Clinical Approach to Muscle Diseases

Carlayne E. Jackson, M.D.

Disclosures

Semin Neurol. 2008;28(2):228-240. 

In This Article

Electrophysiological Studies

Electrodiagnostic studies, consisting of both nerve conduction studies (NCS) and electromyography (EMG), should be part of the routine evaluation of a patient with a suspected myopathy.[20] These studies are helpful in confirming that the muscle is indeed the correct site of the lesion and that weakness is not the result of an underlying motor neuron disease, neuropathy, or neuro-muscular junction disorder. Nerve conduction studies are typically normal in patients with myopathy. Needle EMG examination showing evidence of brief duration, small amplitude motor units with increased recruitment can be extremely helpful in confirming the presence of a myopathy. Needle EMG can also provide a clue to which muscles have had recent or ongoing muscle injury and can be a guide to which muscle to biopsy. It is important to realize, however, that the EMG can be normal in a patient with myopathy, and the results of electrodiagnostic studies need to be evaluated in the context of the patient's history, neurological examination, and other laboratory studies. For example, there are some diseases that mainly produce a myopathy but that may also be associated with a subclinical neuropathy such as mitochondrial disorders, myotonic dystrophy, and inclusion body myositis.

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