidity and mortality — especially anti-MDA5 and antisynthetase. Screen every patient with HRCT and PFTs at diagnosis and serially.[7]

The preventable-harm list, learned the hard way:[1]

• Occult malignancy — strongest with anti-TIF1-gamma or anti-NXP2 in adult DM; rescreen for up to three years.[13]
• Aspiration pneumonia from pharyngeal and oesophageal weakness — assess swallow early, involve speech and language therapy.
• Respiratory muscle weakness — monitor FVC; risk of ventilatory failure requiring non-invasive ventilation.
• Calcinosis cutis — especially juvenile DM; difficult to treat (topical sodium thiosulfate, bisphosphonates, surgical excision of symptomatic deposits).
• Myocarditis and cardiac conduction disease — an under-appreciated but real cause of mortality; baseline ECG and echocardiogram.
• Steroid toxicity — osteoporosis, diabetes, hypertension, infection, cataracts, avascular necrosis. Always co-prescribe bone and gastric protection and PJP prophylaxis.

The four classic diagnostic pitfalls: treating IBM as refractory PM with escalating immunosuppression (futile and harmful); mislabelling a raised AST/ALT as liver disease when it is muscle-derived; confusing PMR (pain, normal CK) with IIM (weakness, high CK); and under-recognition that amyopathic DM carries the same malignancy and ILD risk as classic DM.[2]

Prognosis and disposition

DM and PM generally respond to corticosteroids and steroid-sparing agents, with good functional recovery; modern three- and five-year survival exceeds 80 per cent. Outcomes are driven by three things: ILD, malignancy, and treatment complications.[1][2]

• Anti-MDA5 ILD carries the highest early mortality — death often within three to six months of presentation — and demands early aggressive combination immunosuppression.[7]
• IBM is slowly progressive and refractory; patients accumulate disability (falls, dysphagia, quadriceps weakness requiring a wheelchair) over five to fifteen years. Life expectancy is relatively preserved.[5]
• Juvenile DM does well with modern aggressive therapy, though calcinosis, vasculopathy, and lipodystrophy may persist.
• Cancer-associated myositis prognosis tracks the underlying malignancy — resection may improve the myositis.
• Multidisciplinary follow-up — rheumatology, neurology, respiratory, dermatology, oncology where relevant, and speech and language therapy — with serial CK, PFTs, and malignancy surveillance is essential.[1]

Special populations

• Adult dermatomyositis and malignancy — strongest association; full age-appropriate malignancy screen, intensified if anti-TIF1-gamma or anti-NXP2, rescreen for up to three years.[13]
• Anti-MDA5 disease — rapidly progressive ILD with amyopathic DM; early aggressive combination immunosuppression and close respiratory monitoring, with a low threshold for intensive care.[7]
• Juvenile DM — calcinosis, vasculopathy, and lipodystrophy predominate; aggressive early therapy to prevent calcinosis and contractures; weight-based dosing.
• Inclusion body myositis — older men; steroid-refractory; supportive care, physiotherapy, swallowing therapy; avoid immunosuppression harm.[5]
• Pregnancy — plan conception during remission. Prefer azathioprine or calcineurin inhibitors (comparatively safe in pregnancy and lactation). Avoid methotrexate, mycophenolate, and cyclophosphamide (teratogenic). Hydroxychloroquine and low-dose prednisolone are acceptable. IVIG is safe in pregnancy for flares.
• Statin-exposed patient with very high CK and weakness — consider anti-HMGCR immune-mediated necrotising myopathy; statin withdrawal alone is insufficient and immunosuppression is required.[8]
• Elderly and immunocompromised — higher risk of steroid toxicity, infection, and drug interactions; favour IVIG and lower cumulative steroid exposure.

Evidence, guidelines and regional differences

NICE and the British Society for Rheumatology endorse high-dose oral prednisolone with early methotrexate as first-line steroid-sparing, azathioprine as first-line in pregnancy, IVIG for refractory DM and severe dysphagia, and rituximab for refractory disease. NHS commissioning tightly governs biologic access.[1]

The ACR, the Vasculitis Clinical Research Consortium, and the International Myositis Society favour the 2017 EULAR/ACR criteria, the 2023 International Myositis Society cancer-screening guideline, and a growing role for JAK inhibitors in refractory DM and anti-MDA5 ILD (interferon-axis rationale). The RIM trial, although negative on its primary endpoint, established rituximab safety, and a post-hoc predictor analysis favoured DM.[10]

• Japan and East Asia — anti-MDA5 ILD is particularly aggressive. Japanese centres have pioneered combination immunosuppression (ciclosporin or tacrolimus plus intravenous cyclophosphamide with high-dose steroids) for MDA5-ILD, with whole-body MRI for juvenile DM.[7]
• Europe (EULAR) — the 2017 EULAR/ACR criteria are standard; calcineurin inhibitors are favoured for ILD, and pan-European registries drive antibody-phenotype definition.
• India and resource-limited settings — diagnosis rests on CK plus muscle biopsy plus EMG; access to myositis-specific antibody panels, IVIG, and rituximab is often limited, and management is built around affordable steroid-sparing agents (methotrexate, azathioprine) with opportunistic infection prophylaxis.

Landmark evidence

• 2017 EULAR/ACR classification criteria (Lundberg 2017) — the current data-driven, probability-scored criteria for IIM and its subgroups, superseding the Bohan and Peter scheme; incorporates biopsy and anti-Jo-1.[1]
• Pathophysiology framework (Dalakas 2015, NEJM) — defines the two distinct mechanisms: complement-mediated microangiopathy (DM) versus T-cell-mediated cytotoxicity (PM/IBM), underpinning biopsy interpretation and the steroid-refractory nature of IBM.[2]
• Bohan and Peter 1975 (NEJM, two parts) — the five classic clinical criteria examiners still ask for.[3][4]
• Griggs 1995 and ENMC 2004 IBM criteria — codified the IBM phenotype (quadriceps plus finger-flexor weakness, rimmed vacuoles, steroid-refractory).[5][6]
• Mammen 2011 (anti-HMGCR) — defined the statin-associated autoimmune necrotising myopathy.[8]
• Sato 2005 (CADM-140/anti-MDA5) — described the amyopathic DM antibody that predicts rapidly progressive ILD.[7]
• Anquetil 2019 — placed myositis-specific autoantibodies at the cornerstone of immune-mediated necrotising myopathy.[9]
• Dalakas 1993 (IVIG in DM, NEJM) — the landmark randomised controlled trial establishing IVIG for steroid-resistant DM.[11]
• Aggarwal 2014 (RIM predictors) — rituximab in refractory DM and PM; predictors of improvement favoured DM.[10]
• RESILIENT 2019 (bimagrumab, Lancet Neurology) — negative primary endpoint in IBM; reaffirmed the absence of disease-modifying therapy.[12]
• 2023 International Myositis Society cancer-screening guideline — risk-stratified malignancy screening, intensified for anti-TIF1-gamma and anti-NXP2, rescreen for up to three years.[13]

Ward-round test

Stem 1 — The woman with raised liver enzymes

A 52-year-old woman has six weeks of painless difficulty combing her hair and rising from a chair, a lilac discolouration on her upper eyelids, scaly papules over her knuckles, AST and ALT twice the upper limit of normal, and a CK of 3500. What is the diagnosis, what is the AST/ALT telling you, and what two screens must you order today?[1][2]

Stem 1 — Dermatomyositis; muscle not liver; malignancy and ILD screens

This is dermatomyositis: subacute symmetric proximal weakness with the pathognomonic rash (heliotrope plus Gottron papules) and a markedly raised CK. The AST and ALT are muscle-derived, not hepatic — they are muscle enzymes, and chasing them as liver disease is the classic pitfall. Today, order a myositis-specific antibody panel (especially anti-TIF1-gamma, anti-NXP2, anti-MDA5, anti-Jo-1), a high-resolution CT chest and pulmonary function tests (ILD screen), and a malignancy screen (CT chest, abdomen, and pelvis plus age-appropriate tests), intensified and extended for up to three years if anti-TIF1-gamma or anti-NXP2 are positive.[1][2][13]

Stem 2 — "Refractory polymyositis" at 68

A 68-year-old man is referred as "refractory polymyositis" after three immunosuppressants failed over a year. He has asymmetric quadriceps weakness (frequent falls) and weak finger flexors, a CK of only 700, and a biopsy that was never done. What is the likely diagnosis, what single test settles it, and what should you stop doing?[5]

Stem 2 — Inclusion body myositis; biopsy; stop escalating immunosuppression

This is inclusion body myositis until proven otherwise — older man, asymmetric quadriceps and finger-flexor weakness, mildly raised CK, and a poor steroid response are the signature. A muscle biopsy (looking for endomysial inflammation with rimmed vacuoles) settles it, with MRI to guide the site. Stop escalating immunosuppression — IBM is steroid-refractory, and bimagrumab was negative in RESILIENT. Switch to supportive care: quadriceps-strengthening physiotherapy, fall prevention, swallowing therapy, and assistive devices.[5][12]

Stem 3 — The statin patient with a CK of 12,000

A 64-year-old woman on a statin for two years develops severe proximal weakness; her CK is 12,000. She stopped the statin eight weeks ago and the CK has not budged. Name the entity, the antibody, and why withdrawal alone will fail.[8][9]

Stem 3 — Anti-HMGCR immune-mediated necrotising myopathy; statin withdrawal is insufficient

This is immune-mediated necrotising myopathy due to anti-HMGCR antibodies — very high CK, necrosis with minimal inflammation on biopsy, statin-exposed. The antibody targets HMG-CoA reductase, the rate-limiting enzyme of cholesterol synthesis and the pharmacological target of statins, so the autoimmune process is self-sustaining. Statin withdrawal alone is insufficient: treat with high-dose corticosteroids plus a steroid-sparing agent (methotrexate, azathioprine, or rituximab), often with IVIG.[8][9]

Stem 4 — Amyopathic DM with a dry cough

A 45-year-old woman has the classic DM rash for eight months with normal strength and a normal CK, but now has a dry cough, exertional dyspnoea, and cutaneous palmar papules. What antibody do you fear, what is the threat, and what is the first move?[7]

Stem 4 — Anti-MDA5; rapidly progressive ILD; aggressive combination immunosuppression

Fear anti-MDA5 — amyopathic or mildly myopathic DM with cutaneous ulcers and palmar papules that carries rapidly progressive interstitial lung disease and high early mortality, especially in East Asian populations. The threat is death from respiratory failure within months. The first move is urgent HRCT and pulmonary function tests, then aggressive combination immunosuppression (high-dose corticosteroids plus a calcineurin inhibitor or cyclophosphamide, with rituximab or a JAK inhibitor). Note that amyopathic DM still needs the full malignancy and ILD screen — "amyopathic" is not "benign".[7][2]

Rapid viva checklist

1. Definition plus the three clinical anchors
2. The five subtypes, with the discriminator for each
3. The two immune mechanisms (capillaries versus T cells)
4. Bohan and Peter five criteria and the diagnostic tiers
5. 2017 EULAR/ACR probability score threshold (at least 55 per cent)
6. The myositis-specific antibodies and what each predicts
7. The two mandatory screens in DM (malignancy and ILD)
8. Emergency bundle for rapidly progressive ILD and respiratory failure
9. Definitive therapy with doses — steroids, steroid-sparing, IVIG, rituximab
10. The mantra, and the four classic pitfalls[1][2]

If you cannot answer any ward-round stem above from this page alone, re-read the matching section — the page is built to be self-sufficient for final-prof and NEET-PG, INICET, USMLE, and MRCP questions on dermatomyositis and polymyositis (the idiopathic inflammatory myopathies).[1]

References

1. [1]Lundberg IE, Tjärnlund A, Bottai M, et al. 2017 European League Against Rheumatism/American College of Rheumatology Classification Criteria for Adult and Juvenile Idiopathic Inflammatory Myopathies and Their Major Subgroups Arthritis Rheumatol, 2017.PMID 29106061
2. [2]Dalakas MC. Inflammatory muscle diseases N Engl J Med, 2015.PMID 25923553
3. [3]Bohan A, Peter JB. Polymyositis and dermatomyositis (first of two parts) N Engl J Med, 1975.PMID 1090839
4. [4]Bohan A, Peter JB. Polymyositis and dermatomyositis (second of two parts) N Engl J Med, 1975.PMID 1089199
5. [5]Griggs RC, Askanas V, DiMauro S, et al. Inclusion body myositis and myopathies Ann Neurol, 1995.PMID 7486861
6. [6]Hoogendijk JE, Amato AA, Lecky BR, et al. 119th ENMC international workshop: trial design in adult idiopathic inflammatory myopathies, with the exception of inclusion body myositis, 10-12 October 2003, Naarden, The Netherlands Neuromuscul Disord, 2004.PMID 15099594
7. [7]Sato S, Hirakata M, Kuwana M, et al. Autoantibodies to a 140-kd polypeptide, CADM-140, in Japanese patients with clinically amyopathic dermatomyositis Arthritis Rheum, 2005.PMID 15880816
8. [8]Mammen AL, Chung T, Christopher-Stine L, et al. Autoantibodies against 3-hydroxy-3-methylglutaryl-coenzyme A reductase in patients with statin-associated autoimmune myopathy Arthritis Rheum, 2011.PMID 21360500
9. [9]Anquetil C, Boyer O, Wesner N, et al. Myositis-specific autoantibodies, a cornerstone in immune-mediated necrotizing myopathy Autoimmun Rev, 2019.PMID 30639649
10. [10]Aggarwal R, Bandos A, Reed AM, et al. Predictors of clinical improvement in rituximab-treated refractory adult and juvenile dermatomyositis and adult polymyositis Arthritis Rheumatol, 2014.PMID 24574235
11. [11]Dalakas MC, Illa I, Dambrosia JM, et al. A controlled trial of high-dose intravenous immune globulin infusions as treatment for dermatomyositis N Engl J Med, 1993.PMID 8247075
12. [12]Hanna MG, Badrising UA, Benveniste O, et al. Safety and efficacy of intravenous bimagrumab in inclusion body myositis (RESILIENT): a randomised, double-blind, placebo-controlled phase 2b trial Lancet Neurol, 2019.PMID 31397289
13. [13]Oldroyd AGS, Callen JP, Chinoy H, et al. International Guideline for Idiopathic Inflammatory Myopathy-Associated Cancer Screening: an International Myositis Assessment and Clinical Studies Group (IMACS) initiative Nat Rev Rheumatol, 2023.PMID 37945774