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Dermatomyositis and antisynthetase syndrome

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Diagnostic Criteria: Dermatomyositis (DM) and Antisynthetase Syndrome (ASyS)

Dermatomyositis

1. EULAR/ACR 2017 Classification Criteria (Primary Standard)

The 2017 EULAR/ACR classification criteria replaced the older Bohan and Peter (1975) criteria. They use a probability-based scoring system across 16 variables, run in two models depending on whether muscle biopsy is available. Each variable carries a weighted score; the total determines classification.

Score Variables

Variable	Score (No Biopsy)	Score (With Biopsy)
Age of onset
18 to <40 years	1.3	1.5
≥40 years	2.1	2.2
Muscle Weakness
Symmetric proximal upper extremity weakness	0.7	0.7
Symmetric proximal lower extremity weakness	0.8	0.5
Neck flexors weaker than neck extensors	1.9	1.6
Proximal leg muscles weaker than distal	0.9	1.2
Skin Manifestations
Heliotrope rash	3.1	3.2
Gottron's papules	2.1	2.7
Gottron's sign	3.3	3.7
Other Clinical
Dysphagia or esophageal dysmotility	0.7	0.6
Laboratory
Anti-Jo-1 autoantibody present	3.9	3.8
Elevated CK, LDH, AST, or ALT	1.3	1.4
Muscle Biopsy (if available)
Endomysial infiltration of mononuclear cells (surrounding but not invading myofibers)	-	1.7
Perimysial and/or perivascular infiltration	-	1.2
Perifascicular atrophy	-	1.9
Rimmed vacuoles	-	3.1

Classification Thresholds

Category	Without Biopsy	With Biopsy
Probable IIM (55–90% probability)	≥5.5 and <7.5	≥6.7 and <8.7
Definite IIM (≥90% probability)	≥7.5	≥8.7

Sensitivity 93%, specificity 88% when biopsy is included. - Firestein & Kelley's Textbook of Rheumatology, Table 86.4; Rheumatology 2-Volume Set (2022, Elsevier), p. 4851

2. Subgroup Classification: Dermatomyositis Specifically

A decision tree built into the 2017 criteria classifies IIM into subgroups. DM is identified by its skin features. The dermatomyositis subgroup is distinguished from polymyositis, IBM, juvenile IIM, and overlap myositis primarily by:

Characteristic skin lesions (heliotrope rash, Gottron papules/sign)
Muscle biopsy showing perifascicular atrophy - considered pathognomonic for DM
Myositis-specific antibodies: anti-TIF1-γ (anti-p155/140), anti-Mi-2, anti-MDA5, anti-NXP2

Note: Clinically amyopathic DM (CADM / "DM sine myositis") - skin changes present >6 months without muscle involvement - is a limitation of the 2017 criteria (not well captured). - Rheumatology 2-Volume Set, p. 4857-4858

3. Key Clinical Features Used in Diagnosis

Pathognomonic / Highly Specific Skin Signs

Heliotrope rash: violaceous periorbital confluent erythema, often pruritic - the "cutaneous hallmark" of DM
Gottron papules: violaceous papules over dorsal MCP/IP joints of the hands
Gottron sign: violaceous erythema (not raised) over knuckles, elbows, or knees

Other Characteristic Features

Shawl sign: erythema over posterior neck/shoulders
V-sign: erythema in V-shaped distribution over anterior chest/neck
Mechanic's hands: hyperkeratotic, fissured skin at radial aspect of fingers (more typical of ASyS overlap)
Periungual telangiectasias and dystrophic cuticles
Poikiloderma atrophicans vasculare (photoexposed areas)
Calcinosis cutis (more prevalent in juvenile DM)

Muscle

Symmetric, progressive proximal muscle weakness
Neck flexors > extensors weakness
Dysphagia (up to 30%)
CK elevated (but can be normal in up to one-third, especially in active disease or CADM)
Perifascicular atrophy on biopsy - pathognomonic

Systemic

ILD (interstitial lung disease)
Cardiac involvement (conduction defects, cardiomyopathy)
Raynaud phenomenon
Associated malignancy in ~10-15% of adults (nasopharyngeal in Asian populations; breast, lung, colorectal in Western countries); anti-TIF1-γ strongly associated
Goldman-Cecil Medicine, p. 3042-3050; Bradley and Daroff's Neurology, p. 326-331

4. Key Investigations

Test	Significance
Serum CK, LDH, AST, ALT	Elevated in most; can be normal in CADM
Anti-Mi-2	Specific for DM; associated with classic skin features
Anti-TIF1-γ (anti-p155/140)	DM + malignancy association
Anti-MDA5	DM + rapidly progressive ILD; often amyopathic
Anti-NXP2	DM + calcinosis (juvenile DM)
Anti-Jo-1	More typical of ASyS; present in some DM-overlap
Skin biopsy	Interface dermatitis (cell-poor), epidermal atrophy, mucin deposition
Muscle biopsy	Perifascicular atrophy (pathognomonic), perimysial/perivascular inflammation
MRI (STIR)	Muscle edema in affected muscles
EMG	Myopathic changes

Antisynthetase Syndrome (ASyS)

Diagnostic Criteria (Connors et al. / Murray & Nadel)

ASyS does not have a single universally adopted classification score like EULAR/ACR for IIM. The widely used working criteria (Connors GR et al., Chest 2010) require:

Mandatory: Positive anti-aminoacyl-tRNA synthetase antibody

Plus one or more of the following:

Myositis (by Bohan and Peter criteria)

Interstitial lung disease (ILD by ATS criteria)

Arthritis (by clinical examination, radiographic findings, or patient self-report)

Unexplained, persistent fever

Raynaud phenomenon

Distal digital fissuring ("mechanic's hands")

Murray & Nadel's Textbook of Respiratory Medicine, Table 92.6

Anti-tRNA Synthetase Antibodies (MSA)

Present in 25-35% of all patients with myositis. The full panel:

Antibody	Target tRNA Synthetase	Notes
Anti-Jo-1	Histidyl	Most common (~20-30% of myositis); marker of ILD; present in 50-100% of myositis+ILD cases
Anti-PL-7	Threonyl
Anti-PL-12	Alanyl
Anti-EJ	Glycyl
Anti-OJ	Isoleucyl
Anti-KS	Asparaginyl
Anti-Ha	Tyrosyl
Anti-Zo	Phenylalanyl

Harrison's Principles of Internal Medicine 22E, p. 2957

Clinical Features of ASyS

Feature	Details
Myositis	Proximal muscle weakness; CK usually elevated
ILD	Most important manifestation; reduced FVC and DLCO; HRCT shows honeycomb/ground-glass pattern; may be rapidly progressive
Arthritis	Nonerosive polyarthritis
Mechanic's hands	Hyperkeratotic, cracked, fissured skin at lateral/radial digits
Raynaud phenomenon	Vasospasm of digits
Fever	Persistent, unexplained
Erythematous rash	Some patients; muscle biopsies may show DM-pattern histopathology

Histopathology (Muscle Biopsy in ASyS)

Distinguishing features from pure DM:

Predilection for perimysial damage: perimysial fragmentation + alkaline phosphatase staining
Plasmacytoid dendritic cells and macrophages in perimysium
MAC deposition on capillaries
Perifascicular muscle fiber necrosis (vs. perifascicular atrophy in DM)
MHC-I, MHC-II (HLA-DR) deposits on perifascicular sarcolemma
HLA-DR expression suggests gamma-IFN driven (vs. type I IFN dominant in DM)
Harrison's 22E, p. 2956-2957

Key Differences: DM vs. ASyS

Feature	Dermatomyositis	Antisynthetase Syndrome
Defining antibodies	Anti-Mi-2, anti-TIF1-γ, anti-MDA5, anti-NXP2	Anti-Jo-1 and other anti-tRNA synthetases
Skin	Heliotrope, Gottron papules (hallmark)	Mechanic's hands (hallmark); may overlap with DM rash
ILD	Present in some (especially anti-MDA5)	Prominent, often the dominant feature
Malignancy risk	Yes, ~10-15% adults	No increased risk
Muscle biopsy	Perifascicular atrophy	Perifascicular necrosis
Arthritis	Less prominent	Common
Pathogenesis	Type I IFN dominant	Gamma-IFN dominant
Treatment	Corticosteroids ± azathioprine/MMF	May need early rituximab for ILD

Note on overlap: Many ASyS patients have skin and muscle biopsy features of DM and are sometimes classified under DM. The distinction matters because ILD in ASyS can be severe and progressive, warranting aggressive early immunosuppression (often rituximab). - Harrison's 22E, p. 2957

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