Abstracts

Transcranial electrical stimulation (TCES), used to elicit motor evoked potentials (MEPs) for intraoperative monitoring (IOM), preferentially stimulates cortex under the anode. MEPs attributable to cortical stimulation under the cathode (CSE-MEPs) are sometimes present, and may permit assessment of both corticospinal tracts with a single transcranial stimulus. However, changes in CSE-SEPs may have different implications than changes in MEPs elicited by anodal stimulation. I reviewed IOM data from 43 operations during which MEPs to TCES between electrodes on both sides of the head were used to monitor the spinal cord. The connections to the stimulating electrodes were repeatedly reversed to record MEPs to both L-anode/R-cathode and L-cathode/R-anode stimulation. Stimulus parameters were adjusted to produce clear MEPs contralateral to the TCES anode. During 22 operations, MEPs were recorded from muscles on both sides of the body, but the responses were all contralateral to the TCES anode. In 2 patients, MEPs were only present on one side, and were also produced by anodal stimulation. In 2 patients, no MEPs could be elicited.
MEPs in muscles contralateral to the TCES cathode were recorded during 17 operations. During 8 of these, the CSE-MEPs were small and present only intermittently, and were not considered sufficiently reliable for IOM. Robust CSE-MEPs judged adequate for IOM were recorded during 9 operations (21% of the total). In some of them, the presence of large bilateral MEP responses to a single TCES polarity led to the use of a single polarity for monitoring the spinal cord motor tracts on both sides. The CSE-MEPs remained stable in 5 cases, but became markedly attenuated or disappeared, either transiently or persistently, during 4 operations. In one of these, the loss of CSE-MEPs was associated with adverse changes in both the MEPs mediated by anodal stimulation and the somatosensory evoked potentials, and thus most likely reflected spinal cord compromise. In the other 3 patients, the other IOM measures were stable; when the TCES stimulus polarity was reversed, large MEPs were recorded in the limb(s) in which the CSE-MEPs had deteriorated, and all 3 patients had good postoperative neurologic outcomes. CSE-SEPs suitable for IOM are recorded in a minority of cases of TCES-MEP monitoring. When present, they can be used to assess the motor tracts on both sides of the spinal cord with a single stimulus polarity, reducing the number of transcranial stimuli that are delivered. However, CSE-MEPs may disappear during IOM in the absence of corticospinal tract dysfunction, most likely due to changes in cortical excitability.
Adverse changes in CSE-MEPs should not be interpreted as definite evidence of spinal cord compromise. Instead, one should reverse the TCES stimulus polarity and use the MEPs elicited by anodal stimulation to more accurately assess the spinal cord motor tracts on the side where the CSE-MEPs disappeared.