Cortical and spinal evoked potential response to electrical stimulation in human rectum

Abstract

Aim: To study a novel technique to record spinal and cortical evoked potentials (EPs) simultaneously in response to electrical stimulation in the human rectum.

Methods: Eight male and nine female healthy volunteers participated. Stimulating electrodes were attached to the rectal mucosa at 15 cm and 12 cm above the dentate line. Recording skin electrodes were positioned over vertebrae L4 through S2. The electrical stimulus was a square wave of 0.2 ms duration and the intensity of the stimulus varied between 0 and 100 mA. EP responses were recorded using a Nicolet Viking IV programmable signal conditioner.

Results: Simultaneous recording of cortical and spinal EPs was obtained in > 80% of the trials. The EP responses increased with the intensity of the electrical stimulation, were reproducible overtime, and were blocked by application of Lidocaine jelly at the site of stimulation. The morphology (N1/P1), mean ± SD for latency (spinal N1, 4.6 ± 0.4 ms; P1, 6.8 ± 0.5 ms; cortical N1, 136.1 ± 4.2 ms; P1, 233.6 ± 12.8 ms) and amplitude (N1/P1, spinal, 38 ± 7 μV; cortical 19 ± 3 μV) data for the EP responses were consistent with those in the published literature. Reliable and reproducible EP recordings were obtained with the attachment of the electrodes to the rectal mucosa at predetermined locations between 16 and 8 cm above the anal verge, and the distance between the attachment sites of the electrodes (the optimal distance being approximately 3.0 cm between the two electrodes).

Conclusion: This technique can be used to assess potential abnormalities in primary afferent neural pathways innervating the rectum in several neurodegenerative and functional pain disorders.

Figure 1

Stimulating electrodes. Two electrodes (Medtronic CapSureFix Model 5076) were used in this study. This model had an electrically active helix that was designed to extend easily (up to 2 mm) for active fixation and retraction. The electrodes were positioned at 15 cm and 12 cm above the anal verge.

Figure 2

Representative cortical and spinal evoked potential responses to electrical stimulation in the rectum. Evoked potential (EP) responses demonstrated that the typical morphology for cortical and spinal EP recordings including an N1/P1 wave form that increased with stimulus intensity (threshold intensity, left panel; 2 × threshold intensity, right panel). Note the different scales for measuring the amplitude of EP response in the left panel (cortical, 5 μV/division; spinal, 20 μV/division) and right panel (cortical, 10 μV/division; spinal, 50 μV/division). Summary latency and amplitude results for recordings obtained using an electrical stimulus 1.5 × threshold is presented in Table 2.

Figure 3

Evoked potential responses were reproducible over time. Representative cortical and spinal evoked potential responses from two recording sessions for one volunteer are shown in this figure. No significant changes in the latency and amplitude values were observed compared to the initial recording session. Summary data are presented in Table 3.

Figure 4

Application of Lidocaine jelly to the rectal mucosa at the sites of electrical stimulation resulted in loss of the cortical and spinal evoked potential responses. A representative evoked potential response (before and after application of Lidocaine) is presented in this figure. In addition, all participants reported a loss in the sensory response to electrical stimulation after application of Lidocaine.