Enhanced stability of somatosensory evoked potentials attained in the median nerve by using temporal electrodes for intraoperative recording in patients in the semisitting position

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Object. Findings published in case reports indicate that monitoring of median nerve somatosensory evoked potentials (MN-SSEPs) is unreliable in patients who undergo surgery while in the semisitting position due to the occurrence of changes in the potentials that are unrelated to neurological damage. The present study was designed to test the hypothesis that in these patients MN-SSEPs are more stable when recording electrodes are placed over the temporal region.

Methods. In 30 patients who underwent surgery in the semisitting position, MN-SSEPs were recorded intraoperatively by using electrodes placed over the temporal region as well as those placed at conventional recording sites. The authors analyzed MN-SSEP amplitudes and latencies at different recording sites and at distinct steps of the monitoring procedure.

In 10 of the 30 patients a clinically significant attenuation (>50%) of MN-SSEP amplitude was observed at conventional recording sites and this was obviously not related to neurological damage. In contrast, no significant changes were observed in MN-SSEPs recorded from electrodes located over the temporal region.

Conclusions. In patients who undergo surgery in the semisitting position, the use of additional recording electrodes placed over the temporal region makes intraoperative MN-SSEP monitoring less prone to false-positive alarms and thus enhances the reliability of intraoperative MN-SSEP monitoring.

Article Information

Address reprint requests to: Helmut Wiedemayer, M.D., Neurochirurgische Klinik, Universitätsklinikum Essen, Hufelandstrasse. 55, 45122 Essen, Germany. email: helmut.wiedemayer@uni-essen.de.

© AANS, except where prohibited by US copyright law.

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    Case 119-01. Intraoperative MN-SSEP recordings obtained in a 63-year-old woman who underwent surgery in the semisitting position for a cerebellar metastasis. Consecutive traces of recordings from standard electrode sites (C3′ and C4′) and from temporal electrodes (T3 and T4) are shown. Note the different scales for the amplitudes. After the dura mater had been opened, marked attenuations of the MN-SSEP recorded at standard sites occurred with a decrease in amplitudes at C3′ and C4′ from 4.3 to 1.5µV and from 3.6 to 1.6 µV, respectively. In contrast, the amplitudes recorded at T3 and T4 remained relatively stable with decreases from 1.9 to 1.3 µV and from 1.9 to 1.7 µV, respectively. When the anesthetized patient was repositioned supine at the end of surgery, the amplitudes recorded on both sides recovered simultaneously to baseline values. This case demonstrates the typical changes in MN-SSEPs observed in this study.

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    Graph showing the percentages of amplitude changes when baseline recordings were compared with the minimum amplitude measured after the dura mater had been opened. For each case the percentage of amplitude changes recorded from the standard site C3′ and the temporal site T3 are shown. The limit of a 50% loss of amplitude is outlined by the dashed line. The graph demonstrates that a loss of amplitude that was greater than 50% of baseline was observed in 10 patients when MN-SSEP recordings from the standard site C3′ were considered. In contrast, the loss of amplitude in the MN-SSEP recorded at the temporal site T3 remained well below the limit of 50% in all cases.

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    Case 220-01 (not included in this study because a true-positive alarm occurred intraoperatively). Intraoperative MN-SSEP recording obtained in a 9-year-old boy who underwent surgery in the semisitting position for a recurrent benign astrocytoma. Left: Enhanced magnetic resonance image of the head demonstrating the margins of the tumor (arrows), which was infiltrating the medulla oblongata on the right side. Right: Consecutive traces of intraoperative MN-SSEP recordings. A true-positive monitoring alarm occurred in this case. During dissection of the tumor, marked attenuations of the MN-SSEP amplitudes recorded from both the standard electrode at C3′ and the temporal electrode at T3 were observed. At C3′ and T3 the amplitudes decreased from 6.3 to 0.9 µV and from 3.8 to 0.8 µV, respectively. Postoperatively, the patient suffered from right hemiparesis. On the contralateral side, a moderate loss of amplitude was observed at C4′ (a decrease from 5.7 to 2.4 µV), whereas the MN-SSEP recorded at T4 remained relatively stable (a decrease from 3.6 to 2.4 µV). At the end of surgery after the patient had been repositioned supine, the amplitudes measured at C4′ and T4 recovered completely, whereas the MN-SSEP recorded from C3′ and T3 remained severely attenuated, indicating that the observed changes were not related to the positioning of the patient, but rather to neurological damage. This case demonstrates that temporal electrodes appear to be appropriate for the recording of the MN-SSEP changes caused by neurological damage.

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