Chapter 20 Inflammatory myopathies

Abstract

Inflammatory myopathies are a heterogeneous group of disorders characterized by muscle weakness, elevated serum creatine kinase levels, electrophysiologic abnormalities, and inflammation on muscle biopsy. Inflammatory myopathies are traditionally divided into idiopathic and secondary etiologies. The three major categories of an idiopathic inflammatory myopathy are polymyositis, dermatomyositis, and inclusion body myositis. Electromyography (EMG) findings in inflammatory myopathies are neither sensitive nor specific. In patients with either mild, or treated, disease, the EMG can be normal. Because of the patchy nature of the disease, the investigation of multiple muscles may be necessary to document sufficient abnormalities for a diagnosis. Although characteristic EMG abnormalities have been documented, none of these are specific to inflammatory myopathies. The electromyographic abnormalities associated with inflammatory myopathies do not differentiate one from the other, nor do they differentiate inflammatory muscle disease from other myopathic processes. Nonetheless, in conjunction with a thorough clinical history and exam, they can be helpful in supporting or excluding the clinical suspicion of an inflammatory myopathy.