Inflammatory Myopathies (IIM): Polymyositis and Dermatomyositis “WEAK MUSCLE” Diagnosis, Staging, and Step-Care Treatment Plan By 2017 EULAR/ACR probability score

1 Pathophysiology & Clinical Presentation

Key point	Polymyositis (PM)	Dermatomyositis (DM)
Immune driver	CD8⁺ T-cell–mediated myofibre injury	Humoral + complement attack on endomysial vessels
Core symptom	Symmetric proximal muscle weakness (climbing stairs, hair-combing)	Same muscle weakness plus pathognomonic rashes
Cutaneous clues	—	Heliotrope eyelid rash, Gottron papules, V-sign, shawl sign, holster sign, CMVE

2 Diagnostic Framework

Mnemonic “W-EMB(S)” – Weakness, Enzymes, Myopathic EMG, Biopsy, (Skin for DM).

Criterion	What to check
A — Muscle weakness	Symmetric, proximal, ≥ 4 weeks
B — Muscle enzymes	CK, AST, ALT, LDH, aldolase ↑
C — EMG	Fibrillation potentials, short polyphasic MUAPs
D — Biopsy	PM → endomysial CD8⁺ & necrosis; DM → perifascicular atrophy
E — Skin	Heliotrope, Gottron, CMVE, V/shawl/holster signs

Diagnosis: PM = A–D present, no E. DM = E + ≥ 3 of A–D. Apply the 2017 EULAR/ACR probability score to label possible/probable/definite  IIM; a score ≥ 7.5 (with biopsy) or ≥ 8.7 (without) = definite myositis.org.

Diving Deeper into the Core Diagnostic Criteria for Polymyositis (PM) & Dermatomyositis (DM)

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3 Staging & Activity Monitoring

Domain	Tool (+ timing)	Clinically meaningful change
Muscle strength	MMT-8 every visit (0–80)	Response = ≥ 20 % or ≥ 6-point gain sralab.org
Global disease activity	IMACS Core Set Measures → calculate ACR/EULAR Total Improvement Score (TIS)	TIS ≥ 20 = minimal, ≥ 40 = moderate, ≥ 60 = major improvement academic.oup.comniehs.nih.gov
Skin severity (DM)	CDASI activity score	≤ 14 mild, 15–40 moderate, > 40 severe jaad.org
Permanent damage	Myositis Damage Index (MDI) yearly	Higher score = chronic / polycyclic course
Extra-muscular	HRCT + PFTs (ILD), echo ± RHC (PAH), age-appropriate malignancy screen (anti-TIF1-γ, NXP-2)	—

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4 Investigations – “MIOSITIS” checklist

Letter	Order this	Purpose
M	MRI STIR thighs/shoulders	Localise active muscle for biopsy
I	Immune panel (MSA/MAA)	Phenotype & prognosis (e.g., anti-MDA-5 → RP-ILD)
O	Organ screen – HRCT, PFTs, echo, LFTs	Baseline & follow-up
S	Serum enzymes – CK etc.	Disease activity
I	Infection screen – HBV/HCV, HIV, IGRA	Pre-immunosuppression
T	Tumour search > 40 yrs	CT chest/abd/pelvis, mammogram/colonoscopy/PSA
I	Interval labs q 4–8 w on therapy	Response & toxicity
S	Strength testing (MMT-8 / dynamometer)	Objective follow-up

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5 Management – Treat-to-Target ≤ 6 months

Step	Drug(s) & key points	Escalate when…
1 High-dose steroid	Prednisone 1 mg/kg/day (max 80 mg) ± IV methyl-pred 1 g ×3 if severe dysphagia/RP-ILD	CK or strength plateaus, unable to taper < 20 mg by 3 mo
2 Steroid-sparing csDMARD (start ≤ 4 wks)	Methotrexate 15–25 mg/wk or Azathioprine 2–3 mg/kg/d. ILD: Mycophenolate 1 g bid / Tacrolimus 1–3 mg bid	Persistent activity or steroid-toxicity
3 IVIG 2 g/kg monthly	Rapid weakness, dysphagia, pregnancy, or refractory DM rash	Re-check after 3 cycles
4 Biologic / targeted	Rituximab 1 g × 2 (RIM trial) myositis.org, Abatacept, JAK-i (Tofacitinib) for anti-MDA-5 ILD, Cyclophosphamide for severe ILD
5 Cutaneous DM adjuncts	Hydroxychloroquine 200–400 mg/d, potent topical steroids, sun-block
6 Rehab & prevention	Early physiotherapy, vitamin D + DEXA, vaccines (flu, pneumococcal, zoster, COVID-19)

Toxicity pearls • MTX → hepatotoxicity & pneumonitis • AZA → TPMT/NUDT15-related cytopenia • Mycophenolate → cytopenia & diarrhoea • Rituximab → HBV reactivation.

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6 General Doctor vs Rheumatology – who does what?

Task	GP / Internist	Needs Rheumatologist
Suspicion, baseline CK, ANA, rule-out infection	✓	—
Start high-dose steroid if profound weakness or ILD	✓ (then phone Rheum same day)	Dose-taper & DMARD plan
Vaccination, osteoporosis prophylaxis, BP/DM care	✓	—
Routine labs & CK q 4–8 w	✓ (forward to Rheum)	Adjust drugs / interpret flares
Decisions on IVIG, biologics, cyclophosphamide	✗	✓ – complex immunotherapy
Work-up / management of ILD, PAH, malignancy	Arrange tests	Rheum + Respiratory / Oncology
Refractory disease, pregnancy, juvenile cases	Stabilise & refer	Multidisciplinary care

Bottom line: All confirmed or strongly suspected IIM patients require early Rheumatology co-management; the generalist initiates life-saving steroids, screens for comorbidities, and provides long-term preventive care.

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7 High-yield Dermatomyositis Rashes

Rash	Clinical clue	Exam mnemonic
Gottron papules	Violaceous papules over MCP/PIP	“Got Guitar Knuckles”
Heliotrope	Lilac eyelid discoloration ± oedema	“Helios = sun-purple”
V-sign / Shawl sign	Photodistributed chest / shoulder erythema	Letter shapes on skin
Holster sign	Lateral thigh erythema	“Gun-holster rash”
CMVE	Confluent macular violaceous erythema	Key board-style patch

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One-minute OSCE Script

1. “Mrs D has symmetric proximal weakness, CK 2400 IU/L, heliotrope rash → 2017 EULAR/ACR score 9.1 = definite dermatomyositis.”

2. Stage: MMT-8 54/80 (moderate), TIS baseline 0, CDASI 28 (moderate), HRCT clear.

3. Order MRI thighs, myositis panel, EMG, and malignancy screen.

4. Start Prednisone 1 mg/kg + Methotrexate 15 mg/wk + calcium/vit-D; refer to Rheumatology within a week; physiotherapy day 1.

5. Review CK & strength at 4 weeks; add IVIG if TIS < 20.
   

Use “W-EMB(S)”, TIS 20-40-60, and MIOSITIS check-list to keep both patients and examiners safe.

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Diving Deeper into the Core Diagnostic Criteria for Polymyositis (PM) & Dermatomyositis (DM)

Below, each element of the classic A–E framework is unpacked in detail and cross-mapped to the weightings in the 2017 EULAR/ACR probability score (the current research-grade standard).

Criterion	What to document in practice	Key histo-/physiologic pearls	EULAR/ACR weight (with biopsy)
A — Muscle weakness	• Manual Muscle Testing (MMT-8) or dynamometry of neck-flexors, deltoids, biceps, wrist extensors, iliopsoas, gluteus maximus, quadriceps, ankle dorsiflexors. • Symmetric, proximal, ≥ 4 weeks; neck-flexors weaker than extensors; proximal legs weaker than distal.	CD8⁺ T-cell cytotoxicity in PM; perifascicular ischemia in DM cause predominantly proximal fibre loss.	• Prox UL: 0.7  • Prox LL: 0.5  • Neck-flexor < extensor: 1.6  • Prox > distal leg: 1.2 myositis.org
B — Muscle enzymes	Order CK, aldolase, AST, ALT, LDH at baseline and every taper decision. Typical CK peaks:   • PM / DM active: > 1 000 IU (L) (may reach > 10 000).   • “Amyopathic” DM: CK often normal.	CK tracks necrosis/regeneration; aldolase may rise first as CK normalises. Beware: strenuous exercise, statins and hypothyroidism also raise CK.	CK/LD/AST/ALT↑: 1.4 health.com
C — Electromyography	Needle EMG in ≥ 2 proximal & 1 distal muscle: • Fibrillation/positive sharp waves at rest • Short-duration (< 5 ms), low-amplitude (< 500 µV) polyphasic MUAPs • Early full recruitment with minimal effort	Reflects membrane irritability from inflammatory insult. Complex repetitive discharges & “myotonic-like” potentials bolster diagnosis.	(EMG is not scored in 2017 criteria but remains clinically invaluable) myositis.org
D — Muscle biopsy	Biopsy an MRI-edematous site before steroids if possible. Process fresh-frozen + formalin.	Polymyositis → Endomysial CD8⁺ T-cells & macrophages invading non-necrotic fibres with diffuse MHC-I up-regulation. autoimmunhighlights.biomedcentral.com Dermatomyositis → Perifascicular atrophy, perivascular/ perimysial CD4⁺ T-cells & B-cells, capillary C5b-9 complement deposition. academic.oup.com	• Endomysial inflam (non-invasive): 1.7 • Perivascular/perimysial inflam: 1.2 • Perifascicular atrophy: 1.9 myositis.org
E — Skin manifestations (DM)	Heliotrope rash (eyelids), Gottron papules/sign, V-sign, shawl sign, holster sign, Mechanics hands, periungual telangiectasia. Document photos under good light.	Skin pathology shows interface dermatitis & mucin; MxA and IFN-signature staining aid amyopathic DM Dx.	• Heliotrope: 3.2 • Gottron papule: 2.7 • Gottron sign: 3.7 myositis.org

How the 2017 EULAR/ACR Probability Score Works

1. Assign points for each variable present (table clip above).

2. Sum the points.

3. Convert to a probability of IIM:*Without biopsy: P = 1 / [1 + e^(6.49 – score)]With biopsy: P = 1 / [1 + e^(5.33 – score)] myositis.org

4. Interpret:

   • ≥ 7.5 (no biopsy ≥ 8.7) → Definite IIM (> 90 % probability)

   • 5.5 – 7.4 (no biopsy 6.7 – 8.6) → Probable IIM (> 55 %)

   • 5.3 – 5.4 → Possible IIM (50–55 %)

   • < 5.3 (< 6.5 with biopsy) → Unlikely
     

Shortcut rule still acceptable in clinics: PM = A–D + absence of E |  DM = E + ≥ 3 of A–D (Bohan & Peter).

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Putting It Together – Sample Calculation

Patient: 46-year-old woman with proximal leg weakness (MMT 4/5), heliotrope rash, neck-flexor 3/5, CK 3500, anti-Jo-1⁺, biopsy shows perifascicular atrophy.

Variable	Points
Age ≥ 40	2.2
Prox-LL weakness	0.5
Neck-flexor weaker	1.6
CK↑	1.4
Heliotrope	3.2
Anti-Jo-1	3.8
Perifascicular atrophy	1.9
Total	14.6

With biopsy:P = 1 / [1 + e^(5.33 – 14.6)] ≈ 0.999 → Definite DM.

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Practical Tips for Each Criterion

• Weakness (A) – always test neck flexors; their relative weakness carries the biggest score (1.6 pts).

• Enzymes (B) – trend CK every 4–8 weeks; a fall > 80 % yet persistent weakness suggests steroid-induced myopathy rather than active disease.

• EMG (C) – combine with MRI (STIR) to target biopsy and avoid false-negatives.

• Biopsy (D) – take from deltoid or vastus lateralis; avoid severely atrophic muscle. Request MHC-I, CD3, C5b-9 immunostains.

• Skin (E) – photograph lesions; they may fade after steroids, hampering later scoring.
  

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Key Take-aways

• Weightings matter: heliotrope rash (3.2 pts) or anti-Jo-1 (3.8 pts) can swing a case from “possible” to “definite” quickly.

• Biopsy deepens certainty and allows subtype classification (PM vs DM vs IBM).

• Use the web calculator (link in criteria paper) at the bedside for exact probability.

• Remember the probability concept: the criteria classify for research; clinical judgment still rules when treatment can’t wait for a biopsy.

Feel free to ask for more on biopsy techniques, MRI protocols, or management algorithms!