What is the peripheral nerve compound action potential (CAP) of lower 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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A surface electrode placed in the popliteal fossa in the midline can be used to record the peripheral nerve CAP following posterior tibial nerve stimulation at the ankle. To minimize both electrical stimulus artifact and ECG pickup, a reference electrode on the same leg is used. Possible linkages include a midline electrode 2 cm above the popliteal crease referred to a midline electrode 5 cm above the popliteal crease and a midline popliteal fossa electrode referred to an electrode placed at the lateral aspect of the same knee.

In patients in whom a clear foot twitch is not obtained, the presence of a clear peripheral nerve CAP at the popliteal fossa demonstrates that the posterior tibial nerve has been stimulated adequately. In this case, the absence of more rostrally generated SEP components is evidence of abnormality within the neural somatosensory pathways. Without the peripheral nerve recording channel, the absence of rostral SEPs also could have been due to technical factors that prevented adequate nerve stimulation. Similarly, during intraoperative monitoring, examination of the peripheral nerve recording channel permits a rapid classification of the causes of changes in the more rostrally generated SEP components.

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