Also known as: polymyositis, PM, dermatomyositis, DM, inflammatory myopathy, IIM
Overview
Idiopathic inflammatory myopathies characterized by symmetric proximal muscle weakness. Polymyositis (PM) is an isolated inflammatory myopathy without distinctive skin findings; dermatomyositis (DM) has the same myopathy plus a characteristic cutaneous eruption. Both are part of the broader inflammatory myopathy spectrum that also includes immune-mediated necrotizing myopathy (IMNM), antisynthetase syndrome, and inclusion body myositis (IBM).
Epidemiology
Combined incidence 5-10 per million per year. DM has a bimodal distribution (juvenile 5-15 years and adult 45-65 years); PM peaks in adulthood. Female-to-male ratio approximately 2:1. DM is more frequently associated with occult malignancy than PM.
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Question 1PulmonaryMedium
A 40-year-old woman who has never smoked reports several months of progressive exertional dyspnea. She also describes proximal muscle weakness and cracked, fissured skin over the lateral aspects of the fingers. CT of the chest shows bilateral lower-lobe ground-glass opacities consistent with interstitial lung disease. Serologic testing is positive for anti-Jo-1 antibody. Which of the following is the most likely diagnosis?
AIdiopathic pulmonary fibrosis
BSystemic sclerosis
CHypersensitivity pneumonitis
DSarcoidosis
EAntisynthetase syndrome
Reveal answer & full explanation
Correct answer: E — Antisynthetase syndrome
AIdiopathic pulmonary fibrosis
BSystemic sclerosis
CHypersensitivity pneumonitis
DSarcoidosis
EAntisynthetase syndrome✓
Why antisynthetase syndrome is correct
Antisynthetase syndrome is defined by an anti-aminoacyl-tRNA synthetase antibody (most commonly anti-Jo-1) combined with interstitial lung disease and inflammatory myopathy
The supportive features here complete the classic cluster: mechanic's hands (fissured skin of the lateral fingers), proximal muscle weakness, and bilateral ground-glass interstitial lung disease
Anti-Jo-1 positivity with myositis and interstitial lung disease is essentially specific for this syndrome and drives treatment with corticosteroids plus a steroid-sparing agent such as mycophenolate or azathioprine
Why the others are wrong
Idiopathic pulmonary fibrosis — IPF is a diagnosis of exclusion in older adults showing a usual interstitial pneumonia (honeycombing) pattern with no autoantibodies or myopathy; the anti-Jo-1 antibody and muscle weakness exclude it (anchoring on interstitial lung disease)
Hypersensitivity pneumonitis — this requires an inciting antigen exposure and does not produce anti-Jo-1 antibody, mechanic's hands, or myositis (buzzword-matching on ground-glass opacities)
Sarcoidosis — sarcoidosis typically causes bilateral hilar adenopathy with non-caseating granulomas, not anti-Jo-1-associated myositis with mechanic's hands (confused with another interstitial lung disease)
Systemic sclerosis — systemic sclerosis also causes lower-lobe ground-glass interstitial lung disease in middle-aged women and involves the fingers, but its skin finding is sclerodactyly with Raynaud phenomenon and its ILD-associated antibody is anti-Scl-70 (anti-topoisomerase I); anti-Jo-1 with mechanic's hands and proximal weakness defines antisynthetase syndrome instead (confused with another connective tissue disease-ILD)
Question 2MusculoskeletalMedium
A 65-year-old woman has 6 months of progressive proximal muscle weakness, a heliotrope rash, and Gottron papules. Creatine kinase is 4,200 U/L, and muscle biopsy shows perimysial inflammation consistent with dermatomyositis. Myositis-specific antibody testing returns positive for anti-TIF1-gamma. Beyond initiating immunosuppression, which of the following is the most important next step in her evaluation?
AHigh-resolution chest CT for interstitial lung disease
BBaseline swallow evaluation for aspiration risk
CWhole-body MRI to map skeletal muscle involvement
Why Comprehensive age-appropriate malignancy screening is correct
Adult dermatomyositis is a recognized paraneoplastic syndrome, and anti-TIF1-gamma (anti-transcriptional intermediary factor 1-gamma) is the myositis-specific antibody most strongly associated with underlying malignancy
A large fraction of older adults positive for anti-TIF1-gamma have a concurrent or soon-to-be-diagnosed cancer, with risk concentrated in the first 3 years after diagnosis
Because identifying and treating the tumor is central to controlling the myositis, comprehensive age- and sex-appropriate screening (e.g., CT chest/abdomen/pelvis, mammography, colonoscopy, pelvic imaging) is mandatory at diagnosis
This aligns with the 2023 IMACS international guideline on myositis-associated cancer screening, which classifies anti-TIF1-gamma dermatomyositis as high risk and calls for enhanced screening at diagnosis
Why the others are wrong
Baseline swallow evaluation for aspiration risk — pharyngeal weakness does occur in dermatomyositis, but no dysphagia, choking, or nasal regurgitation is described, so this does not displace cancer screening in an anti-TIF1-gamma-positive older adult (right-concept-wrong-priority)
Whole-body MRI to map skeletal muscle involvement — muscle MRI is used to select a biopsy site or gauge activity when the diagnosis is unsettled, but biopsy and serology have already secured it here, so it would not change management (redundant testing)
High-resolution chest CT for interstitial lung disease — ILD screening is driven by anti-MDA5 and anti-synthetase antibodies, whereas anti-TIF1-gamma carries low ILD risk and high cancer risk (confused-with-MDA5)
DXA scan before prolonged glucocorticoid therapy — glucocorticoids are the backbone of initial dermatomyositis therapy and baseline bone health assessment is recommended to prevent glucocorticoid-induced osteoporosis, but it can be completed within the first months of treatment, whereas anti-TIF1-gamma positivity in an older adult makes finding an occult malignancy the time-sensitive priority (anchoring on treatment toxicity)
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Autoimmune attack on muscle tissue. DM is characterized by complement-mediated perifascicular atrophy with humoral and microvascular injury. PM features endomysial CD8+ T-cell infiltrates surrounding non-necrotic muscle fibers. Multiple myositis-specific autoantibodies (anti-Jo-1, anti-Mi-2, anti-MDA5, anti-TIF1-gamma, anti-NXP2, anti-HMGCR) define clinically distinct subsets with different organ involvement and cancer risk.
Clinical presentation
Symptoms
Symmetric proximal muscle weakness — difficulty rising from a chair, climbing stairs, lifting arms overhead, combing hair
Insidious onset over weeks to months
Myalgia in 25-50 percent (less prominent than weakness)
Dysphagia from pharyngeal muscle involvement
Dyspnea from interstitial lung disease or respiratory muscle weakness
Signs / physical exam
Symmetric proximal weakness on confrontation testing
Dermatomyositis skin findings: heliotrope rash (violaceous discoloration of the eyelids), Gottron papules (scaly violaceous plaques over MCP and PIP joints), shawl sign, V-sign, holster sign, mechanic's hands, periungual erythema with nailfold capillary changes
Calcinosis cutis in juvenile DM
Subcutaneous edema in active DM
Classic findings
Middle-aged woman with several months of progressive symmetric proximal weakness, a violaceous (heliotrope) rash over the upper eyelids, and Gottron papules over the knuckles.
Differential diagnosis
Inclusion body myositis — Older men, asymmetric distal and proximal weakness (finger flexors, quadriceps), poor response to steroids; rimmed vacuoles on biopsy
Statin-associated myopathy / IMNM — Anti-HMGCR antibodies; necrosis with minimal inflammation on biopsy; persists after statin discontinuation
Polymyalgia rheumatica — Older adults with proximal stiffness rather than weakness, markedly elevated ESR, dramatic response to low-dose steroids, normal CK
Hypothyroid myopathy — Elevated TSH, low T4, generalized weakness with cramps, elevated CK
Muscular dystrophy — Earlier onset, family history, distinctive muscle biopsy findings, dystrophin or other gene mutations
Drug-induced myopathy — Statins, glucocorticoids (no CK elevation), colchicine, antiretrovirals
Diagnostic workup
Diagnostic criteria
Bohan and Peter criteria (historic) and EULAR/ACR 2017 criteria. Classic features include symmetric proximal weakness, elevated muscle enzymes, characteristic EMG findings (small polyphasic motor unit potentials, fibrillations, positive sharp waves), and muscle biopsy demonstrating perifascicular atrophy with perimysial and perivascular inflammation (DM) or endomysial CD8+ infiltrates (PM). Skin findings are mandatory for DM diagnosis.
Labs
Creatine kinase (CK) — typically markedly elevated (10-50x normal), though may be normal in chronic disease and amyopathic DM
Aldolase, LDH, AST, ALT (muscle origin) often elevated alongside CK
TSH, CMP (electrolytes can affect CK and weakness interpretation)
Age-appropriate cancer screening for adult DM (and PM to a lesser degree) — chest CT, mammography, pelvic ultrasound, colonoscopy, CA-125; consider PET-CT for high-risk antibodies (TIF1-gamma)
Imaging
MRI of affected muscles — T2 hyperintensity reflecting edema; guides biopsy site
High-resolution chest CT for interstitial lung disease, particularly if anti-Jo-1 or anti-MDA5 positive
Pulmonary function testing including DLCO
Echocardiogram if cardiac involvement suspected
Diagnostic algorithm
Feature
Polymyositis
Dermatomyositis
Inclusion Body Myositis
Demographics
Adults
Bimodal (juvenile and adult)
Men >50
Weakness pattern
Symmetric proximal
Symmetric proximal
Asymmetric, finger flexors and quadriceps
Skin
Absent
Heliotrope, Gottron papules
Absent
CK
Elevated
Variably elevated
Mildly elevated or normal
Cancer association
Modest
Strong (especially adult)
None
Steroid response
Good
Good
Poor
Comparison of inflammatory myopathies.
Treatment
First-line
High-dose corticosteroids — prednisone 1 mg/kg/day (up to 60-80 mg) until objective improvement, then taper over 9-12 months
Methylprednisolone pulses for severe weakness, dysphagia, or rapidly progressive interstitial lung disease
Steroid-sparing agent introduced early to limit cumulative steroid exposure: methotrexate or azathioprine
Physical therapy and home exercise to preserve function and prevent contracture
Sun protection, topical corticosteroids or calcineurin inhibitors, and hydroxychloroquine for DM skin disease (hydroxychloroquine does not treat the myositis)
Second-line / adjunct
Mycophenolate mofetil for refractory disease or ILD
IV immunoglobulin (IVIG) for refractory or severe disease, particularly with dysphagia (PRODERM trial supports DM use)
Rituximab for refractory PM/DM
Dihydropyridine calcium channel blockers for Raynaud phenomenon; PDE5 inhibitors or IV prostanoids with low-dose aspirin for severe digital ischemia
Cyclophosphamide for severe ILD or vasculitis
Complications
Aspiration pneumonia from pharyngeal weakness
Interstitial lung disease (especially with anti-Jo-1 antisynthetase syndrome and anti-MDA5 — the latter associated with rapidly progressive ILD)
Adult DM is associated with occult malignancy in 15-30 percent of cases; comprehensive cancer screening is mandatory at diagnosis and should be repeated annually for 3 years.
Anti-MDA5 DM is associated with amyopathic disease, distinctive skin ulcerations, and rapidly progressive ILD with high mortality — aggressive immunosuppression from the outset.
Statin-associated immune-mediated necrotizing myopathy (anti-HMGCR) does not improve after statin discontinuation and requires immunosuppression.
Inclusion body myositis should be considered in older men with asymmetric weakness involving finger flexors and quadriceps that fails to respond to steroids.
Antisynthetase syndrome (anti-Jo-1 or another anti-tRNA synthetase antibody) combines myositis with interstitial lung disease, mechanic's hands, Raynaud phenomenon, inflammatory arthritis (usually non-erosive), and fever; ILD affects most patients, may precede the myositis, and drives morbidity and mortality, so obtain HRCT and PFTs with DLCO at diagnosis even without respiratory symptoms.
Myositis antibody phenotypes: anti-Mi-2 = classic DM with prominent skin and nailfold changes (ragged cuticles, periungual erythema), high CK, good steroid response, and low ILD risk; anti-MDA5 = amyopathic DM with skin ulcers and rapidly progressive ILD; anti-TIF1-gamma and anti-NXP2 = cancer-associated adult DM (NXP2 also calcinosis); anti-SRP and anti-HMGCR = immune-mediated necrotizing myopathy.
References
EULAR/ACR 2017 — Lundberg IE et al., 2017 European League Against Rheumatism/American College of Rheumatology classification criteria for adult and juvenile idiopathic inflammatory myopathies (Ann Rheum Dis 2017)
PRODERM Trial — Aggarwal R et al., Trial of Intravenous Immune Globulin in Dermatomyositis (NEJM 2022)
IMACS 2023 — Oldroyd AGS 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;19:805-817)
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