Key Facts

Dermatomyositis is the most recognisable inflammatory myopathy due to its characteristic skin manifestations. The association with malignancy mandates thorough cancer screening in adults.

Epidemiology

• Incidence: 1-2 per 100,000/year
• F:M 2:1; bimodal peak (children 5-15, adults 40-60)
• Malignancy association predominantly in adults >40

Aetiology

• Autoimmune: humoral (complement-mediated) immune response targeting endothelial cells
• Genetic: HLA-DRB1*0301
• Environmental: UV light, drugs (statins, penicillamine), infections
• Paraneoplastic: particularly anti-TIF1γ antibodies

Pathophysiology

• Complement-mediated microangiopathy → capillary dropout in muscle and skin
• Perifascicular atrophy on biopsy (B-cell and CD4+ T-cell infiltrate, perivascular)
• Distinct from polymyositis (CD8+ T-cell-mediated direct myofibre attack)
• IFN-driven pathway increasingly recognised

Skin (Often Precedes Muscle Disease)

• Heliotrope rash: lilac/purple discolouration of upper eyelids with periorbital oedema
• Gottron papules: violaceous, flat-topped papules over MCP, PIP, DIP extensor surfaces (also knees, elbows)
• Gottron sign: erythematous/violaceous rash over extensor surfaces without papules
• Shawl sign: V-shaped erythema over upper back/chest (photosensitive)
• Mechanic's hands: rough, cracked, hyperkeratotic skin on lateral fingers
• Nail fold changes: periungual erythema, dilated capillaries, cuticular overgrowth
• Calcinosis: especially in juvenile DM; subcutaneous calcium deposits

Muscle

• Symmetric proximal muscle weakness: difficulty rising from chairs, climbing stairs, lifting arms above head
• Gradual onset over weeks to months
• Myalgia in ~50%
• Pharyngeal/oesophageal weakness: dysphagia, aspiration risk
• Respiratory muscle weakness: dyspnoea

Systemic

• ILD: 20-40%; may be subclinical; anti-Jo-1 and anti-MDA5 associated
• Arthralgia/arthritis: non-erosive
• Cardiac: myocarditis, conduction defects (rare)
• Raynaud phenomenon: 20-30%

Red Flags

• Adult >40 with DM → malignancy screening essential
• Rapidly progressive ILD (anti-MDA5) → aggressive immunosuppression + consider rituximab/tacrolimus
• Dysphagia → aspiration risk; may need NG/PEG feeding

Bloods

• CK: elevated (often 5-50× ULN); most sensitive muscle enzyme
• LDH, AST, ALT, aldolase: also elevated (muscle-derived)
• Myositis-specific antibodies: anti-Mi-2 (classic DM), anti-MDA5 (amyopathic DM, ILD), anti-TIF1γ (malignancy), anti-NXP2 (calcinosis, malignancy), anti-Jo-1 (antisynthetase syndrome)
• ANA: positive in ~60%
• ESR/CRP: may be elevated
• FBC, U&Es, LFTs: baseline

Imaging

• MRI of proximal muscles (thighs most commonly): muscle oedema on STIR sequences; guides biopsy site
• HRCT chest: ILD screening (NSIP most common pattern)
• CT chest/abdomen/pelvis: malignancy screening

Neurophysiology

• EMG: myopathic pattern (short-duration, low-amplitude polyphasic motor unit potentials, fibrillations)

Biopsy

• Muscle biopsy: perifascicular atrophy (pathognomonic), perivascular inflammation (B cells, CD4+ T cells, complement deposits)
• Skin biopsy: interface dermatitis (can be similar to SLE)

Malignancy Screening (Adults >40)

• CT CAP, mammography, pelvic USS, PSA, colonoscopy (age-appropriate)
• Repeat annually for 3 years
• Anti-TIF1γ positive: particularly high malignancy risk (~60%)

Pharmacological

First-line:

• Prednisolone 1mg/kg/day (max 60mg): taper over months as CK normalises
• Methotrexate 15-25mg weekly: steroid-sparing (preferred for skin and muscle)
• Azathioprine 2-3mg/kg/day: alternative steroid-sparing agent

Refractory:

• IV immunoglobulin (IVIg): 2g/kg over 2-5 days, then monthly maintenance; particularly effective for skin
• Rituximab: for refractory disease (RIM trial showed benefit)
• Mycophenolate mofetil: for ILD
• Ciclosporin/tacrolimus: for refractory disease, ILD

ILD:

• Mycophenolate mofetil 2-3g/day: first-line for DM-ILD
• Anti-MDA5 ILD: aggressive combination (steroids + tacrolimus + cyclophosphamide; rituximab)
• Nintedanib: if progressive fibrosis

Skin:

• Sunscreen: photosensitive rash
• Hydroxychloroquine: for skin manifestations
• Topical steroids/tacrolimus: for localised skin disease

Non-pharmacological

• Physiotherapy: graded exercise programme once CK improving
• Speech and language therapy: if dysphagia
• Malignancy screening: at diagnosis + annually for 3 years

Referral Criteria

• All suspected inflammatory myopathy → rheumatology
• ILD → respiratory medicine
• Dysphagia → gastroenterology/ENT
• Malignancy identified → oncology

• 5-year survival: ~85-90% (improved with modern immunosuppression)
• Malignancy-associated DM: prognosis depends on underlying cancer
• Anti-MDA5 with rapidly progressive ILD: mortality 40-50% without aggressive treatment
• Juvenile DM: generally good prognosis; calcinosis is major long-term morbidity
• Inclusion body myositis (if initially misdiagnosed as DM): poor response to treatment
• Most patients achieve remission or good control on combination therapy
• CK levels correlate with disease activity and guide treatment

Myositis-Specific Antibodies

DM vs PM