Polymyositis and Dermatomyositis

Symptoms

• Symmetric proximal muscle weakness — difficulty rising from a chair, climbing stairs, lifting arms overhead, combing hair
• Insidious onset over weeks to months
• Myalgia in 25-50 percent (less prominent than weakness)
• Dysphagia from pharyngeal muscle involvement
• Dyspnea from interstitial lung disease or respiratory muscle weakness

Signs / physical exam

• Symmetric proximal weakness on confrontation testing
• Dermatomyositis skin findings: heliotrope rash (violaceous discoloration of the eyelids), Gottron papules (scaly violaceous plaques over MCP and PIP joints), shawl sign, V-sign, holster sign, mechanic's hands, periungual erythema with nailfold capillary changes
• Calcinosis cutis in juvenile DM
• Subcutaneous edema in active DM

Classic findings

Middle-aged woman with several months of progressive symmetric proximal weakness, a violaceous (heliotrope) rash over the upper eyelids, and Gottron papules over the knuckles.

Diagnostic criteria

Bohan and Peter criteria (historic) and EULAR/ACR 2017 criteria. Classic features include symmetric proximal weakness, elevated muscle enzymes, characteristic EMG findings (small polyphasic motor unit potentials, fibrillations, positive sharp waves), and muscle biopsy demonstrating perivascular inflammation (DM) or endomysial CD8+ infiltrates (PM). Skin findings are mandatory for DM diagnosis.

Labs

• Creatine kinase (CK) — typically markedly elevated (10-50x normal), though may be normal in chronic disease and amyopathic DM
• Aldolase, LDH, AST, ALT (muscle origin) often elevated alongside CK
• ANA, myositis-specific antibodies (Jo-1, Mi-2, MDA5, TIF1-gamma, NXP2, HMGCR) and myositis-associated antibodies (Ro52, PM-Scl, Ku)
• TSH, CMP (electrolytes can affect CK and weakness interpretation)
• Age-appropriate cancer screening for adult DM (and PM to a lesser degree) — chest CT, mammography, pelvic ultrasound, colonoscopy, CA-125; consider PET-CT for high-risk antibodies (TIF1-gamma)

Imaging

• MRI of affected muscles — T2 hyperintensity reflecting edema; guides biopsy site
• High-resolution chest CT for interstitial lung disease, particularly if anti-Jo-1 or anti-MDA5 positive
• Pulmonary function testing including DLCO
• Echocardiogram if cardiac involvement suspected

First-line

• High-dose corticosteroids — prednisone 1 mg/kg/day (up to 60-80 mg) until objective improvement, then taper over 9-12 months
• Methylprednisolone pulses for severe weakness, dysphagia, or rapidly progressive interstitial lung disease
• Steroid-sparing agent introduced early to limit cumulative steroid exposure: methotrexate or azathioprine
• Physical therapy and home exercise to preserve function and prevent contracture
• Sun protection and topical corticosteroids or calcineurin inhibitors for DM skin disease

Second-line / adjunct

• Mycophenolate mofetil for refractory disease or ILD
• IV immunoglobulin (IVIG) for refractory or severe disease, particularly with dysphagia (PRODERM trial supports DM use)
• Rituximab for refractory PM/DM
• Calcium channel blockers and warfarin/aspirin for severe Raynaud or vasculopathy
• Cyclophosphamide for severe ILD or vasculitis