Basic principles in EMG

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The primary goals of the EMG study are to localize the lesion, characterize the underlying nerve pathophysiology, quantitate the severity of the lesion, and assess the temporal course of the disorder (Preston et al., 1998).

Localizing a lesion within the peripheral nervous system is best determined with the use of an EMG study. The EMG study can be tailored in such a fashion as to specifically localize the lesion to the nerve roots, plexus, trunks, or individual peripheral nerves. The clinician designing the EMG study must have intimate knowledge of the anatomy of the peripheral nervous system for precise lesion localization.

EMG studies can often identify the underlying pathologic process involving a nerve lesion. The EMG study can determine whether the pathology leading to the clinical deficit is secondary to axonal loss, demyelination or if the underlying disorder is secondary to muscle disease or neuromuscular dysfunction. This differentiation between pathologic processes allows for a narrowing of the differential diagnosis.

The EMG study can also assess the degree or extent of axonal loss versus demyelination, which then allows the clinician to predict the extent of recovery from a particular lesion and the expected time frame in which this recovery should take place. A lesion that primarily involves axonal loss will carry a worse prognosis and therefore a less complete recovery would be expected. A lesion that is the result of demyelination can be expected to recover fully when given the required time for remyelination. Lesions with mixed pathology will recover in an intermediate time frame.

The EMG study can provide data on the temporal course and rate of recovery of a lesion. Abnormalities seen on the EMG can separate lesions into hyperacute

(less than 1 week), acute (less than a few weeks), or chronic. The electromyographer must therefore be aware of the patient's clinical time course to accurately interpret the abnormalities observed during the study.

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