The three components of electrodiagnosis useful in evaluation of the peripheral nervous system and spinal cord include electromyography, electroneurography (nerve conduction studies), and somatosensory evoked potentials.
Electromyographic examination involves introduction of a special recording needle into a muscle belly. Two types of needles are used by nearly all electromyographers: monopolar, composed of a steel wire insulated with Teflon; and concentric, a hollow needle through which runs a fine wire. Monopolar needles may be preferred because they cause less discomfort to the patient, but they require a surface reference electrode. Concentric needles have a more uniform exposed surface, and so may be more useful for quantification and comparison of motor unit potentials.
Electrical potentials located within a few millimeters of the needle are picked up by the electrode, then transmitted from the muscle to a preamplifier placed near the...