16- POLYMYOSITIS & DERMATOMYOSITIS

• Polymyositis (PM)


  A rare muscle disorder of unknown etiology in which there is inflammation and necrosis of skeletal muscle fibers
• Dermatomyositis (DM)


  When Polymyositis involves the skin, it is called Dermatomyositis
• Polymyositis


  • Immune-mediated (CD8)
  • Anti-SRP antibodies
• Dermatomyositis


  • Immune-mediated (B cell and CD4)
  • Anti-Mi2 antibodies
• Epidemiology


  • Affects adults and children
  • More common in women (2:1)
  • Peak age 40-50
• Polymyositis


  • Symmetrical progressive muscle weakness and wasting affecting the proximal muscles of the shoulder and pelvic girdle
  • Difficulty squatting, going upstairs, rising from a chair and raising the hands above the head
  • Involvement of pharyngeal, laryngeal and respiratory muscles can lead to head drop, dysphagia, dysphonia and respiratory failure (late)
  • Esophageal muscle involvement leading to dysphagia
  • Arthralgia, fatigue, morning stiffness, low grade fever
  • Raynaud's phenomenon
  • Normal sensation, reflexes, ocular
  • Histological heart involvement
  • Persistent low grade myoglobinuria (may lead to RF so we must hydrate)
  • ILD, arthritis, mechanic hands, Raynaud's phenomenon in Jo1 positive (anti-synthetase syndrome)
• Symptoms of DM


  • Same as PM
  • Characteristic skin changes:
  • Heliotrope (purple) discoloration of the eyelids
  • Scaly erythematous plaque over the knuckles (Gottron's papules)
  • V sign
  • Shawl sign
  • Periungual erythema
  • Raynaud's phenomenon
  • Subcutaneous calcifications
• Symptoms of DM


  Increased incidence of underlying malignancy (ovarian, breast, colon, NHL, melanoma)
• Muscle biopsy


  • Definitive test
  • Shows inflammatory cell infiltration and necrosis of muscle cells
• Serum muscle enzymes


  • Creatine kinase, aminotransferase, aldolase are elevated
• ANA


  • Positive in more than 75%
• Anti-JO antibodies (anti-tRNA synthetase)


  • Positive
  • More common in DM
• ESR


  • Usually not raised
• Electromyography (EMG)


  • Shows short duration, low amplitude polyphasic units on voluntary activation
  • Shows increased spontaneous activity with fibrillations, complex repetitive discharges, positive sharp waves
• MRI


  • Can demonstrate areas of muscle inflammation (must be very large)
• Biopsy


  • Should be proximal, deep, at least 4 cm chunk
  • Will show infiltration of myocytes by Th1 and cytotoxic T cells
• Diagnosis of PM & DM
• Oral prednisolone


  Treatment of choice: 0.5-1 mg/kg body weight continued for at least 1 month after myositis has become clinically and enzymatically inactive then tapered gradually down
• Other treatments


  • Anti-osteoporosis Tx
  • Immunosuppressive therapy (azathioprine, methotrexate, ciclosporin) if there is disease relapse on steroid tapering
  • IVIG every 6-8 weeks if you don't want steroids
  • Plasmapheresis if refractory