Polymyositis

Diseases, ,

Clinical features:

Proximal muscle weakness; may be asymmetric. Muscle wasting is late
Shoulder girdle tenderness (may be absent)
Dysphagia & neck muscle weakness may occur
Spares face muscles & ocular muscles

Muscle biopsy:

Endomysial (within the fascicles): muscle fibre necrosis, lymphocytes (absence of eosinophils & plasma cells). Absence of rimmed vacuoles. Surrounding healthy fibres.
If chronic: increased connective tissue, positive reactivity with alkaline phosphatase
Immunohistochemistry: CD8+ T cell lymphocytes that are associated with myofibres that express MHC-1. MHC-1 expression persists even after corticosteroids.

Other points:

Part of antisynthetase syndrome (anti-tRNA synthetase): polymyositis, associated with:

  • Interstitial lung disease, Raynaud’s phenomenon, nonerosive arthritis, skin rash (mechanic’s hand)

Antisynthetase syndrome antibodies:

  • Anti-Jo1 (dermatomyositis or polymyositis), anti-PL-7, anti-EJ,

May be a Primary disease or secondary to:

  • Retroviral infections e.g. HIV
  • Overlap syndromes:
    • Limited cutaneous SSc +polymyositis: Anti-PM/Scl formerly anti-PM1
  • Anti-Ku

Polymyositis rarely affects children
CK, follows disease activity: Elevated

Nerve conduction studies/electromyography NCS/EMG:

MUAP Myopathic recruitment On activation: Short duration, low amplitude polyphasic units
Spontaneously: increased activity with fibrillations, complex repetitive charges & positive sharp waves PSW

Investigations to consider:

  • FBC, blood chemistry panel
  • CK: usually increased, marker of activity
  • EMG: myopathic changes
  • Antibodies:
    • Anti-Jo-1: dermatomyositis
    • Anti PM/Scl: polymyositis systemic sclerosis overlap syndrome
  • HIV testing
  • Muscle biopsy
  • CT chest, abdomen & pelvis

Monitor:

  • Weakness not CK

Treatment:

General measures:

  • Physical therapy to prevent contractures
  • Rest during exacerbations

Glucocorticoids:

  • Prednisolone +/- potassium supplements +/- antacids
  • High dose, then taper over 10 weeks, Maintenance for 2-3 yrs
  • If resistant (>3 months on glucocorticoids) or rapid:
  • Azathioprine P.O.
  • Methotrexate weekly

Plasmapharesis
IVIG
 

Causes of Polymyositis:

Idiopathic
Secondary:

  • Zidovudine
  • Penicillinamine
  • Overlap syndrome with other connective tissue disease

DDx. of polymyositis:

Chronic weakness:

  • Dermatomyositis
  • Inclusion body myositis
  • Motor neuron disease (amyotrophic lateral sclerosis and others)
  • Spinal muscular atrophy
  • Endocrine myopathy:
    • Cushing’s syndrome
    • Hyper or hypo- thyroidism
    • Hyper- or hypo- parathyroidism
  • Drug induced myopathy:
    • True myositis:
      • Penicillamine, Procainamide
      • Zidovudine (mitochondrial myopathy)
    • Glucocorticoids
    • Statins: Lovastatin, simvastatin
    • Fibrates: Clofibrate
  • Metabolic myopathy:
    • Some Glycogen storage diseases, Some Lipid storage diseases, Some mitochondrial myopathy
  • Fascioscapulohumeral muscular dystrophy
  • Paraneoplastic neuromyopathy

Acute weakness:

  • Alcoholism
  • Neuropathy
  • Some Glycogen storage diseases
  • Parasitic polymyositis
  • Pyomyositis a.k.a. Tropical polymyositis
  • Other bacteria:
    • Borrelia burgdorferi ‘Lyme disease’
    • Legionella pneumophila ‘Legionnaire’s disease’
  • Pain and muscle tenderness:
    • Polymyalgia rheumatica
    • Fibromyalgia
    • Chronic fatigue syndrome

 

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