What is the peripheral nerve compound action potential (CAP) of upper limb somatosensory evoked potentials (SEPs)?

Updated: Feb 26, 2019
  • Author: Sombat Muengtaweepongsa, MD, MSc; Chief Editor: Selim R Benbadis, MD  more...
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Answer

Answer

During clinical diagnostic studies of the upper limb SEP, a surface electrode at Erbs point is used to record the peripheral nerve CAP as it traverses the brachial plexus. N9, the initial negative peak (labeled EP in the image below), reflects the CAP within the most rapidly conducting subset of the afferent fibers. Multiple negative peaks, reflecting peripheral nerve fiber populations with different conduction velocities, may be recorded in some subjects, most often in children. When this occurs, the earliest negative peak should be interpreted as the N9 peak. A smaller P9 far-field peak, which most likely also arises within the brachial plexus, may be seen in scalp-to-noncephalic recordings; it has a slightly shorter latency than N9.

Normal median nerve somatosensory evoked potential Normal median nerve somatosensory evoked potentials (SEPs) recorded using the minimal (4-channel) recording montage recommended by the American EEG Society (AEEGS) guidelines. Negativity at input 1 is shown as an upward deflection. Courtesy of American Electroencephalographic Society, 1994.

Erbs point-recording electrodes have several disadvantages during intraoperative monitoring, such as their proximity to the sterile field, ease of dislodgment, and pickup of ECG artifact. A useful alternative recording site is over the peripheral nerve in the antecubital fossa (see image below).

Somatosensory evoked potentials (SEPs) recorded du Somatosensory evoked potentials (SEPs) recorded during resection of a posterior fossa tumor (intradural extension of a clear-cell tumor of the right middle ear) in a 46-year-old woman. The peripheral nerve compound action potentials (CAPs) to left median nerve stimulation, recorded at the elbow, and the simultaneously recorded cortical SEPs both displayed marked amplitude attenuation. The stimulating electrodes at the left wrist were replaced; the peripheral nerve and cortical SEPs both returned to their baseline values and remained there through the end of the operation. Courtesy of Legatt, 1995.

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