Definition (Curr Rheumatol Rep 2011;13:216)
• Dermatomyositis: Weakness in proximal muscles and pathognomonic skin rashes
• Polymyositis: The skin is spared polymyositis; very rare in childhood
Pathophysiology (Pediatr Rev 1990;12:117; Pediatr Rev 1984;6:163)
• Dermatomyositis: 2/2 formation of thrombi from lesions of endothelial cells of intramuscular capillaries, arterioles, and veins resulting in infarct of muscle
• Perivascular infiltrates generally present on biopsy. Vasculitic involvement
• Polymyositis: Inflammatory infiltrates, including lymphocytes and plasma cells, in perivascular regions and w/ muscle fascicles; bx may show muscle necrosis and phagocytosis, but endothelial cells of the blood vessels are normal
Epidemiology
• Dermatomyositis is the most common idiopathic inflam myopathy of childhood (80% cases)
• Bimodal distribution: 5–14 yo (avg age 7 yr) and 45–64 yo; 2:1 F:M predominance
• Combined freq 1–3.2 cases per 1,000,000 children <17 yo; Caucasian > Hispanic > AA
• Dermatomyositis is 10–20× more common than polymyositis
Criteria for Diagnosis (Curr Rheumatol Rep 2011;13:216)
• Exclusion of other rheumatic diseases
• Dermatomyositis: Characteristic rash (heliotrope dermatitis and Gottron’s papules) and 3 of the following, sym prox muscle weakness, ↑ muscle enzymes, muscle histopathology on bx; EMG w/ denervation & myopathy. MRI w/ contrast often used in place of EMG/biopsy in typical cases. Pyomyositis essentially as above but w/o dermatologic findings
Clinical Manifestations (Curr Rheumatol Rep 2011;13:216)
• Gottron’s rash (91%), Heliotrope rash (83%), nail fold capillary Δs (80%), malar/facial rash (42%), myalgia/arthralgia (25%), dysphonia or dysphagia (24%); only 16% p/w fever
• Weakness usually prox and symmetrical (hip and shoulder girdles), LE > UE (difficulty climbing up stairs, getting up from chair, or combing one’s hair)
• Motor weakness seen in ∼100% of pts; DTRs may be absent, nml, or hyperactive
• Extraocular and facial muscles almost never involved but do see dysphagia/dysphonia
• Skin manifestations: Erythematous areas over MCP and IP joints (Gottron’s papules), violaceous scaly rash often w/ lichenification, and violaceous discoloration of upper eyelids w/ periorbital edema (heliotrope rash), nail fold capillary involvement, can also see ulcerative disease (<10% and predicts a severe course of illness & persistent weakness)
• Lipodystrophy develops in 14–25%, hypertriglyceridemia & insulin resistance (50%)
• Vasculopathy can affect any part of GI tract and 1/3 pts w/ pulmonary involvement 2/2 respiratory muscle weakness
Diagnostic Studies (Pediatr Rev 1990;12:117; Pediatr Rev 1984;6:163)
• Look for genetic disorders in family, which excludes dermatomyositis/polymyositis
• ↑ in muscle enzymes (CPK, aldolase, LDH, SGOT, SGPT); ↑ CPK is the most frequently seen. Inc vWF ag sugg active vasculitis
• MRI w/ contrast of quadriceps useful to eval for muscle inflammation
• Abn EMGs, nml nerve-conduct studies; fibrillation potentials, sharp waves at rest
• Short duration and abundant single motor unit potentials on min volitional mvmt
• High-frequency bizarre discharges
• Muscle bx generally of biceps, quadriceps, or deltoid muscle; abn in ∼90%
• Findings include perivascular infiltrates, basophilia, moth-eaten appearance of many muscle fibers, and +alkaline–phosphatase reaction—not always done
• Skin bx can be done if normal mm enzymes
• Ancillary studies including ESR, neopterin, PFTs, UA, or HLA B8 may be helpful
• DMS in children not assoc with inc malignant risk as in adults
Management (Curr Rheumatol Rep 2011;13:216; Lancet 2008;371:2201)
• Prednisone 1st line, usually w/ initial induction phase w/ high dose of steroids followed by a lower dose for mtnce; Rx for 2–6 mo. Hydroxychloroquine for skin
• One suggested regimen; 2 mg/kg IV divided q8 × 6 wk, when improved strength/rash and normalized muscle enzymes taper to bid then daily and tapered 10% q2wk
• W/ dysphagia/dysphonia, pulmonary or GI dz then IV methylpred pulses
• Often methotrexate added as steroid-sparing agent. IVIG in severe cases
• Rituximab failed to show benefit in RCT, however has helped some patients. Cyclophosphamide & cyclosporine effective
Complications (Pediatr Rev 1990;12:117; Pediatr Rev 1984;6:163)
• Untreated pts can develop respiratory involvement, GIB, joint contractures, severe muscle wasting, calcinosis, and death
• Calcinosis can occur if unRx’d or Rx’d → pain, ↓ motion, skin ulcer, and abscesses
• Generally 4 diff patterns: Small nodules on skin, subcutaneous masses usually near joints, sheet-like deposits in fascial planes, and a pattern resembling an exoskeleton
• Relapses reported in pts w/ dermatomyositis several yrs after treatment
• Steroid-induced complications
• Dz can be monophasic or chronic; monophasic often can do well
• Outcome does not appear to correlate with age or sex; since introduction of steroids mortality rate <10% w/ improved functional outcomes