Dermatomyositis

1. Overview

Dermatomyositis (DM) is an idiopathic inflammatory myopathy characterised by the combination of proximal muscle weakness and distinctive cutaneous manifestations. It represents one of the major subgroups of inflammatory myopathies, distinguished from polymyositis and inclusion body myositis by its pathognomonic skin findings and unique pathophysiology. [1,2]

The condition affects both children and adults, with bimodal age distribution. Adult-onset dermatomyositis carries significant implications due to its well-established association with underlying malignancy, occurring in 15-30% of cases. The discovery of myositis-specific autoantibodies has revolutionised understanding of disease phenotypes, with specific antibodies predicting clinical features, complications, and prognosis. [3,4]

Dermatomyositis is now recognised as a complement-mediated microangiopathy affecting muscle, skin, and potentially other organs including the lungs and heart. Early recognition is critical, as prompt immunosuppressive therapy can prevent irreversible muscle damage, and systematic malignancy screening may detect cancers at earlier, more treatable stages. The advent of antibody testing and improved imaging has enhanced diagnostic accuracy and prognostic stratification. [5,6]

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2. Epidemiology

Incidence and Prevalence

Dermatomyositis is a rare condition with an estimated incidence of 5-10 cases per million population per year in European and North American populations. [7] The prevalence ranges from 10-20 per million, though geographic and ethnic variations exist. Higher rates have been reported in African American populations compared to Caucasians. [8]

Age Distribution

Dermatomyositis exhibits a bimodal age distribution with two distinct peaks. The first peak occurs in childhood between ages 5-15 years (juvenile dermatomyositis), while the second peak affects adults between ages 40-60 years. [7,8] Adult-onset disease carries the significant concern of malignancy association, whereas juvenile disease is more commonly associated with calcinosis and has a lower malignancy risk.

Sex Distribution

There is a clear female predominance with a female to male ratio of approximately 2:1 in adult dermatomyositis. This female preponderance is consistent with other autoimmune conditions and suggests hormonal or genetic factors in disease susceptibility. [7,8]

Risk Factors

Unlike many autoimmune conditions, specific environmental triggers or genetic predispositions for dermatomyositis remain poorly defined. Associations have been reported with:

• HLA-DRB1 alleles (particularly HLA-DRB1*03:01) in Caucasian populations [13]
• UV radiation exposure (may trigger or exacerbate cutaneous manifestations)
• Certain medications (statins, hydroxyurea, though rare)
• Infections (viral triggers proposed but not definitively proven)
• Silica exposure (weak association in some studies)

The strongest association remains with underlying malignancy, particularly in older adults and those with specific antibody profiles.

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3. Aetiology and Pathophysiology

Aetiology

The precise aetiology of dermatomyositis remains unknown, but evidence supports an autoimmune process triggered by environmental factors in genetically susceptible individuals. Unlike polymyositis, where T-cell-mediated muscle fibre destruction predominates, dermatomyositis is characterised by complement-mediated microangiopathy. [1,2]

Proposed Triggers:

• Viral infections (influenza, coxsackievirus, HIV)
• Malignancy-associated antigens triggering immune response
• UV radiation (for cutaneous manifestations)
• Certain medications (checkpoint inhibitors, statins)

Pathophysiology

Complement-Mediated Microangiopathy

The fundamental pathophysiological process in dermatomyositis involves activation of the complement cascade with deposition of the membrane attack complex (MAC, C5b-9) on capillary endothelium in muscle and skin. This leads to:

1. Capillary destruction: Complement-mediated lysis of endomysial capillaries
2. Ischaemia: Reduced capillary density causing muscle ischaemia
3. Perifascicular atrophy: Characteristic pattern of muscle fibre atrophy in watershed zones
4. Inflammatory infiltrates: Secondary B-cell and T-cell infiltration in perivascular and perimysial regions

Interferon Signature

Type I interferon pathway activation is a hallmark of dermatomyositis pathogenesis. Elevated expression of interferon-inducible genes has been demonstrated in muscle and skin tissue, correlating with disease activity. This interferon signature distinguishes dermatomyositis from other inflammatory myopathies and provides therapeutic targets. [14,15]

Myositis-Specific Autoantibodies

Several myositis-specific autoantibodies (MSAs) have been identified, each associated with distinct clinical phenotypes:

Anti-Mi-2 (10-15% of DM patients)

• Target: Nuclear helicase protein
• Phenotype: Classic dermatomyositis with prominent cutaneous features
• Prognosis: Excellent response to treatment, low malignancy risk
• Mechanism: Unknown pathogenic role

Anti-TIF1γ (Transcriptional Intermediary Factor 1-gamma, 15-20% of adult DM)

• Target: Nuclear protein involved in TGF-β signalling
• Phenotype: Classic dermatomyositis
• Association: Strong association with malignancy (40-70% in adults)
• Clinical utility: Mandates comprehensive malignancy screening

Anti-MDA5 (Melanoma Differentiation-Associated protein 5, 10-30% depending on ethnicity)

• Target: Cytoplasmic viral RNA sensor
• Phenotype: Amyopathic or hypomyopathic dermatomyositis
• Association: Rapidly progressive interstitial lung disease (RP-ILD)
• Prognosis: High mortality if RP-ILD develops (30-50% mortality at 6 months)
• More common in Asian populations

Anti-NXP2 (Nuclear Matrix Protein 2, 15-25% of juvenile DM)

• Target: Nuclear matrix protein
• Phenotype: Severe muscle disease, calcinosis
• Association: Malignancy in adult cases, peripheral oedema
• Prognosis: More resistant to treatment

Anti-SAE (Small Ubiquitin-like Modifier Activating Enzyme)

• Target: Enzymes in SUMO pathway
• Phenotype: Skin-predominant disease, dysphagia
• Association: Low malignancy risk

[3,4,14]

Muscle Pathology

Histopathological examination reveals characteristic features:

• Perifascicular atrophy (pathognomonic finding)
• Perivascular and perimysial inflammatory infiltrates (B-cells, CD4+ T-cells, plasmacytoid dendritic cells)
• Capillary loss and endothelial damage
• MAC deposition on capillaries (demonstrated by C5b-9 immunostaining)
• Regenerating fibres
• Variable necrosis (less prominent than polymyositis)

Skin Pathology

Cutaneous biopsies show:

• Interface dermatitis with vacuolar changes at dermoepidermal junction
• Lymphocytic infiltration
• Mucin deposition in dermis
• MAC deposition on dermal vessels

Malignancy Association

The association between dermatomyositis and malignancy is well-established and clinically significant. The standardised incidence ratio for cancer in dermatomyositis patients is approximately 3-6 times that of the general population. [9,10]

Key Facts:

• Malignancy detected in 15-30% of adult dermatomyositis cases
• Risk highest in first 3-5 years after DM diagnosis
• Bidirectional association: cancer may precede or follow DM diagnosis
• Most cancers are within 1 year before or after DM diagnosis

Most Common Associated Malignancies:

• Ovarian cancer (highest association)
• Lung cancer
• Colorectal cancer
• Gastric cancer
• Nasopharyngeal cancer (particularly in Asian populations)
• Breast cancer
• Non-Hodgkin lymphoma

High-Risk Features for Malignancy:

• Age > 40 years
• Male sex
• Absence of interstitial lung disease
• Cutaneous necrosis or ulceration
• Anti-TIF1γ antibody positivity
• Rapid disease onset
• Elevated CRP (unusual in uncomplicated DM)

[9,10]

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4. Clinical Presentation

Cutaneous Features

The skin manifestations of dermatomyositis are often the presenting feature and may precede muscle symptoms by weeks to months. Some patients have skin-predominant disease (amyopathic dermatomyositis) with minimal or no muscle involvement.

Pathognomonic Cutaneous Signs:

Gottron Papules

• Erythematous to violaceous papules or plaques overlying the metacarpophalangeal (MCP) and interphalangeal (IP) joints
• May extend to extensor surfaces of elbows, knees, and ankles
• Often scaly with hyperkeratosis
• May ulcerate in severe cases
• Pathognomonic when present (highly specific)

Gottron Sign

• Erythematous macules over extensor surfaces of joints (without papule formation)
• Less specific than Gottron papules

Heliotrope Rash

• Violaceous to erythematous discolouration of periorbital skin
• Often accompanied by periorbital oedema
• May be subtle and require careful examination
• Named after heliotrope flower colour
• Highly specific when present

Characteristic but Non-Pathognomonic Features:

Shawl Sign

• Photosensitive erythema over upper back, shoulders, and posterior neck
• May extend in V-distribution
• Often indicates active disease

V-Sign

• Erythema in V-distribution over anterior chest and neck
• Photodistributed

Mechanic's Hands

• Hyperkeratotic, cracked, fissured skin on palmar and lateral aspects of fingers
• Resembles manual labourer's hands
• Associated with anti-synthetase antibodies (Jo-1), suggesting overlap syndrome
• May indicate higher risk of ILD

Additional Cutaneous Features:

• Poikiloderma (atrophic skin with hyper/hypopigmentation and telangiectasia)
• Calcinosis cutis (subcutaneous calcium deposits, more common in juvenile DM)
• Periungual changes (dilated capillary loops, cuticular overgrowth)
• Scalp involvement (pruritic erythema, scaling, alopecia)
• Panniculitis (rare)
• Cutaneous ulceration (poor prognostic sign, suggests vasculopathy)

Muscle Features

Proximal Muscle Weakness

The classic presentation involves symmetrical proximal muscle weakness affecting:

• Shoulder girdle: Difficulty lifting arms above head, combing hair, reaching overhead
• Pelvic girdle: Difficulty rising from chair, climbing stairs, getting out of car

The weakness typically develops subacutely over weeks to months, distinguishing it from more acute presentations like viral myositis or rapid presentations of polymyositis.

Functional Limitations:

• Difficulty with activities of daily living requiring arm elevation
• Reduced mobility and falls risk
• Neck flexor weakness (may cause difficulty lifting head from pillow)
• Respiratory muscle weakness (diaphragm, intercostals) in severe cases

Important Characteristics:

• Distal muscles typically spared initially
• Reflexes usually preserved (unless severe atrophy)
• Sensation intact (distinguishes from neuropathies)
• Muscle pain (myalgia) present in 30-50% but not universal
• Muscle tenderness on palpation variable

Dysphagia and Oropharyngeal Involvement

Dysphagia occurs in 30-60% of dermatomyositis patients and results from involvement of pharyngeal and upper oesophageal striated muscles. [16]

Clinical Features:

• Difficulty initiating swallowing
• Nasal regurgitation
• Aspiration risk (potentially life-threatening)
• Dysphonia (vocal cord involvement)
• Oesophageal dysmotility affecting upper two-thirds

Significance:

• Marker of disease severity
• Aspiration pneumonia risk
• Nutritional impairment
• Requires specific assessment and management

Systemic and Extramuscular Manifestations

Interstitial Lung Disease (ILD)

ILD represents one of the most significant complications, occurring in 30-40% of dermatomyositis patients. [11] It is the leading cause of death in anti-MDA5-positive patients.

Patterns:

• Non-specific interstitial pneumonia (NSIP) - most common
• Organising pneumonia (OP/COP)
• Usual interstitial pneumonia (UIP) - worse prognosis
• Diffuse alveolar damage (in rapidly progressive cases)

Clinical Presentation:

• Progressive dyspnoea on exertion
• Dry cough
• Bibasilar inspiratory crackles
• May be asymptomatic (detected on screening)

Anti-MDA5-Associated RP-ILD:

• Rapidly progressive course over weeks
• Extensive ground-glass opacities on CT
• Often amyopathic (minimal muscle disease)
• High mortality (30-50% at 6 months despite aggressive treatment)
• More common in Asian populations
• Requires urgent aggressive immunosuppression

Cardiac Involvement

Cardiac manifestations occur in 10-30% of cases but are often subclinical. [17]

Manifestations:

• Myocarditis (acute or chronic)
• Arrhythmias (conduction defects, atrial/ventricular arrhythmias)
• Congestive heart failure
• Pericarditis (rare)
• Coronary vasculitis (rare)

Detection:

• ECG abnormalities (non-specific ST-T changes, arrhythmias)
• Elevated troponin or BNP
• Echocardiographic abnormalities
• Cardiac MRI (T2-weighted imaging showing oedema)

Arthropathy

Joint involvement occurs in 25-50% of patients:

• Symmetrical polyarthritis resembling rheumatoid arthritis
• Usually non-erosive
• Affects small and large joints
• More common with anti-synthetase antibodies (overlap syndrome)

Constitutional Symptoms

• Fatigue (very common, often debilitating)
• Low-grade fever
• Weight loss
• Raynaud phenomenon (especially in overlap syndromes)

Amyopathic Dermatomyositis

A subset of patients (10-20%) present with characteristic cutaneous features but no clinical muscle weakness despite at least 6 months of observation. Subclinical muscle involvement may be detected on MRI or biopsy. This phenotype is strongly associated with anti-MDA5 antibodies and carries risk of rapidly progressive ILD. [18]

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5. Differential Diagnosis

Inflammatory Myopathies

Polymyositis

• Proximal muscle weakness without characteristic DM rash
• T-cell-mediated muscle fibre destruction (different pathophysiology)
• Endomysial inflammatory infiltrates on biopsy
• No perifascicular atrophy
• No capillary MAC deposition
• Diagnosis increasingly questioned (may represent unrecognised IBM or overlap syndromes)

Inclusion Body Myositis (IBM)

• Age > 50 years typically
• Insidious onset (slower progression)
• Distal and proximal weakness (finger flexors, knee extensors)
• Asymmetric weakness common
• Dysphagia frequent
• Poor response to immunosuppression
• Biopsy shows rimmed vacuoles and inclusions

Immune-Mediated Necrotising Myopathy (IMNM)

• Prominent muscle necrosis on biopsy
• Minimal inflammation
• Associated with anti-SRP or anti-HMGCR antibodies
• Statin-associated in some cases
• Severe weakness with very high CK (often > 10,000 U/L)

Overlap Syndromes and Connective Tissue Diseases

Anti-Synthetase Syndrome

• Myositis + ILD + arthritis + mechanic's hands + Raynaud + fever
• Anti-Jo-1 (most common) or other anti-synthetase antibodies
• ILD often more prominent than muscle disease
• Distinct clinical syndrome requiring specific recognition

Systemic Lupus Erythematosus (SLE)

• Malar rash, photosensitivity, mucosal ulcers
• Arthritis, serositis, renal involvement
• Positive ANA, anti-dsDNA, low complement
• Myalgia common but true myositis less frequent

Systemic Sclerosis (Scleroderma)

• Skin thickening and tightness
• Raynaud phenomenon (almost universal)
• Digital ulcers, calcinosis
• ILD common
• Myopathy can occur in overlap syndromes

Mixed Connective Tissue Disease (MCTD)

• Features of SLE, SSc, and myositis
• Anti-U1-RNP antibodies
• Raynaud, puffy hands, arthritis

Muscular Dystrophies

Age-appropriate consideration of genetic muscle diseases:

• Limb-girdle muscular dystrophies
• Facioscapulohumeral dystrophy
• Dysferlinopathy

Distinguishing Features:

• Family history of muscle disease
• Earlier onset (often childhood/adolescence)
• Slowly progressive or static course
• Specific distribution patterns (e.g., facial weakness in FSHD)
• No inflammatory markers or rash
• Biopsy shows dystrophic rather than inflammatory changes
• Genetic testing confirmatory

Drug-Induced Myopathies

Statin-Induced Myopathy

• Exposure to statins
• Usually resolves with drug cessation
• May trigger immune-mediated necrotising myopathy (anti-HMGCR) requiring immunosuppression
• No rash

Checkpoint Inhibitor-Induced Myositis

• Exposure to pembrolizumab, nivolumab, ipilimumab
• Can be severe with cardiac involvement
• Often concurrent myasthenia gravis
• Requires immunosuppression

Other Drugs:

• Hydroxychloroquine (vacuolar myopathy)
• Colchicine (neuromyopathy)
• Alcohol (acute alcoholic myopathy)

Infectious Myositis

Viral Myositis

• Influenza, HIV, Coxsackie virus
• Acute onset
• Self-limiting course usually
• Specific viral testing

Pyomyositis

• Bacterial infection (usually Staphylococcus aureus)
• Localised muscle abscess
• Fever, localised pain and swelling
• MRI shows fluid collection
• More common in tropical regions or immunocompromised

Endocrine and Metabolic Myopathies

Hypothyroid Myopathy

• Proximal weakness, muscle cramps, myalgia
• Elevated CK
• TSH elevated, T4 low
• Resolves with thyroid replacement

Vitamin D Deficiency

• Proximal myopathy, bone pain
• Low 25-OH vitamin D
• Common and often overlooked

Cushing Syndrome

• Steroid-induced myopathy (including exogenous steroids)
• Proximal weakness without CK elevation
• Cushingoid features

Other Considerations

Malignancy-Related

• Paraneoplastic syndrome distinct from DM-associated cancer
• Lambert-Eaton myasthenic syndrome (fatiguable weakness)
• Cancer cachexia

Critical Illness Myopathy

• ICU setting, prolonged immobilisation
• Neuromuscular blocking agents, corticosteroids

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6. Investigations

Muscle Enzymes

Creatine Kinase (CK)

• Elevated in 70-90% of patients with active muscle disease
• Degree of elevation variable (100-10,000+ U/L)
• May be normal in amyopathic DM or chronic/burnt-out disease
• Useful for monitoring disease activity and treatment response
• Less elevated than in immune-mediated necrotising myopathy

Other Muscle Enzymes:

• Aldolase (may be elevated when CK normal)
• LDH (lactate dehydrogenase)
• AST and ALT (often elevated, can be confused with hepatic origin)

Electromyography (EMG)

Needle EMG demonstrates myopathic changes:

• Short-duration, low-amplitude, polyphasic motor unit potentials
• Early recruitment
• Spontaneous fibrillations and positive sharp waves (indicate active muscle damage)
• Distribution: proximal > distal muscles

Utility:

• Confirms myopathic process
• Guides biopsy site selection (abnormal but not end-stage muscle)
• Differentiates from neuropathic disorders
• Less commonly used now with availability of MRI

Magnetic Resonance Imaging (MRI)

MRI has become increasingly valuable in dermatomyositis evaluation. [19]

T2-Weighted and STIR Sequences:

• High signal intensity indicating muscle oedema/inflammation
• Identifies active disease
• Guides biopsy location
• Monitors treatment response
• More sensitive than clinical examination for subclinical involvement

Patterns:

• Patchy or diffuse signal abnormalities
• Predominant proximal muscle involvement
• Perifascicular pattern (specific for DM)
• Subcutaneous oedema

Advantages over Biopsy:

• Non-invasive
• Surveys entire muscle groups
• Distinguishes active inflammation from chronic fibrosis
• Can detect improvement with therapy

Muscle Biopsy

Muscle biopsy remains the gold standard for definitive diagnosis, providing histopathological confirmation and excluding alternative diagnoses.

Characteristic Histological Features:

Perifascicular Atrophy (Pathognomonic)

• 2-10 layers of atrophic fibres at periphery of fascicles
• Results from watershed ischaemia due to capillary loss
• Specific for dermatomyositis (not seen in polymyositis or IBM)

Perivascular and Perimysial Inflammation

• B-lymphocytes, CD4+ T-cells, plasmacytoid dendritic cells
• Predominantly perivascular location
• Perimysial distribution (between fascicles)
• Contrast with endomysial infiltrates in polymyositis

Capillary Changes

• Reduced capillary density (capillary dropout)
• Endothelial hyperplasia
• MAC (C5b-9) deposition on capillaries (immunohistochemistry)
• Diagnostic specificity

Additional Features:

• Variable muscle fibre necrosis and regeneration
• Type 2 fibre atrophy
• Increased connective tissue (chronic cases)
• Alkaline phosphatase positivity at perifascicular fibres

Biopsy Technique:

• Open surgical biopsy preferred (adequate tissue)
• Select muscle: clinically weak but not end-stage (MRI guidance)
• Avoid muscles recently sampled by EMG (artefactual changes)
• Common sites: deltoid, quadriceps, biceps brachii

Autoantibody Testing

Comprehensive myositis antibody panel testing is now standard in suspected dermatomyositis. [3,4]

Myositis-Specific Antibodies (MSAs):

Myositis-Associated Antibodies (MAAs):

• Anti-Ro52 (often concurrent with MSAs, associated with ILD)
• Anti-PM/Scl (overlap with systemic sclerosis)
• Anti-Ku (overlap with systemic sclerosis/SLE)

Anti-Synthetase Antibodies (suggest overlap syndrome):

• Anti-Jo-1 (most common, 20-30% of myositis overall, less common in pure DM)
• Anti-PL-7, Anti-PL-12, Anti-EJ, Anti-OJ, Anti-KS

Routine Autoantibodies:

• ANA (antinuclear antibody): positive in 50-80%, usually speckled or nucleolar pattern
• RF (rheumatoid factor): occasionally positive
• Anti-dsDNA: negative (helps exclude SLE)

Pulmonary Function Testing

Essential screening given 30-40% ILD prevalence. [11]

Tests:

• Spirometry: restrictive pattern (reduced FVC)
• Lung volumes: reduced TLC, RV
• Gas transfer: reduced DLCO (most sensitive early marker)

Indications for HRCT Chest:

• Abnormal PFTs
• Respiratory symptoms
• Anti-MDA5 or anti-synthetase antibodies (high ILD risk)
• Consider screening all DM patients

HRCT Patterns:

• Ground-glass opacities
• Reticular abnormalities
• Consolidation (organising pneumonia)
• Subpleural distribution
• Basal predominance typically

Malignancy Screening

Comprehensive age-appropriate cancer screening is mandatory in adult dermatomyositis. [9,10]

Standard Screening (All Adult DM Patients):

• Detailed history and physical examination
• CT chest/abdomen/pelvis
• Age and sex-appropriate cancer screening:
  • Mammography (women > 40)
  • Cervical smear (women)
  • Colonoscopy (adults > 50 or earlier with symptoms)
  • PSA (controversial, discuss with patient)
  • Testicular examination
• Baseline and serial CA-125 (ovarian cancer - highest association)
• Nasopharyngoscopy in Asian populations
• Faecal occult blood testing

Enhanced Screening in High-Risk:

• Anti-TIF1γ positive patients
• Age > 60 years
• Rapid onset DM
• Skin necrosis or ulceration
• Elevated CRP (unusual in uncomplicated DM)

Consideration of PET-CT:

• High-risk cases
• Negative initial screening with high suspicion
• Detects occult malignancies

Surveillance:

• Repeat comprehensive screening at 6 months, 12 months, then annually for 3-5 years
• Highest cancer detection within 3 years of DM diagnosis

Cardiac Assessment

Screening Investigations:

• ECG: arrhythmias, conduction defects, non-specific ST-T changes
• Echocardiography: systolic/diastolic dysfunction, regional wall motion abnormalities
• Troponin: may be elevated in myocarditis
• BNP/NT-proBNP: heart failure marker

Advanced Imaging (if cardiac involvement suspected):

• Cardiac MRI: gold standard for myocarditis detection
  • "T2-weighted: oedema"
  • "Late gadolinium enhancement: fibrosis/necrosis"
  • "T1 mapping: diffuse fibrosis"

Other Laboratory Investigations

Inflammatory Markers:

• ESR: often normal or mildly elevated in uncomplicated DM
• CRP: typically normal (elevation suggests infection, malignancy, or overlap syndrome)

Organ Function:

• Renal function: baseline before immunosuppression
• Liver function: monitor for drug toxicity
• Bone profile: calcium, phosphate, vitamin D

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7. Diagnostic Criteria

2017 EULAR/ACR Classification Criteria

The 2017 EULAR/ACR classification criteria for idiopathic inflammatory myopathies provide a probability-based scoring system for dermatomyositis. [2]

Scoring Domains:

• Age of symptom onset
• Muscle weakness pattern
• Skin manifestations
• Laboratory findings (muscle enzymes)
• Muscle biopsy features
• Other clinical features

Dermatomyositis-Specific Features (High Weights):

• Heliotrope rash
• Gottron papules
• Gottron sign

Probable DM: Score ≥5.5 without biopsy, ≥6.7 with biopsy Definite DM: Score ≥7.5 without biopsy, ≥8.7 with biopsy

Bohan and Peter Criteria (Historical)

The Bohan and Peter criteria (1975) were historically used but are less specific. [20]

Criteria:

1. Symmetrical proximal muscle weakness
2. Elevated serum muscle enzymes
3. Myopathic changes on EMG
4. Characteristic muscle biopsy abnormalities
5. Typical skin rash (for DM)

Classification:

• Definite DM: Criterion 5 + any 3 of criteria 1-4
• Probable DM: Criterion 5 + any 2 of criteria 1-4

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8. Management

The management of dermatomyositis requires a multidisciplinary approach involving rheumatologists, dermatologists, respiratory physicians, oncologists, and rehabilitation specialists. Treatment aims to suppress immune-mediated inflammation, prevent complications, screen for malignancy, and optimise functional outcomes.

General Principles

Treatment Goals:

• Suppress muscle and skin inflammation
• Prevent irreversible muscle damage and atrophy
• Screen for and treat associated malignancy
• Manage extramuscular manifestations (ILD, cardiac)
• Minimise corticosteroid exposure
• Maintain/restore functional capacity
• Prevent complications

Monitoring Disease Activity:

• Clinical assessment: muscle strength (manual muscle testing, MMT-8)
• Functional measures: getting out of chair, climbing stairs, lifting arms
• CK levels (trend more important than absolute value)
• Global disease activity scores
• MRI for objective assessment

Pharmacological Management

First-Line Immunosuppression

High-Dose Corticosteroids

Corticosteroids remain the cornerstone of initial therapy for dermatomyositis.

Regimen:

• Prednisolone 1 mg/kg/day (maximum 80-100 mg) orally
• Continue high dose for 4-8 weeks until clinical improvement and CK declining
• Gradual taper over 6-12 months to minimum effective dose (typically 5-10 mg/day maintenance)
• Rapid tapering risks disease flare

Severe Cases (respiratory muscle weakness, dysphagia, severe ILD):

• IV methylprednisolone 1 g daily for 3-5 days (pulse therapy)
• Followed by oral high-dose predniso lone

Side Effects Requiring Prophylaxis:

• Osteoporosis: bisphosphonates + calcium/vitamin D
• Gastric protection: PPI
• Hyperglycaemia: monitoring, antidiabetic agents if needed
• Infection prophylaxis: consider PCP prophylaxis (cotrimoxazole) if high dose > 3 months
• Cataracts, glaucoma: ophthalmology review

Steroid-Sparing Agents

Steroid-sparing immunosuppressants should be initiated early (often with or shortly after corticosteroids) to facilitate steroid tapering and prevent long-term steroid toxicity.

Methotrexate

• First-line steroid-sparing agent
• Dose: 15-25 mg weekly (oral or subcutaneous)
• Onset of action: 6-12 weeks
• Efficacy: effective for muscle and skin disease
• Monitoring: FBC, renal/liver function every 4-8 weeks
• Contraindications: pregnancy, significant renal/liver impairment, ILD (relative contraindication)
• Folate supplementation: 5 mg weekly (separate day from MTX)

Azathioprine

• Alternative first-line agent
• Dose: 2-3 mg/kg/day (maximum 200 mg)
• Onset: 8-12 weeks
• Check TPMT levels before starting (guides dosing, toxicity risk)
• Monitoring: FBC, liver function every 4-8 weeks
• Well-tolerated, safe in pregnancy (Category D)

Mycophenolate Mofetil

• Increasingly used as first-line or second-line
• Dose: 2-3 g daily in divided doses
• Efficacy: particularly for ILD-associated disease
• Monitoring: FBC every 4-8 weeks
• Contraindication: pregnancy

Calcineurin Inhibitors (Tacrolimus, Cyclosporine)

• Effective for refractory muscle and skin disease
• Particularly effective for anti-MDA5-associated DM and ILD
• Tacrolimus: 2-4 mg daily (target trough 5-10 ng/mL)
• Monitoring: renal function, drug levels, blood pressure
• Side effects: nephrotoxicity, hypertension, diabetes

Intravenous Immunoglobulin (IVIG)

IVIG is highly effective for refractory dermatomyositis, particularly for cutaneous and muscle manifestations.

Indications:

• Refractory muscle weakness despite conventional immunosuppression
• Refractory skin disease (especially pruritus, ulceration)
• Dysphagia (particularly effective)
• Patients unable to tolerate conventional immunosuppressants
• Pregnancy (safe)

Regimen:

• 2 g/kg divided over 2-5 days per month
• Response within 2-4 infusions typically
• Long-term maintenance often required

Evidence:

• RCT evidence supports efficacy
• Well-tolerated
• Expensive, access may be limited

Biological Therapies

Rituximab (Anti-CD20)

• Most evidence-based biological therapy for myositis
• Indications: refractory disease despite conventional treatment
• Regimen: 1 g IV at weeks 0 and 2, repeat at 6 months
• Alternative: 375 mg/m² weekly for 4 weeks
• Efficacy: effective in 80-90% of refractory cases
• Particularly effective for anti-synthetase syndrome
• Monitor: immunoglobulin levels, infusion reactions, PML risk (extremely rare)

Other Biologics (Less Evidence):

• Anti-TNF agents (infliximab, adalimumab): limited efficacy, may worsen disease
• Abatacept (CTLA4-Ig): case series show promise
• Tocilizumab (anti-IL-6): case reports
• JAK inhibitors (tofacitinib, baricitinib): emerging evidence, particularly for refractory skin disease

Management of Rapidly Progressive ILD (Anti-MDA5)

Anti-MDA5-positive dermatomyositis with rapidly progressive ILD requires urgent aggressive immunosuppression.

Regimen:

• High-dose IV methylprednisolone 1 g daily × 3-5 days
• Followed by oral prednisolone 1 mg/kg
• Plus dual or triple immunosuppression:
  • Tacrolimus (target trough 5-10 ng/mL)
  • Mycophenolate mofetil 2-3 g daily OR cyclophosphamide (500-750 mg/m² monthly)
  • Consider IVIG
• Rituximab in refractory cases

Monitoring:

• Serial PFTs and HRCT
• Clinical deterioration despite treatment common
• High mortality (30-50% at 6 months)

Cutaneous Disease Management

Topical Therapies:

• High-potency corticosteroids for skin lesions
• Tacrolimus ointment for facial rash

Photoprotection:

• Broad-spectrum sunscreen (SPF 50+)
• Protective clothing, avoid peak sun hours
• UV radiation exacerbates cutaneous disease

Systemic Therapy for Refractory Skin Disease:

• Hydroxychloroquine 200-400 mg daily (first-line for skin)
• Methotrexate
• IVIG (particularly effective for pruritus)
• JAK inhibitors (tofacitinib): emerging evidence

Malignancy Management

Treatment of underlying malignancy is essential in malignancy-associated dermatomyositis.

Principles:

• Definitive cancer treatment per oncology protocols
• Dermatomyositis may improve with successful cancer treatment
• Continue immunosuppression for DM during cancer therapy (coordination with oncology)
• Complete cancer remission may lead to DM remission in some cases

Rehabilitation and Supportive Care

Physiotherapy:

• Early mobilisation (avoid bed rest)
• Graduated exercise program
• Isometric exercises during acute phase
• Progressive resistance training during recovery
• Prevents contractures and improves functional outcomes

Occupational Therapy:

• Assessment of activities of daily living
• Assistive devices
• Energy conservation techniques

Swallowing Assessment (if dysphagia present):

• Speech and language therapy evaluation
• Modified diet consistency
• Swallowing strategies
• Consider enteral feeding if severe aspiration risk

Respiratory Physiotherapy:

• Breathing exercises
• Secretion clearance if ILD
• Monitor respiratory muscle strength

Management of Specific Complications

Calcinosis:

• Difficult to treat
• Options: diltiazem, bisphosphonates, IVIG, surgical excision if symptomatic

Cardiac Involvement:

• Myocarditis: high-dose corticosteroids, immunosuppression
• Arrhythmias: cardiology referral, antiarrhythmic agents, pacemaker if indicated
• Heart failure: standard HF management

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9. Complications

Mortality

Overall 5-year mortality ranges from 10-30% depending on series and era. [12] Major causes of death include:

• Rapidly progressive ILD (especially anti-MDA5)
• Malignancy
• Respiratory failure
• Cardiac complications
• Infection (often iatrogenic from immunosuppression)

Favourable Prognostic Factors:

• Younger age
• Anti-Mi-2 positivity
• Isolated skin or muscle involvement
• Good response to initial therapy

Poor Prognostic Factors:

• Anti-MDA5 with RP-ILD
• Anti-TIF1γ with malignancy
• Older age
• Significant ILD
• Cardiac involvement
• Delay in diagnosis and treatment

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10. Prognosis

With modern immunosuppressive therapy, the majority of dermatomyositis patients achieve disease control and functional improvement. However, prognosis varies significantly based on clinical phenotype, antibody profile, and complications.

Overall Outcomes:

• 60-80% achieve good disease control with treatment
• 10-20% have refractory disease requiring multiple agents
• Functional recovery possible but may be incomplete
• Many patients require long-term low-dose maintenance therapy
• Relapses common with rapid steroid tapering

Quality of Life:

• Fatigue often persistent even with disease control
• Chronic pain in subset of patients
• Skin manifestations may be disfiguring
• Functional limitations improve but may not normalise

Long-Term Monitoring:

• Regular follow-up for disease activity assessment
• Surveillance for malignancy (first 3-5 years)
• Monitoring for treatment toxicity
• Bone density scans (corticosteroid-induced osteoporosis)
• Cardiovascular risk management

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11. Prevention and Screening

Primary Prevention

No specific primary prevention strategies exist given unclear aetiology.

General Measures:

• Photoprotection (may reduce cutaneous flares)
• Smoking cessation (general health, reduces ILD risk)
• Avoid unnecessary medications that may trigger myositis

Screening

At-Risk Populations: No specific high-risk populations identified for screening, as genetic predisposition is weak.

Malignancy Screening: As detailed in Investigations section, comprehensive age-appropriate cancer screening is mandatory for all adult dermatomyositis patients at diagnosis and during follow-up.

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12. Key Guidelines

2017 EULAR/ACR Classification Criteria for Adult and Juvenile Idiopathic Inflammatory Myopathies

• Lundberg IE et al., Ann Rheum Dis 2017 [2]
• Provides validated classification criteria
• Facilitates uniform patient selection for trials and studies

EULAR Recommendations for Management of Adult Dermatomyositis and Polymyositis

• Not yet published as comprehensive guideline
• Based on expert consensus and evolving evidence

British Society for Rheumatology Guidelines

• Recommends early aggressive immunosuppression
• Steroid-sparing agents to minimize steroid toxicity
• Multidisciplinary approach

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13. Examination Focus

Common Exam Questions

Written Examination Questions:

1. "A 55-year-old woman presents with 3-month history of proximal muscle weakness and a violaceous rash around her eyes. CK is elevated. What is the most likely diagnosis and what is your initial management?"

2. "What are the indications for malignancy screening in dermatomyositis? Which malignancies are most commonly associated?"

3. "Describe the histopathological features that distinguish dermatomyositis from polymyositis on muscle biopsy."

4. "A patient with newly diagnosed dermatomyositis is found to have anti-MDA5 antibodies. What are the clinical implications and how would you manage this patient?"

5. "List the characteristic cutaneous features of dermatomyositis. Which are pathognomonic?"

Clinical Examination Scenarios:

1. "Examine this patient's hands"

• Looking for Gottron papules, mechanic's hands, nail fold changes

2. "This patient has muscle weakness. Perform a focused neurological examination"

• Looking for proximal weakness, preserved reflexes and sensation

3. "Examine this patient's skin"

• Looking for heliotrope rash, shawl sign, V-sign, Gottron papules

Viva Topics:

1. Myositis-specific antibodies and their clinical associations
2. Differential diagnosis of proximal myopathy
3. Management of rapidly progressive ILD in anti-MDA5-positive DM
4. Malignancy screening protocol in dermatomyositis
5. Distinguishing dermatomyositis from polymyositis (clinical, histological, immunological)

Viva Points

Opening Statement:

"Dermatomyositis is an idiopathic inflammatory myopathy characterised by proximal muscle weakness and distinctive cutaneous manifestations, most notably Gottron papules and heliotrope rash. It affects approximately 5-10 per million population annually with a female predominance and bimodal age distribution affecting children and adults aged 40-60."

Key Facts to Mention:

Pathophysiology:

• Complement-mediated microangiopathy affecting endomysial capillaries
• Type I interferon signature
• Perifascicular atrophy on muscle biopsy (pathognomonic)

Clinical Features:

• Gottron papules (pathognomonic): erythematous plaques over MCP/IP joints
• Heliotrope rash: violaceous periorbital discoloration
• Proximal muscle weakness: shoulder and hip girdles
• Dysphagia in 30-60%

Myositis-Specific Antibodies:

• Anti-Mi-2: classic DM, good prognosis
• Anti-TIF1γ: high malignancy association (40-70% in adults)
• Anti-MDA5: rapidly progressive ILD, high mortality
• Anti-NXP2: calcinosis, severe disease

Investigations:

• CK elevated (70-90%)
• EMG: myopathic changes
• MRI: muscle edema, guides biopsy
• Muscle biopsy: perifascicular atrophy, perimysial inflammation, MAC deposition
• Myositis antibody panel
• Malignancy screening: mandatory in adults

Management:

• High-dose corticosteroids: prednisolone 1 mg/kg/day
• Steroid-sparing agents: methotrexate, azathioprine (initiated early)
• IVIG for refractory disease
• Rituximab for refractory cases
• Aggressive immunosuppression for RP-ILD (tacrolimus, mycophenolate, cyclophosphamide)

Complications:

• ILD (30-40%), rapidly progressive in anti-MDA5
• Malignancy (15-30% adults)
• Aspiration pneumonia
• Cardiac involvement

Prognosis:

• 60-80% achieve disease control
• 5-year mortality 10-30%
• Anti-Mi-2: excellent prognosis
• Anti-MDA5 with RP-ILD: 30-50% mortality at 6 months

Common Mistakes

❌ Mistakes That Fail Candidates:

1. Missing malignancy screening: Failing to recognise mandatory comprehensive cancer screening in adult dermatomyositis, especially with anti-TIF1γ positivity

2. Confusing DM with polymyositis: Not recognising that skin findings are essential for DM diagnosis; polymyositis has no rash

3. Not recognising anti-MDA5 implications: Missing the urgency of rapidly progressive ILD in anti-MDA5-positive patients requiring immediate aggressive immunosuppression

4. Inadequate dysphagia assessment: Underestimating aspiration risk in patients with dysphagia; this is potentially life-threatening

5. Monotherapy with steroids alone: Not initiating steroid-sparing agents early, leading to steroid toxicity

6. Missing cardiac involvement: Failing to screen for cardiac complications which are a significant cause of mortality

7. Describing polymyositis biopsy findings for DM: Confusing endomysial inflammation (polymyositis) with perimysial/perivascular inflammation and perifascicular atrophy (DM)

8. Wrong antibody associations: Confusing antibody-phenotype associations (e.g., stating anti-Jo-1 is common in DM when it's more associated with anti-synthetase syndrome)

Model Answers

Q: Describe your approach to a patient presenting with proximal muscle weakness and a rash suggestive of dermatomyositis.

Model Answer:

"I would approach this systematically with three parallel aims: confirming the diagnosis, assessing disease severity and complications, and screening for malignancy.

Initial Assessment: First, I would take a detailed history focusing on the onset and progression of muscle weakness, functional limitations such as difficulty climbing stairs or lifting objects, any dysphagia or respiratory symptoms, and constitutional symptoms. I would ask about cutaneous features and photograph the rash for documentation. A drug history is essential to exclude drug-induced myopathy, and I would inquire about symptoms suggesting malignancy.

Examination: I would perform a full examination including careful assessment of characteristic skin lesions - looking specifically for Gottron papules over the MCP and IP joints, heliotrope rash around the eyes, shawl sign, V-sign, and mechanic's hands. Muscle examination would assess proximal muscle groups, looking for symmetrical weakness of shoulder and hip girdles, while checking that reflexes and sensation are preserved. I would assess for dysphagia, respiratory muscle weakness, and examine for signs of ILD with chest auscultation.

Investigations: Blood tests would include CK (elevated in 70-90%), muscle enzymes (aldolase, LDH, AST/ALT), inflammatory markers (ESR usually normal, CRP elevation suggests complication), renal and liver function, and a comprehensive myositis antibody panel including anti-Mi-2, anti-TIF1γ, anti-MDA5, anti-NXP2, and anti-synthetase antibodies.

MRI of affected muscle groups would demonstrate oedema on T2/STIR sequences and guide biopsy site selection. Muscle biopsy would be performed to confirm diagnosis and demonstrate perifascicular atrophy, perimysial inflammation, and MAC deposition on capillaries.

Screening for complications would include pulmonary function tests with DLCO, HRCT chest if PFTs abnormal or high ILD risk (anti-MDA5, anti-synthetase), ECG and echocardiography, and swallowing assessment if dysphagia present.

Malignancy Screening: Given the 15-30% malignancy association in adults, I would perform comprehensive age-appropriate cancer screening including CT chest/abdomen/pelvis, age-specific screening (mammography, colonoscopy, cervical smear), CA-125 for ovarian cancer, and consider PET-CT in high-risk cases particularly with anti-TIF1γ positivity.

Management: I would initiate high-dose corticosteroids with prednisolone 1 mg/kg/day along with bone protection, PPI, and monitoring for hyperglycaemia. Concurrently, I would start a steroid-sparing agent such as methotrexate 15-25 mg weekly with folic acid, or azathioprine 2 mg/kg/day after checking TPMT levels. For refractory disease, I would consider IVIG or rituximab.

If anti-MDA5 antibodies are positive, I would urgently assess for ILD and if rapidly progressive ILD is present, initiate aggressive triple immunosuppression with IV methylprednisolone, tacrolimus, and mycophenolate or cyclophosphamide.

I would arrange multidisciplinary input including rheumatology, dermatology, respiratory medicine if ILD present, and physiotherapy for rehabilitation. Regular monitoring of disease activity through clinical assessment, CK levels, and functional measures would guide treatment adjustments. Surveillance for malignancy would continue for 3-5 years with repeat screening at 6 months, 12 months, and annually."

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14. References

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3. Satoh M, Tanaka S, Ceribelli A, Calise SJ, Chan EK. A Comprehensive Overview on Myositis-Specific Antibodies: New and Old Biomarkers in Idiopathic Inflammatory Myopathy. Clin Rev Allergy Immunol. 2017;52(1):1-19. doi:10.1007/s12016-015-8510-y

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5. Ladislau L, Suarez-Calvet X, Toquet S, et al. JAK inhibitor improves type I interferon induced damage: proof of concept in dermatomyositis. Brain. 2018;141(6):1609-1621. doi:10.1093/brain/awy105

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7. Bendewald MJ, Wetter DA, Li X, Davis MD. Incidence of dermatomyositis and clinically amyopathic dermatomyositis: a population-based study in Olmsted County, Minnesota. Arch Dermatol. 2010;146(1):26-30. doi:10.1001/archdermatol.2009.328

8. Meyer A, Meyer N, Schaeffer M, et al. Incidence and prevalence of inflammatory myopathies: a systematic review. Rheumatology (Oxford). 2015;54(1):50-63. doi:10.1093/rheumatology/keu289

9. Hill CL, Zhang Y, Sigurgeirsson B, et al. Frequency of specific cancer types in dermatomyositis and polymyositis: a population-based study. Lancet. 2001;357(9250):96-100. doi:10.1016/S0140-6736(00)03540-6

10. Fiorentino DF, Chung LS, Christopher-Stine L, et al. Most patients with cancer-associated dermatomyositis have antibodies to nuclear matrix protein NXP-2 or transcription intermediary factor 1γ. Arthritis Rheum. 2013;65(11):2954-2962. doi:10.1002/art.38093

11. Bonella F, Costabel U. Interstitial lung disease in idiopathic inflammatory myopathies. Curr Opin Pulm Med. 2014;20(5):445-451. doi:10.1097/MCP.0000000000000084

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13. Rothwell S, Cooper RG, Lundberg IE, et al. Dense genotyping of immune-related loci in idiopathic inflammatory myopathies confirms HLA alleles as the strongest genetic risk factor and suggests different genetic background for major clinical subgroups. Ann Rheum Dis. 2016;75(8):1558-1566. doi:10.1136/annrheumdis-2015-208119

14. Ladislau L, Suarez-Calvet X, Toquet S, et al. JAK inhibitor improves type I interferon induced damage: proof of concept in dermatomyositis. Brain. 2018;141(6):1609-1621. doi:10.1093/brain/awy105

15. Galindo-Feria AS, Albrecht I, Fernandes-Cerqueira C, et al. Proinflammatory Histidyl-Transfer RNA Synthetase-Specific CD4+ T Cells in the Blood and Lungs of Patients With Idiopathic Inflammatory Myopathies. Arthritis Rheumatol. 2020;72(1):179-191. doi:10.1002/art.41075

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17. Diederichsen LP, Simonsen JA, Diederichsen AC, Hvidsten S, Hougaard M, Junker P, Davidsen ML, Hansen V, Iversen LV, Hilberg O. Cardiac abnormalities in adult patients with polymyositis or dermatomyositis as assessed by noninvasive modalities. Arthritis Care Res (Hoboken). 2016;68(7):1012-1020. doi:10.1002/acr.22772

18. Gerami P, Schope JM, McDonald L, Walling HW, Sontheimer RD. A systematic review of adult-onset clinically amyopathic dermatomyositis (dermatomyositis siné myositis): a missing link within the spectrum of the idiopathic inflammatory myopathies. J Am Acad Dermatol. 2006;54(4):597-613. doi:10.1016/j.jaad.2005.10.041

19. Pipitone N, Notarnicola A, Levrì L, Morra E, Salvarani C. Do we really need muscle biopsy in suspected idiopathic inflammatory myopathies? Clin Exp Rheumatol. 2021;39(5):1153-1161.

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Last Reviewed: 2026-01-05 | MedVellum Editorial Team