Diagnosis

Dermatomyositis

Etiology

Cause

• The cause is unknown

• Genetic factors

• Immunologic factors

• Environmental triggers:

  • Infections (virus-induced autoimmunity): cellular proteins, breakdown of self-tolerance, an unmasking of previously hidden epitopes, autoantibody induced B cell activation, and molecular mimicry

    • Coxsackie B virus, enterovirus, parvovirus

  • Drugs:

    • antineoplastic drugs (hydroxyurea, cyclophosphamide), anti-infectious agents (penicillin, sulfonamides, isoniazid), NSAIDs (diclofenac, phenylbutazone), D-penicillamine, statins, and certain vaccines

  • UW Radiation

Pathophysiology [qbo]

1. A humoral attack against the muscle capillaries and small arterioles

2. Complement activation followed by the formation of membrane attack complex (MAC)

3. Skin lesions: Capillaries are destroyed

4. Muscular symptoms: Hypoxic injury to the muscle fibers --> microinfarction --> atrophy

   • Symmetric proximal skeletal muscle weakness

5. Extramacular manifestations:

   • Esophageal dysfunction

   • Interstitial lung disease

Complications [qbo]

• Respiratory failure: hypoventilation, aspiration pneumonia, or interstitial lung disease

• Malignancy

• Heart Disease: myocarditis, congestive heart failure, or coronary artery disease

• Esophageal Disease: dysphagia, malnutrition, aspiration

• Other: calcinosis, muscle atrophy, and contractures

Epidemiology

Incidence per 100.000 [spb][iat]

Prevalence per 100.000 [spb][iat]

Approach

• Blood tests: ANA, Myositis-specific antibodies, creatine kinase, aldolase, aspartate aminotransferase, lactic dehydrogenase

• ECG

• MRI

• Chest x-ray

• Barium swallow

• Ultrasound muscles

• EMG

• CT

• Muscle biopsy

• Skin biopsy

Treatment

1. Sun avoidance

2. Medications: Hydroxychloroquine, Corticosteroids, Methotrexate, Azathioprine, Cyclophosphamide, Cyclosporine, Mycophenolate mofetil, Leflunomide, Chlorambucil, Tacrolimus, intravenous immune globulin (IVIG), Rituximab (anti-CD20+ B cells).

Differential diagnoses

Amyotrophic lateral sclerosis, Drug side effects, Hypothyroidism, Lichen planus, Multicentric reticulohistiocytosis, Myasthenia gravis, Parapsoriasis, Pityriasis rubra pilaris, Polymorphic light eruption, Polymyalgia rheumatica, Polyneuropathy, Psoriasis, Rosacea, Sarcoidosis, Systemic lupus erythematosus, Tinea corporis