= most common inflammatory myopathy with diffuse nonsuppurative inflammation of striated muscle + skin

Cause: cell-mediated (type IV) autoimmune attack on striated muscle

Pathophysiology: damaged chondroitin sulfate no longer inhibits calcification

Path: atrophy of muscle bundles followed by edema and coagulation necrosis, fibrosis, calcification

Histo: mucoid degeneration with round cell infiltrates concentrated around blood vessels

Age: bimodal: 5–15 and 50–60 years; M÷F = 1÷2

• elevated muscle enzymes (creatinine kinase, aldolase)
• myositis-specific autoantibodies: anti-Jo-1
  1. anti-aminoacyl-tRNA synthetase
     • arthritis, Raynaud phenomenon, fever, fatigue
     • interstitial lung disease
       Prognosis: requires prolonged treatment
  2. anti-Mi-2 antibodies:
     • V-shaped chest rash (= shawl rash)
     • cuticular overgrowth
       Prognosis: good response to medication
  3. anti-signal recognition particle antibodies
     • abrupt onset myositis ± heart involvement

• Skeletal musculature
  Location: thigh (vastus lateralis + intermedius m. with relative sparing of rectus + biceps femoris m.) >pelvic girdle >upper extremity >neck flexors >pharyngeal muscles
  • bilateral symmetric edema in pelvic + thigh muscles
  • fatty infiltration + muscle atrophy (over months to years)
  • sheetlike confluent calcifications in soft tissues of extremities (quadriceps, deltoid, calf muscles), elbows, knees, hands, abdominal wall, chest wall, axilla, inguinal region) in 75%
• Skeleton
  • pointing + resorption of terminal tufts
  • rheumatoid-like arthritis (rare)
  • “floppy-thumb” sign
  
  Cx: flexion contractures; soft-tissue ulceration
• Chest
  • aspiration pneumonia is the most common occurrence ← pharyngeal muscle weakness
  • hypoventilation + respiratory failure ← respiratory muscle weakness
  • disseminated pulmonary infiltrates (reminiscent of scleroderma)
  • diaphragmatic elevation with reduced lung volumes + basilar atelectasis
  • interstitial fibrosis (5–30%), most severe at lung bases:
    • may precede development of myositis
  • associated with presence of anti-Jo-1 antibodies
  
  Patterns: NSIP, cryptogenic organizing pneumonia, UIP, diffuse alveolar damage
  • fine reticular pattern progressing to coarse reticulonodular pattern + honeycombing
  
  HRCT:
  • predominantly linear abnormalities + ground-glass attenuation
  • air-space consolidation in middle + lower lung zones with peribronchial + subpleural distribution
• Myocardium
  • changes similar to those of skeletal muscle involvement
• GI tract
  • dysphagia ← progressive weakness of proximal striated m.
  • atony + dilatation of esophagus
  • atony of small intestines + colon

Clinical forms:

1. ACUTE FORM = childhood-onset form
   • fever, joint pain, lymphadenopathy, splenomegaly, subcutaneous edema
   • more severe dermatomyositis
   
   Prognosis: death within a few months
2. CHRONIC FORM = adult-onset form
   = insidious onset with periods of spontaneous remission and relapse
   • low-grade fever, muscular aches + pains, edema
   • muscle weakness (1^st symptom in 50%) ← active inflammation, necrosis, muscle atrophy with fatty replacement, steroid-induced myopathy
   • skin erythema (1^st symptom in 25%): heliotrope rash (= dusky blue-purple erythema of eyelids) with periorbital edema, Gottron papules (= raised scaly violaceous rash over knuckles, major joints and upper body)

Cx: increased prevalence of malignant neoplasms of lung, breast, prostate, ovary, GI tract, kidney

Dx: measurement of serum muscle enzyme concentration; electromyography; muscle biopsy (normal in up to 15%)