Table 2 Bohan and Peter criteria for the diagnosis of polymyositis (PM) and dermatomyositis (DM)

1. Proximal muscle weakness, usually symmetrical

2. Elevated serum muscle enzymes

3. Electromyographic abnormalities

 a. Common: myopathic potential – low amplitude, short duration and polyphasic action potentials

 b. Characteristic triad: (i) myopathic potentials; (ii) fibrillations, positive sharp waves, increased insertional activity; (iii) complex repetitive discharges

4. Muscle biopsy findings typical of PM or DM: necrosis, phagocytosis, regeneration, inflammation

5. Dermatological features of DM: Gottron’s sign or papules, or heliotrope rash

Definite: PM – four criteria without rash. DM – four criteria including rash

Probable disease: PM – three criteria without rash. DM – three criteria including rash

Possible disease: PM – two criteria without rash. DM – two criteria including rash