TABLE 1

Classifications of idiopathic inflammatory myositis

Classification	Type of myositis	Subtype	Required assessments^#	Description
Bohan and Peter's criteria	PM	Definite PM	2	Classical PM
Bohan and Peter's criteria		Probable PM	2
		Possible PM	2
	DM	Definite DM	2	Classical DM
		Probable DM	2
		Possible DM	2
	Overlap myositis	CTM	2	Myositis with another CTD, irrespective of DM phenotype
	CAM	CAM	2	Myositis associated with cancer within 3 years
ENMC criteria	PM	Definite PM	1	Classical PM
		Probable PM	3	Classical PM
	DM	Definite DM	4	Classical DM
		Probable DM	3	Classical DM
		Amyopathic DM	3	DM rash, positive skin biopsy
		Possible DM sine dermatitis	3	Typical biopsy without cutaneous involvement
	Immune mediated necrotising myopathy		3	Necrotic muscle fibres, sparse inflammatory cells, MAC deposition on vessels at biopsy
	Nonspecific myositis		3	No DM rash, nonspecific abnormalities at biopsy
Troyanov's criteria	PM	Pure PM	5	Myositis without extramuscular signs or antibodies
	DM	Pure DM	5	DM as the first manifestation, rashes persist until diagnosis, high cutaneous score, DM-specific antibodies
		Overlap myositis with DM features	5	Myositis as the first manifestation, low cutaneous score, no DM-specific antibodies
		Adermatomyopathic DM	5	DM rash or calcinosis, positive skin biopsy
		Clinically amyopathic DM	5	Amyopathic or hypomyopathic DM
	Overlap myositis	Overlap myositis	5	Myositis with at least one clinical overlap feature and/or an overlap autoantibody
	CAM	CAM	5	Clinical paraneoplastic features without an overlap autoantibody or anti-Mi-2

ENMC: European neuromuscular centre; PM: polymyositis; DM: dermatomyositis; CTM: myositis with other defined connective tissue disease; CTD: connective tissue disease; CAM: cancer associated myositis; MAC: membrane attack complex. ^#: the assessments are as follows. 1: physical examination, creatine phosphokinase and muscle biopsy; 2: assessments listed in 1 plus electromyography; 3: assessments listed in 2 plus muscular magnetic resonance imaging and autoantibodies; 4: physical examination and biopsy; 5: assessments listed in 2 plus autoantibodies.