Differentiation between inclusion body myositis and polymyositis

	Inclusion body myositis	Polymyositis
Sex	Male > female	Female > male
Age	Uncommon before 50	Common before 50
Onset	Insidious	Acute or subacute
Course	Slowly progressive	More rapid
Distribution of weakness	Usually asymmetric finger flexor and proximal leg weakness	Proximal, symmetric
Creatine kinase	Normal or <10x normal	Often >10x normal
EMG	Myopathic or mixed myopathic and neurogenic	Myopathic
Muscle biopsy	Inflammation, rimmed vacuoles, inclusions, p62 + inclusions, cytochrome oxidase negative fibers	Inflammation, fiber necrosis
Response to therapy	Generally poor	Expected
Autoantibodies	Anti-cytosolic 5' nucleotidase IA (NT5C1A)	No specific autoantibodies

EMG: electromyography.

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