About Myositis

Myositis is an inflammation of the muscles.

  • Myositis abbreviations to know:
    • PM: Polymyositis
    • DM: Dermatomyositis
    • JDM: Juvenile dermatomyositis
    • IBMP: Inclusion body myositis
    • IMNM: Immune mediated necrotizing myositis
    • CADM: Clinically amyopathic dermatomyositis
    • ILD: Interstitial lung disease
  • Ages of onset:
    • DM: Bimodal
      5-15 yo, 45-65 yo
    • PM: 50-60 yo
    • IBM: >50 yo

  • Dermatomyositis > polymyositis can be associated with malignancy

  • Symmetrical, proximal muscle weakness= hallmark of inflammatory myositis. If pain >>> weakness: consider alternate etiology (rhabdo, endocrinopathy)
  • Classic rash distributions (Gottron’s papules over dorsum knuckles, heliotrope rash, etc.) with muscle weakness should prompt work up for dermatomyositis
  • Vast majority of JDM cases have skin involvement
  • HyperCKemia doesn’t equate to myositis – different sexes and races can have different “normal limits” also.
    • Exercise and dehydration can also impact CK levels.
  • Inclusion body myositis is most common myositis of elderly.
    • Remember distal and proximal muscle weakness with neuropathic features
  • Dermatomyositis and antibodies to NXP and TIF1gamma in adults highly associated with malignancy, but dermatomyositis rarely associated with cancer in children

Proximal muscle WEAKNESS >> pain

Clinical Phenotypes

  • PM
  • DM: Myositis with rash
  • CADM: Typical skin rashes of DM, without weakness
  • JDM: Juvenile onset
  • IBM:
    • Proximal+distal mm
    • Asymmetrical
    • Inflammation and degeneration
  • IMNM:
    • Necrotizing myositis
    • HMG-CoA reductase antibody in >90%, often with statin use

DM Rashes

Antisynthetase Syndrome

  • Myositis + skin (mechanic hands) + interstitial lung disease (view images)

Other Organs

  • Pulm (interstitial lung)
  • GI (dysphagia)
  • Cardiac (myocarditis)

Lab Workup

  • Creatine kinase high
  • Aldolase high(some)
  • ESR/CRP high
  • AST/ALT elevation
  • Myglobinuria 
  • +ANA in a subset 
  • Myositis-specific antibodies per rheumatologist

Imaging

  • Muscle MRI with contrast can show muscle edema of affected muscles
  • Chest CT if interstitial lung disease is suspected
    • ILD CT protocol = high resolution, prone with deep inspiration

Additional Tests

  • EMG shows myopathic changes
    • High sensitivity, low specificity
  • Muscle biopsy can be diagnostic.
  • Swallow study if dysphagia is reported.

Rheumatic

  • Polymyalgia rheumatica
  • Overlap with SLE, MCTD, etc.

Endocrinologic

  • Diabetes
  • Hypo/Hyper- thyroidism

Viral Infections

Neuromuscular

  • Muscular dystrophies
  • Neuromuscular junction disorder
  • Denervating disorders

Metabolic myopathy

Rhabdomyolysis

Drug Toxicity

  • Glucocorticoids
  • Statins
  • Colchicine
  • Cocaine, Alcohol

Fibromyalgia

Initial Treatment

  • Steroids: PO/IV Prednisone equivalent to 1-2 mg/kg/day, then slow taper over months
  • IVIg

Long-term Immunosuppression per Rheumatology

PT

Physical therapy and rehabilitation are very important to continue long-term.

Monitoring

  • Labs to follow:
    • CK/aldolase
    • CRP
  • Muscle strength
  • Monitor for signs of lung siease
    • Follow up with PFTs + lung imaging
  • Screen for underlying malignancy with age appropriate screening and SPEP/UPEP (esp for dertmatomyositis)

Prognosis

  • IBM and IMNM tend to have more difficult response to treatment