[Approach to the differential diagnosis of inflammatory myopathies]

Inflammatory myopathies are a group of idiopathic diseases characterized by an inflammatory infiltrate of the skeletal muscles that includes Polymyositis, Dermatomyositis, and Inclusion Body Myositis, each one displaying distinctive histopathological, immunological, and pathogenic features. The diagnosis of Inflammatory Myopathies is suspected on the basis of clinical features and supported by evidence obtained from laboratory tests, plasma levels of muscle enzymes, detection of autoantibodies, electromyography and, recently, magnetic resonance and ultrasonographic image studies have been included into the diagnostic arsenal. A definitive diagnosis relies on the findings in the muscle biopsy, performed before treatment and preferably before severe muscle atrophy has developed. Precision in diagnosis plays a fundamental role in the evaluation of these patients, allowing conclusions to be drawn regarding the response to treatment and prognosis.