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
- True myositis:
- 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