What is dermatomyositis?

Dermatomyositis: Definition and Clinical Features

Dermatomyositis is a rare systemic inflammatory disease characterized by proximal muscle weakness and distinctive skin manifestations, including an erythematous photosensitive rash, Gottron papules, periorbital edema, heliotrope rash, and periungual telangiectasias. 1

Key Clinical Features

• Cutaneous manifestations include characteristic rashes involving the face, neck, torso, fingers, and extensor surfaces of extremities 1
• Symmetric proximal muscle weakness develops over weeks to months with elevated muscle enzyme levels 1
• Electromyographic abnormalities include polyphasic motor unit action potentials of short duration and low amplitude with increased insertional and spontaneous activity 1
• Multiple organ systems may be involved, including vascular, pulmonary, gastrointestinal, and cardiac systems 1
• Dysphagia related to cricopharyngeal weakness or spasm is a common manifestation 1

Disease Subtypes

Adult Dermatomyositis

• Typically affects individuals between 45-64 years of age 2
• Associated with malignancy in approximately 20% of adult cases 3
• May present with interstitial lung disease, particularly nonspecific interstitial pneumonitis 1

Juvenile Dermatomyositis (JDM)

• Affects children younger than 18 years 1
• Characterized by proximal muscle weakness, calcinosis cutis, cutaneous vasculitis, ulcerations, and vasculopathy affecting the gastrointestinal tract 1
• Higher frequency of neck muscle involvement and subcutaneous calcifications compared to adult DM 2
• Generally has better outcomes than adult-onset disease 2

Amyopathic and Hypomyopathic Dermatomyositis

• Amyopathic DM presents with classic rash without evidence of muscle inflammation 1
• Hypomyopathic DM presents without muscle weakness but may show evidence of muscle inflammation through mild CK elevation, EMG abnormalities, or MRI findings 1
• Both subtypes can be associated with interstitial lung disease and malignancy 1

Pathogenesis

• Multifactorial disease influenced by genetic, environmental, and immunological factors 3
• Immune response characterized by activation of innate and adaptive immune mechanisms 3
• Strong activation of type I interferon pathway 3
• Myositis-specific autoantibodies define subgroups and predict extramuscular organ involvement and long-term prognosis 1
• Triggers may include viral infections, cancer, connective tissue diseases, certain medications, and UV radiation 4

Diagnostic Approach

• Magnetic resonance imaging (T1-weighted, T2-weighted, and sequences using fat suppression techniques) provides useful information to identify muscle inflammation, guide biopsy site selection, and monitor treatment response 1
• Myositis-specific autoantibodies help define clinical phenotypes and predict outcomes 1, 3
• Muscle biopsy remains the gold standard for diagnosis, showing inflammatory infiltrates and characteristic patterns of muscle damage 5
• Screening for associated malignancy is essential in adult patients 3

Treatment

• For adult patients, initial treatment typically includes high-dose corticosteroids (1.5-2.0 mg/kg/day) concurrent with a steroid-sparing agent 1, 4
• Steroid-sparing agents include methotrexate, azathioprine, or mycophenolate mofetil 1
• For juvenile dermatomyositis without severe complications, corticosteroids at 2 mg/kg (maximum 60 mg/day) with subcutaneous methotrexate (15 mg/m² weekly) is recommended 1
• For severe disease with extensive extramuscular involvement, high-dose methylprednisolone plus intravenous immunoglobulin, cyclophosphamide, rituximab, or cyclosporine may be necessary 1
• Newer therapies include biologics (anti-CD20, anti-TNFα, anti-interferon antibodies) and JAK inhibitors 4, 3

Prognosis

• Treatment response rates have improved from 50% to 90% with the use of high-dose corticosteroids 4
• Juvenile DM generally has better outcomes than adult-onset disease 2
• Early diagnosis and treatment are essential for improving prognosis 3
• Autoantibody profiles, myositis subtype, overlap with other collagen vascular disorders, presence of malignancy, and patient age all impact course and prognosis 4