Inflammatory Myopathies
Inflammatory Myopathies
What Do You Need to Know?
- Four major types: Dermatomyositis (DM), Polymyositis (PM), Immune-Mediated Necrotizing Myopathy (IMNM), and Inclusion Body Myositis (IBM) — PM is now rare/diagnosis of exclusion; many previously labeled PM are actually IMNM
- IBM is the most common inflammatory myopathy in patients >50: Insidious, asymmetric, targets finger flexors (FDP) + quadriceps; DOES NOT respond to immunosuppression
- DM = skin + proximal weakness + cancer risk: Heliotrope rash, Gottron papules; 10–15% malignancy in adults >40; TIF1-γ antibody has strongest cancer association
- IMNM = very high CK + necrosis without inflammation: Anti-SRP (severe, cardiac) and anti-HMGCR (statin-associated); requires aggressive immunosuppression
- Biopsy is diagnostic: Perifascicular atrophy = DM (pathognomonic); rimmed vacuoles = IBM; necrosis/regeneration with MAC = IMNM; endomysial CD8+ T cells = PM
- Antisynthetase syndrome: Myositis + ILD + arthritis + Raynaud + mechanic's hands + fever; Jo-1 most common antibody; ILD may dominate
- Treatment: DM/PM/IMNM → steroids + steroid-sparing agent ± IVIg/rituximab; IBM → no proven effective therapy (exercise + supportive care)
Classification
Four Major Types of Inflammatory Myopathy
| Type | Key Distinction |
|---|---|
| Dermatomyositis (DM) | Skin rash + proximal weakness; complement-mediated microangiopathy; perifascicular atrophy on biopsy |
| Polymyositis (PM) | Proximal weakness without skin rash; CD8+ T cell-mediated; now rare — diagnosis of exclusion after ruling out IBM, IMNM, and dystrophy |
| Immune-Mediated Necrotizing Myopathy (IMNM) | Severe proximal weakness + very high CK; necrosis/regeneration with minimal inflammation; anti-SRP or anti-HMGCR antibodies |
| Inclusion Body Myositis (IBM) | Insidious asymmetric weakness (finger flexors + quadriceps); rimmed vacuoles on biopsy; refractory to immunosuppression |
💎 Board Pearl
- PM is now considered a diagnosis of exclusion — many historical PM cases were actually IMNM or IBM; always check for IMNM antibodies and consider biopsy before labeling PM
- If a “PM” patient does not respond to steroids, reconsider the diagnosis — think IBM (if older + distal weakness) or IMNM (if very high CK)
DM vs PM vs IBM vs IMNM — Master Comparison
| Feature | DM | PM | IMNM | IBM |
|---|---|---|---|---|
| Age of onset | Any age (adults + children) | >18 years | Any age | >50 years |
| Sex | F > M (2:1) | F > M | F > M | M > F (3:1) |
| Onset | Subacute (weeks–months) | Subacute | Acute/subacute | Insidious (years) |
| Weakness pattern | Proximal, symmetric | Proximal, symmetric | Proximal, symmetric, severe | Proximal + distal, asymmetric (finger flexors + quadriceps) |
| Skin involvement | Yes (hallmark) | No | No | No |
| CK level | Elevated (5–50×) | Elevated (10–50×) | Very high (often >10,000) | Mild (1–10×) |
| Pathology | Perifascicular atrophy; perimysial inflammation; complement (MAC) on capillaries | Endomysial CD8+ T cells invading non-necrotic fibers | Necrosis/regeneration; minimal/absent inflammation; MAC on capillaries | Rimmed vacuoles; intracellular inclusions (amyloid, TDP-43, p62); endomysial CD8+ T cells |
| Key antibodies | Mi-2, MDA5, TIF1-γ, NXP-2, SAE | Antisynthetases (Jo-1) | Anti-SRP, anti-HMGCR | Anti-cN1A (~60%) |
| Cancer risk | 10–15% (highest with TIF1-γ) | Low | Moderate (especially anti-HMGCR) | None |
| Treatment response | Good | Good | Variable (often aggressive Rx needed) | Poor/none |
| Dysphagia | 20–30% | Uncommon | Uncommon | ~60% |
💎 Board Pearl
- Older man + slow progressive weakness + finger flexors + quadriceps + rimmed vacuoles = IBM — the most tested inflammatory myopathy on boards
- Very high CK (>10,000) + minimal biopsy inflammation = IMNM, not PM
- Skin rash precedes or accompanies weakness = DM — if only skin and no weakness, it is amyopathic DM (check MDA5)
Dermatomyositis — Clinical Features
Skin Findings (May Precede or Accompany Weakness)
| Finding | Description |
|---|---|
| Heliotrope rash | Violaceous/lilac discoloration of upper eyelids ± periorbital edema |
| Gottron papules | Erythematous/violaceous papules over MCP, PIP, DIP joints (also knees, elbows) |
| V-sign | Erythema over anterior chest in V-distribution |
| Shawl sign | Erythema over posterior shoulders, upper back, nape of neck |
| Mechanic’s hands | Cracked, roughened skin on palmar and lateral aspects of fingers — strong association with antisynthetase syndrome |
| Periungual erythema | Dilated capillary loops at nail folds (nailfold capillaroscopy abnormal) |
| Calcinosis cutis | Subcutaneous calcium deposits — more common in juvenile DM (especially NXP-2) |
Systemic Associations
- ILD: 20–40% of DM; especially with antisynthetase and MDA5 antibodies; can be rapidly fatal (RP-ILD with MDA5)
- Cardiac: Myocarditis, arrhythmias, conduction defects (uncommon but serious)
- Malignancy: 10–15% in adults >40 years; ovarian, lung, GI, breast, lymphoma — risk highest in first 3 years after diagnosis
- Amyopathic DM (CADM): Classic skin findings WITHOUT significant weakness; still at risk for ILD and malignancy
🎯 Clinical Pearl
- Gottron papules are pathognomonic for DM — distinguish from Gottron sign (flat erythema over the same joints without raised papules)
- Skin findings may precede weakness by months to years — do not wait for weakness to diagnose DM
DM Antibodies
Myositis-Specific Antibodies in DM
| Antibody | Clinical Phenotype | Cancer Risk | Key Association |
|---|---|---|---|
| Mi-2 | Classic DM with prominent rash + proximal weakness | Low | Good prognosis; excellent treatment response |
| MDA5 (CADM-140) | Amyopathic/hypomyopathic DM; skin ulceration; mechanic’s hands; oral ulcers | Low | Rapidly progressive ILD (RP-ILD); high ferritin; POOR prognosis; may need triple immunosuppression |
| TIF1-γ (p155/140) | DM with extensive skin involvement | HIGHEST (~60% in adults >40) | Strongest cancer association of any myositis antibody; good DM prognosis if no cancer |
| NXP-2 (MJ) | DM with prominent edema, weakness | Moderate (20–30%) | Calcinosis (especially in juvenile DM); second-highest cancer risk after TIF1-γ |
| SAE | Skin-predominant onset → later develops weakness | Low–moderate | Dysphagia common; older adults; skin precedes weakness by months |
| Jo-1 | Antisynthetase syndrome: myositis + ILD + arthritis + Raynaud + mechanic’s hands + fever | Low | Most common antisynthetase antibody; ILD may dominate over myositis |
💎 Board Pearl
- TIF1-γ = cancer — if an adult >40 with DM tests positive for TIF1-γ, pursue aggressive malignancy screening (CT chest/abdomen/pelvis, PET-CT, age-appropriate cancer screens)
- MDA5 = lungs, not muscles — amyopathic DM + rapidly progressive ILD + high ferritin = MDA5; mortality can exceed 50% without early aggressive treatment
- Mi-2 = best prognosis — classic DM that responds well to treatment; the “good antibody”
- NXP-2 = calcinosis — especially in juvenile DM; also associated with cancer in adults
Polymyositis
Overview
- Definition: Subacute proximal symmetric weakness WITHOUT skin rash or inclusion bodies
- CK: Elevated 10–50× normal
- EMG: Irritable myopathy (fibrillations, positive sharp waves, short-duration polyphasic MUAPs, early recruitment)
- Pathology: Endomysial CD8+ cytotoxic T lymphocytes invading non-necrotic muscle fibers; MHC-I upregulation on fibers
- Now rare: True PM accounts for <5% of inflammatory myopathies after reclassification of IMNM and IBM
Diagnostic Requirements (Diagnosis of Exclusion)
- Rule out IBM (check for distal weakness, rimmed vacuoles)
- Rule out IMNM (check SRP, HMGCR antibodies; review biopsy for necrosis pattern)
- Rule out muscular dystrophy (especially LGMD — genetic testing if atypical)
- Rule out DM (no skin findings; no perifascicular atrophy on biopsy)
- Must have endomysial CD8+ T cell infiltrate invading non-necrotic fibers on biopsy
💎 Board Pearl
- PM that doesn’t respond to steroids → reconsider the diagnosis — most likely IBM or IMNM
- The hallmark biopsy finding of PM is CD8+ T cells surrounding and invading non-necrotic fibers — this distinguishes it from IMNM (necrosis without inflammation)
Immune-Mediated Necrotizing Myopathy (IMNM)
Overview
- Presentation: Acute/subacute severe proximal symmetric weakness
- CK: Very high — often >10,000 IU/L (highest among inflammatory myopathies)
- Pathology: Necrosis and regeneration with minimal or absent inflammatory infiltrate; MAC deposition on capillaries
- Mechanism: Complement-mediated — antibodies target muscle fiber surface proteins
IMNM Antibodies
| Antibody | Clinical Features | Treatment Response |
|---|---|---|
| Anti-SRP | Severe, rapidly progressive weakness; cardiac involvement (myocarditis, cardiomyopathy); younger patients; NOT statin-associated | Poor — often refractory; may need triple therapy |
| Anti-HMGCR | Statin-exposed patients (but can occur without statin use); moderate–severe weakness; may have cancer association | Better than SRP; responds to steroids + IVIg ± rituximab |
| Seronegative | ~20% of IMNM; similar clinical presentation; associated with cancer, connective tissue disease | Variable |
Treatment
- Aggressive immunosuppression required: Steroids alone often insufficient
- Typical regimen: Prednisone + IVIg + steroid-sparing agent (methotrexate, azathioprine, or mycophenolate)
- Refractory cases: Rituximab; some require triple therapy (steroids + IVIg + rituximab)
- HMGCR-positive: Discontinue statin (but weakness persists — statin removal alone is not sufficient)
- Monitor CK as a treatment response marker (CK normalizes before strength improves)
💎 Board Pearl
- Statin + persistent weakness + very high CK + necrotic biopsy = think anti-HMGCR IMNM — stopping the statin is necessary but not sufficient; requires immunosuppression
- Anti-SRP = severe + cardiac involvement + poor prognosis — the “bad” IMNM antibody
- IMNM biopsy is the opposite of PM: necrosis everywhere, inflammation nowhere
Inclusion Body Myositis (IBM)
Overview
- Most common inflammatory myopathy in patients >50
- Male predominance (M:F ratio 3:1)
- Insidious onset over years (not weeks–months like DM/PM/IMNM)
- CK: Mild elevation (1–10× normal) — CK may even be normal
Characteristic Weakness Pattern
| Muscle Group | Details |
|---|---|
| Finger flexors (FDP) | Weak grip; difficulty opening jars, turning keys — earliest and most specific finding |
| Wrist flexors | Often affected early alongside finger flexors |
| Quadriceps | Knee buckling, falls, difficulty rising from chair; often asymmetric |
| Ankle dorsiflexors | Foot drop — contributes to falls |
| Pharyngeal muscles | Dysphagia in ~60% — major source of morbidity (aspiration risk) |
- Asymmetric involvement is characteristic (unlike DM/PM/IMNM)
- Combined proximal + distal weakness (unlike DM/PM which are purely proximal)
Pathology
- Rimmed vacuoles (on modified Gomori trichrome stain) — hallmark finding
- Intracellular inclusions: amyloid-β, phosphorylated tau, TDP-43, p62
- Endomysial CD8+ T cells (similar to PM) + degenerative features (unique to IBM)
- Combined inflammatory + degenerative pathology — explains treatment resistance
Diagnostics
- Anti-cN1A (anti-cytosolic 5′-nucleotidase 1A): Positive in ~60% of IBM; supports diagnosis but not 100% specific (can be positive in Sjögren syndrome, SLE)
- EMG: Mixed myopathic + neuropathic features (long-duration MUAPs in a myopathy = IBM clue)
- MRI: Selective atrophy/fatty replacement of quadriceps and forearm flexors
Treatment
- No proven effective immunotherapy — steroids, IVIg, methotrexate, and rituximab have all failed in trials
- Exercise and physical therapy — the most important intervention; resistance training slows functional decline
- Dysphagia management: Speech therapy, dietary modifications, cricopharyngeal dilation or myotomy for severe cases
- Assistive devices for mobility and ADLs
💎 Board Pearl
- Man >50 + insidious weakness + finger flexors (FDP) + quadriceps + asymmetric + mild CK + rimmed vacuoles = IBM — the highest-yield inflammatory myopathy on boards
- IBM does NOT respond to immunosuppression — if the question asks about treatment, the answer is exercise/supportive care
- Mixed myopathic + neuropathic EMG in an inflammatory myopathy = think IBM
- Anti-cN1A is supportive but not diagnostic alone — biopsy with rimmed vacuoles remains the gold standard
Antisynthetase Syndrome
Clinical Features
- Classic hexad: Myositis + ILD + arthritis + Raynaud phenomenon + mechanic’s hands + fever
- Not all features need to be present — ILD or arthritis may be the dominant/presenting manifestation
- ILD may dominate over myositis — some patients have minimal weakness but severe lung disease
Antisynthetase Antibodies
| Antibody | tRNA Target | Key Features |
|---|---|---|
| Jo-1 (anti-histidyl) | Histidine | Most common (~60% of antisynthetase); myositis often prominent; ILD in ~70% |
| PL-7 (anti-threonyl) | Threonine | ILD-predominant; less myositis than Jo-1 |
| PL-12 (anti-alanyl) | Alanine | ILD-predominant; minimal/no myositis |
| EJ (anti-glycyl) | Glycine | Rare; ILD + myositis |
| OJ (anti-isoleucyl) | Isoleucine | Rare; ILD-predominant |
| KS (anti-asparaginyl) | Asparagine | Rare; severe ILD with minimal myositis |
Treatment
- Steroids (prednisone 1 mg/kg) as initial therapy
- Steroid-sparing agent: Mycophenolate preferred for ILD component; azathioprine or methotrexate for myositis-predominant disease
- Rituximab for refractory cases (especially ILD)
- Avoid methotrexate if significant ILD (risk of pulmonary toxicity)
💎 Board Pearl
- Mechanic’s hands + ILD + arthritis + Raynaud + myositis = antisynthetase syndrome — check Jo-1 first, then extended panel
- Non-Jo-1 antisynthetases (PL-7, PL-12, KS) tend to be ILD-predominant with little or no myositis — may present to pulmonology, not neurology
- Perimysial pathology (inflammation + perimysial fragmentation) on biopsy is the histologic pattern most associated with antisynthetase syndrome
Muscle Biopsy Patterns
Key Biopsy Findings by Disease
| Biopsy Pattern | Diagnosis | Key Details |
|---|---|---|
| Perifascicular atrophy | DM | Pathognomonic; small fibers at periphery of fascicle; complement (MAC) on perimysial capillaries; perimysial CD4+ T cells and B cells |
| Endomysial CD8+ T cell invasion of non-necrotic fibers | PM | Cytotoxic T lymphocytes surrounding and invading intact (non-necrotic) muscle fibers; MHC-I upregulation |
| Rimmed vacuoles + intracellular inclusions | IBM | Rimmed vacuoles on Gomori trichrome; 15–18 nm tubulofilamentous inclusions; amyloid-β, TDP-43, p62 deposits; CD8+ T cells also present |
| Necrosis/regeneration + MAC on capillaries | IMNM | Scattered necrotic and regenerating fibers; minimal/absent inflammatory infiltrate; complement deposition on non-necrotic fibers and capillaries |
| Perimysial inflammation + fragmentation | Antisynthetase | Perimysial connective tissue inflammation and fragmentation (“perimysial pathology”); perifascicular necrosis without atrophy |
💎 Board Pearl
- Perifascicular atrophy = DM — this is pathognomonic and the single most tested biopsy finding in inflammatory myopathies
- Rimmed vacuoles = IBM — the second most tested finding; if you see rimmed vacuoles, the answer is IBM
- Necrosis + NO inflammation = IMNM — the absence of inflammatory cells is the distinguishing feature from PM
- CD8+ T cells invading non-necrotic fibers = PM or IBM — add rimmed vacuoles to get IBM; without rimmed vacuoles = PM
Treatment Overview
Treatment by Disease
| Disease | First-Line | Second-Line / Refractory | Key Points |
|---|---|---|---|
| DM | Prednisone (1 mg/kg) + steroid-sparing agent (AZA, MTX, or MMF) | IVIg (FDA-approved for DM); rituximab | IVIg is the only FDA-approved therapy for DM; PE generally NOT effective |
| PM | Prednisone + steroid-sparing agent (AZA or MTX) | IVIg; rituximab | If no response, reconsider diagnosis (IBM? IMNM? dystrophy?) |
| IMNM | Prednisone + IVIg + steroid-sparing agent | Rituximab; triple therapy often needed | Aggressive treatment required upfront; SRP-positive often refractory |
| IBM | No proven effective immunotherapy | Exercise, PT, supportive care | Do NOT treat long-term with steroids — side effects without benefit; manage dysphagia |
| Antisynthetase | Prednisone + MMF (preferred for ILD) or AZA | Rituximab; avoid MTX if significant ILD | ILD component drives treatment decisions |
Steroid-Sparing Agents
| Agent | Onset | Best For | Key Consideration |
|---|---|---|---|
| Azathioprine | 3–6 months | PM, DM (myositis-predominant) | Check TPMT genotype before starting |
| Methotrexate | 4–8 weeks | PM, DM, arthritis-predominant | Avoid if ILD (pulmonary toxicity risk) |
| Mycophenolate | 2–3 months | ILD-predominant, antisynthetase | Preferred steroid-sparing for ILD; teratogenic |
| IVIg | Days–weeks | DM (FDA-approved), IMNM, dysphagia | Rapid onset; requires repeated infusions; costly |
| Rituximab | 2–4 weeks | Refractory DM/PM/IMNM/antisynthetase | B-cell depletion; screen for hepatitis B; monitor CD19/CD20 |
Cancer Screening in DM
- All adults with DM — mandatory cancer screening at diagnosis
- Workup: CT chest/abdomen/pelvis, age-appropriate screening (mammography, colonoscopy, pelvic/testicular US), consider whole-body PET-CT (especially TIF1-γ positive)
- Repeat screening at 6–12 month intervals for at least 3 years
- Highest risk: TIF1-γ (>60% malignancy in adults >40) and NXP-2 (20–30%)
💎 Board Pearl
- IVIg has an FDA indication for DM — this is commonly tested; it is NOT FDA-approved for PM, IBM, or IMNM
- Plasma exchange (PE) is generally NOT effective for DM — unlike many other antibody-mediated diseases
- IBM + steroids = wrong answer on boards — no immunotherapy has shown benefit; exercise is the intervention
- All adult DM requires cancer screening — if the question mentions DM in an adult >40, always screen for malignancy
References
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- Mariampillai K, Granger B, Amelin D, et al. Development of a new classification system for idiopathic inflammatory myopathies based on clinical manifestations and myositis-specific autoantibodies. JAMA Neurol 2018;75(12):1528–1537.
- Greenberg SA. Inclusion body myositis: clinical features and pathogenesis. Nat Rev Rheumatol 2019;15(5):257–272.
- Allenbach Y, Mammen AL, Benveniste O, Stenzel W. Clinico-seropathological classification of immune-mediated necrotizing myopathies. Neuromuscul Disord 2018;28(2):87–99.
- Aggarwal R, Rider LG, Ruperto N, et al. 2016 American College of Rheumatology/European League Against Rheumatism criteria for minimal, moderate, and major clinical response in adult dermatomyositis and polymyositis. Arthritis Rheumatol 2017;69(5):898–910.
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- Carstens PO, Schmidt J. Diagnosis, pathogenesis and treatment of myositis: recent advances. Clin Exp Immunol 2014;175(3):349–358.
- Aggarwal R, Charles-Schoeman C, Engel WK, et al. Trial of intravenous immune globulin in dermatomyositis. N Engl J Med 2022;387(14):1264–1278.