Electromyography

Nerve Conduction Studies

NCS allow for noninvasive evaluation of the characteristics of peripheral nerves. Some of the characteristics evaluated include onset of response (latency), amount of response (amplitude), and speed of response (velocity).

Recording electrodes are placed over the area of interest, and stimulation is given at a predetermined distance. Data collected include latency, amplitude, and velocity. These data can then be compared with control values (“normal” values). Generalized controls are published in a variety of texts, but each EMG lab will ideally establish its own controls based on its equipment and testing methods. The above characteristics can be affected by temperature, age, gender, and height, all of which must be taken into account when grading the data.

Testing can be further divided into the study of sensory and motor nerves. There are multiple methods and sites to test the peripheral nerves. This helps eliminate false-negative or-positive testing and ensure accuracy of results. Many states allow [Page 424]physicians as well as qualified (and in some cases certified) nonphysicians, such as physical therapists or chiropractors, to perform nerve conduction testing.

The electrical stimulation lasts less than 1 millisecond and is typically felt as an “electric shock.” It is generally not regarded as painful but rather as an unpleasant stimulation.

Needle Examination

EMG, or needle testing, quantifies the electrical potential of muscles at rest and muscles in action. Muscles at rest should be electrically silent and when stimulated can be studied to determine the type of pathology. Muscle is also tested with a slight and then full contraction for further characterization. The contraction produces a motor unit action potential (MUAP). This unit of muscle activity can be analyzed for its speed of firing, size of contraction, and duration of firing. These data can help determine whether pathology is originating from the nerve or muscle. The size of the MUAP can help characterize the onset of findings, grossly measured as newer or older than 6 months.

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