2026 Workshop Handout Bundle-W55C - Myopathies EDX Approach

W55C - Myopathies EDX Approach - Kushlaf

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The electrodiagnostic (EDX) evaluation of suspected myopathy should be tailored to the patient’s symptoms and examination, including the pattern of weakness. EDX can support a myopathic process, assess its severity and distribution, and help select a muscle for biopsy, but it generally cannot establish the specific diagnosis. A normal study does not exclude myopathy, as can occur in steroid myopathy.<br /><br />Evaluation usually begins with nerve conduction studies. Sensory responses are generally normal unless another disorder, such as polyneuropathy, is present. Motor studies are often normal, though abnormalities may occur in myopathies affecting distal muscles. In critical illness myopathy, compound muscle action potentials may be low in amplitude and prolonged in duration, typically without the temporal dispersion seen in demyelinating neuropathy.<br /><br />Needle EMG should sample several muscles because myopathic involvement can be patchy or selective. Weak muscles should be examined; thoracic paraspinals can be informative in early or axial myopathy. Findings may include abnormal spontaneous activity and short-duration, small-amplitude, often polyphasic motor unit action potentials, with early recruitment. Chronic myopathies can also show long-duration, high-amplitude units from reinnervation, sometimes mixed with myopathic units.<br /><br />Clinical context is essential: some findings can resemble those of other disorders, and targeted testing—such as exercise testing or repetitive nerve stimulation—may be needed. EDX findings should be interpreted alongside clinical assessment and, when indicated, muscle biopsy.

Keywords

electrodiagnostic

myopathy

nerve conduction studies

ncs

needle EMG

muscle biopsy

motor unit action potentials

critical illness myopathy

steroid myopathy

polyneuropathy

muscle weakness