Primary muscle diseases

This chapter on primary muscle diseases explains how analysis of compound muscle action potential (CMAP) amplitude, abnormal spontaneous activity on needle electromyography (EMG), and motor unit action potentials (MUAP) characteristics may be used to give an indication of pathophysiological processes, and goes on to describe the combination and distribution of abnormalities that may be expected in the more commonly encountered myopathies. The conditions considered in detail are inflammatory myopathy (including myositis), critical illness myopathy, disorders with myotonia, inherited myopathy (including muscular dystrophy), and endocrine, metabolic and toxic disorders. Each of these has a characteristic combination of CMAP, spontaneous EMG, and MUAP findings, but the systematic approach to clinical neurophysiology as a way of understanding muscle pathophysiology can be used to investigate the myriad of rare myopathies that may be encountered in clinical practice.