Measurement Of Sensory Potentials In

Waveforms that are displayed during sensory NCS reflect the passage of current beneath surface electrodes that are at least a few millimeters distant from the current generators (that is, the depolarizing axons). Routine studies usually use a bipolar recording technique in which both the active and reference electrodes lie above the nerve. The potential measured by the active electrode is compared with that measured by the reference electrode, and both are compared with a ground electrode lying elsewhere on the patient. The reference electrode makes an important contribution to the observed compound sensory nerve action potential (SNAP) (Fig. 1). The nerve current is not directly measured in NCS but overlying loops of current in the soft tissue reflect the actual action potentials. This situation is referred to as volume conduction and is further described in Chapter 4.

Temporal dispersion refers to the phenomenon that, as a sensory conduction study is performed over longer and longer segments of nerve, the recorded SNAP loses its sharpness (high amplitude and short duration) and becomes a broader, lower-amplitude potential. This occurs normally in any sensory NCS because individual sensory axons conduct at slightly different rates and because these slight differences are magnified when a NCS is performed over a large segment of nerve. In diseased states, such as polyneuropathies, temporal dispersion can be enhanced (abnormal temporal dispersion). Because sensory NCS are especially vulnerable to the effects of temporal dispersion in both health and disease, sensory studies are usually performed over shorter segments of nerve than motor studies, which tend to be less susceptible to these effects (described in Section 3).

Sensory responses can be recorded with an orthodromic or an antidromic technique. In ortho-dromic studies, the recording electrodes are proximal to the stimulation site, and, in antidromic studies, stimulation is proximal to the recording position. In the hands, ring electrodes are used around the fingers to stimulate (orthodromic) or record (antidromic). Although orthodromic

L MEDIAN, APB

L MEDIAN, APB

10 mV

Fig. 2. A median compound muscle action potential. Recordings made with wrist and elbow stimulating APB, abductor pollicis brevis.

10 mV

Fig. 2. A median compound muscle action potential. Recordings made with wrist and elbow stimulating APB, abductor pollicis brevis.

responses reflect more accurately the physiology of sensory responses via afferent conduction, upper extremity orthodromic responses are usually considerably smaller than antidromic responses.

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