Myositis

Lab Workup

• Creatine kinase high
• Aldolase high (some)
• ESR/CRP high
• AST/ALT elevation
• Myoglobinuria
• +ANA in a subset 
• Myositis-specific antibodies such as Mi2, MDA5, NXP2, PM-SCL, SRP, TIF-1, anti-HMG CoA reductas per rheumatology

Imaging

• Muscle MRI can show muscle edema of affected muscles
• Chest CT if ILD is suspected
  • ILD CT protocol = high resolution, without contrast, supine and prone views with deep inspiration and expiration
• Cardiac imaging (TTE, cardiac MRI, cardiac PET) to assess for cardiac muscle involvement if myocarditis is suspected

Additional Tests

• EMG shows myopathic changes
  • High sensitivity, low specificity
• Muscle biopsy can be diagnostic
• Swallow study, if dysphagia is reported
• NIFs/FVC to assess respiratory musculature
  • Further evaluation can be performed with MEP and MIP in full spirometry to differentiate extra thoracic from intrathoracic cause of restrictive lung involvement (i.e. ILD vs diaphragmatic involvement from myositis)

Rheumatic

• Polymyalgia rheumatica
• Overlap with systemic lupus erythematosus, mixed connective tissue disease, systemic sclerosis, etc.

Endocrinologic

• Diabetes mellitus
• Hypo-/hyper-thyroidism
• Adrenal insufficiency

Viral Infections

Neuromuscular

• Muscular dystrophies
• Neuromuscular junction disorder
• Denervating disorders
• Hereditary myopathies that can be adult onset (e.g., desferlinopathies)

Metabolic Myopathy

Rhabdomyolysis

Drug Toxicity

• Glucocorticoids
• Statins
• Colchicine
• Cocaine, alcohol

Electrolyte Imbalances

• Hyper/hypokalemia
• Hyper/hypocalcemia

Fibromyalgia

Initial Treatment

• Steroids: PO/IV prednisone equivalent to 1-2 mg/kg/day, then slow taper over months
• IVIG in select cases ((life threatening weakness, oropharyngeal dysphagia, respiratory involvement)

Long-Term Immunosuppression per Rheumatology

• Azathioprine
• Methotrexate
• Mycophenolate
• Cyclophosphamide
• Rituximab
• Tacrolimus
• JAK-inhibitors – dermatomyositis only

PT

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

Monitoring

• Labs to follow:
  • CK/aldolase
  • CRP
  • Live enzymes
• Muscle strength
• Monitor for signs of lung disease
  • Follow up with PFTs + lung imaging
• Screen for underlying malignancy based on age-recommendations and risk stratification to the International Guideline for Idiopathic Inflammatory Myopathy-Associated Cancer Screening
  • High risk factors = dermatomyositis, anti-TIF1 gamma antibodies, anti NXP2 antibodies, age >40 yo at IIM onset, persistently high disease activity, mod-severe dysphagia, cutaneous necrosis

Prognosis

• IBM and IMNM tend to have more difficult response to treatment.
• There are currently no FDA approved therapies for IBM; patients tend to develop slowly progressive functional limitations over time.

• 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 workup 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”
  • Exercise and dehydration can also impact CK levels
• Inclusion body myositis:
  • Most common myositis of elderly
  • Insidious weakness of hand flexors (FDP) and knee extensors (quadriceps); findings can be asymmetric initially
  • Remember distal and proximal muscle weakness with neuropathic features
• Dermatomyositis and antibodies to NXP and TIF1 gamma in adults highly associated with malignancy, but dermatomyositis rarely associated with cancer in children