Definition And Epidemiology

Etiology And Pathogenesis

Autoimmune vasculopathic inflammatory disease.
Affects capillaries of skin, muscles, and gastrointestinal tract.
Genetic susceptibility associated with human leukocyte antigen alleles DRB1_0301, DQA1_0501, and DQA1*0301.
Maternal microchimerism proposed as etiologic factor, resembling graft-versus-host disease.
Type I interferons mediate disease pathogenesis.
Endoplasmic reticulum stress leads to degradation of contractile proteins.

Rash develops as initial symptom in half of patients.
Heliotrope rash presents as blue-violet discoloration around orbit with periorbital edema.
Facial erythema typically crosses nasolabial folds.
Gottron papules present as bright-pink or pale, shiny, thickened plaques over metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joints.
Shawl sign demonstrates erythema over chest and neck.
Extreme photosensitivity causes generalized erythema in sun-exposed areas.
Mechanic's hands feature thickened, scaly, erythematous rash over palms and soles.
Nailfold capillaroscopy reveals dilated capillary loops, dropouts, and architectural disorganization.
Cutaneous ulcers develop on toes, fingers, or axillae due to severe vascular inflammation.

Insidious onset of symmetric proximal muscle weakness.
Selectively involves neck flexors, shoulder girdle, and hip flexors.
Patients demonstrate inability to perform sit-ups, head lag, and positive Gower sign.
Muscle tenderness present in half of affected children.

Pharyngeal muscle weakness causes dysphagia, dysphonia, and gastroesophageal reflux.
Respiratory muscle weakness manifests as hypercarbia and potential respiratory failure.
Gastrointestinal vasculitis causes mesenteric ischemia, crampy abdominal pain, gastrointestinal bleeding, and perforation.
Cardiac involvement includes pericarditis, myocarditis, and conduction defects.
Amyopathic dermatomyositis presents with classic rash without apparent muscle weakness.

Diagnosis requires classic rash and at least three signs of muscle inflammation.

Criteria Category	Specific Findings
Rash (Required)	Heliotrope rash of eyelids. Gottron papules.
Weakness	Symmetric proximal weakness.
Muscle Enzymes	Elevation of $\ge$ 1 enzyme (Creatine kinase, Aspartate transaminase, Lactate dehydrogenase, Aldolase).
Electromyography	Short, small polyphasic motor unit potentials. Fibrillations. Positive sharp waves. Insertional irritability.
Muscle Biopsy	Necrosis. Inflammation. Perifascicular atrophy.

Creatine kinase, aldolase, aspartate transaminase, and lactate dehydrogenase correlate with active inflammation.
Alanine transaminase frequently elevated initially while creatine kinase may remain normal.

Antinuclear antibody positivity seen in >80% of children.
Myositis-specific antibodies define distinct clinical subsets and predict prognosis.

Myositis-Specific Autoantibody	Clinical Association
Anti-p155/140 (TIF-1$\gamma$)	Severe cutaneous disease, photosensitive rashes, ulceration, lipodystrophy.
Anti-NXP2 (MJ)	Calcinosis cutis, muscle cramps, severe weakness, joint contractures, gastrointestinal ulceration.
Anti-MDA5	Interstitial lung disease, oral and cutaneous ulcers, arthritis, mild muscle involvement.
Anti-Jo-1 (Antisynthetase)	Antisynthetase syndrome, interstitial lung disease, mechanic's hands, arthritis.

Magnetic Resonance Imaging utilizes Short-tau inversion recovery sequence and T2-weighted images to identify active disease.
Imaging demonstrates symmetric hyperintensities and subcutaneous edema.
Muscle biopsy reserved for doubtful cases.
Histology reveals focal necrosis, phagocytosis of muscle fibers, perifascicular atrophy, endomysial proliferation, and tubuloreticular inclusion bodies within endothelial cells.

Condition	Distinguishing Features
Juvenile Polymyositis	Proximal muscle weakness lacks characteristic dermatological manifestations. Requires biopsy for diagnosis.
Systemic Lupus Erythematosus	Malar rash typically spares nasolabial folds. Anti-dsDNA or Anti-Smith antibodies present. Prominent renal involvement.
Overlap Myositis / MCTD	Mixed features of juvenile dermatomyositis, systemic lupus erythematosus, and scleroderma. High titer anti-U1 RNP.
Muscular Dystrophies	Progressive weakness without acute inflammatory skin changes or elevated inflammatory markers.

Corticosteroids act as mainstay of treatment.
Oral prednisolone initiated at 2 mg/kg/day.
Intravenous methylprednisolone pulse therapy (30 mg/kg/day, maximum 1 g, for 3-5 days) utilized for severe weakness, respiratory involvement, or life-threatening conditions.
Corticosteroids slowly tapered over 12 months.
Methotrexate serves as primary steroid-sparing agent.
Methotrexate administered weekly via oral, intravenous, or subcutaneous route at 15-20 mg/m$^2$.
Co-administered with folic acid 1 mg daily to reduce toxicity.
Intravenous Immunoglobulin (2 g/kg every 2-4 weeks) utilized for severe refractory disease or skin-predominant amyopathic disease.
Hydroxychloroquine (4-6 mg/kg/day) treats cutaneous manifestations and maintains remission.
Refractory disease managed with cyclophosphamide, mycophenolate mofetil, cyclosporine, rituximab, or Janus kinase inhibitors (tofacitinib).

Calcinosis cutis occurs in up to 50% of patients.
Characterized by dystrophic deposition of calcium phosphate or hydroxyapatite crystals in subcutaneous plaques.
Calcium extrusion results in painful cutaneous ulceration.
Highly associated with Anti-NXP2 antibodies.
Lipodystrophy causes progressive loss of subcutaneous and visceral fat over face and upper body.
Delay in diagnosis and treatment initiation strongly associated with development of severe complications.
Mortality rate reduced to approximately 1% with corticosteroid therapy.
Good functional outcome achieved in 75-80% of patients with long-term therapy.