Diagnostic criteria of polymyositis:
Notes:
Myopathic muscle weakness, affecting symmetrical proximal muscles more than distal one and sparing eye and facial muscle, is characterized by a subacute onset (weeks to months) and rapid progression in patients who have no family history of neuromuscular disease, no endocrinopathy, no exposure to myotoxic drugs or toxins, and no biochemical muscle disease (excluded on the basis of muscle biopsy findings).
An adequate trial of prednisolone or other immunosuppressive drugs is warranted in probable cases. If in retrospect, the disease is unresponsive to therapy, another muscle biopsy should be considered to exclude other diseases or possible evolution in inclusion body myositis.
Needle EMG shows myopathic potentials characterized by short duration, low-amplitude polyphasic units on voluntary activation and increased spontaneous activity with fibrillation, complex repetitive discharges, and positive sharp waves.
Source:
- Harrison’s Principles of Internal Medicine 2 volumes 19th Edition: 2199 (2916), Table 388-2
- Davidsons Principles and Practice of Medicine 23rd edition: 1040 (1062)
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