Faculty Bibliography

PURPOSE/OBJECTIVE:To report a case of motor evoked potential changes and spinal cord injury during the initial dissection in scoliosis surgery. METHODS:Motor evoked potentials to transcranial electrical stimulation were recorded from multiple muscles. Somatosensory evoked potentials to limb nerve stimulation were recorded from the scalp. RESULTS:Clear motor evoked potentials were initially present in all monitored muscles. The patient was then pharmacologically paralyzed for the initial dissection. More than usual bleeding was encountered during that dissection, prompting transfusion. As the neuromuscular blockade subsided, motor evoked potentials persisted in the hand muscles but disappeared and remained absent in all monitored leg muscles. The spine had not been instrumented. A wake-up test demonstrated paraplegia; the surgery was aborted. There were no adverse somatosensory evoked potential changes. MRI showed an anterior spinal cord infarct. CONCLUSIONS:Copious soft tissue bleeding during the initial dissection might have lowered pressures in critical segmental arteries enough to cause spinal cord infarction through a steal phenomenon. The lack of somatosensory evoked potential changes reflected sparing of the dorsal columns. When neuromuscular blockade is used during the initial soft tissue dissection, motor evoked potentials should be assessed after this, but before spinal instrumentation, to determine whether there had been any spinal cord compromise during the initial dissection.

Median nerve somatosensory evoked potential monitoring is commonly used during carotid endarterectomy to permit selective shunting in only those patients who are determined to have inadequate collateral flow after carotid cross-clamping. The N20 component is recorded from the CPc (contralateral centroparietal) electrode; either CPi (ipsilateral centroparietal) or Fpz (forehead) can be used as the reference. Because of the distribution of the subcortically generated N18 component, the CPc-Fpz derivation might record both the N20 and the N18 components and might therefore inadequately detect hemispheric ischemia after carotid cross-clamping. Somatosensory evoked potentials recorded were compared using these 2 derivations during 38 carotid endarterectomies to assess their ability to detect neurophysiologic changes after carotid cross-clamping. Although, as expected, the baseline N20 component was significantly larger when recorded with the CPc-Fpz derivation than with the CPc-CPi derivation (3.1 vs. 2.4 μV in the hemisphere ipsilateral to the clamped carotid, P < 0.001), there was no significant difference in the postclamp amplitude decline between the 2 derivations (8.7% vs. 8.6%, P = 0.82). It is concluded that CPc-Fpz is an acceptable derivation for recording postclamp hemispheric somatosensory evoked potential changes during carotid endarterectomy and may be advantageous because it provides a larger amplitude somatosensory evoked potential than the CPc-CPi derivation.

American Clinical Neurophysiology Society (ACNS) guidelines recommend recording P14 between an ipsilateral centroparietal electrode (CPi) and a noncephalic reference, typically the contralateral Erb's point (EPc) (American Clinical Neurophysiology Society. Guideline 9D: guidelines on short-latency somatosensory evoked potentials. J Clin Neurophysiol. 2006;23(2):168-179). We investigated the utility of a forehead (Fpz)-to-inion derivation for recording P14. We analyzed 74 median nerve somatosensory-evoked potential (SEP) studies (148 nerves) with bilaterally normal peripheral and central conductions. The presence of an identifiable P14 and its amplitude and latency were assessed in both the CPi-EPc and Fpz-inion derivations. In 7 of the 148 recordings, P14 was not identifiable in either derivation. The P14 was only identifiable in CPi-EPc in 9 recordings, and only identifiable in Fpz-inion in 4 recordings. In the remaining 128 recordings, the mean P14 latency was 13.2 ± 1.1 ms in both derivations. The mean P14 amplitude using CPi-EPc was 2.0 ± 0.6 µV, significantly larger than that using Fpz-inion, 1.2 ± 0.6 µV (P < .001). In conclusion, the CPi-EPc derivation and the Fpz-inion derivation both record the same P14 component, and latency norms based on either derivation are interchangeable. Although the CPi-EPc derivation typically yields a larger and more identifiable P14, occasionally Fpz-inion yields a larger P14, and rarely P14 is only identifiable using Fpz-inion. Thus, recording of the Fpz-inion derivation may be a useful adjunct during median nerve SEP testing.