Neurosurgery

of spinal deformity, in which both NMEPs and SSEPs were generated and monitored. In three of the cases, surgical manipulation resulted in sudden loss of both NMEPs and SSEPs. In these cases, the electrodes used for elicitation of NMEP were moved along the spinal cord until the precise level of involvement was appreciated, with subsequent laminectomy and decompression at that level. Two of the three patients exhibiting evoked potential loss were asymptomatic following the procedure, while a third was left paraplegic. However, in two additional operations, isolated changes in NMEP were seen without a concomitant change in SSEP. In both of these cases, the surgical procedure was altered accordingly and potential neurological damage was avoided. In addition, the data from NMEP monitoring, requiring no time averaging, were acquired more quickly than data from SSEP, allowing for more timely interventions in the face of pending injury [15].

Monitoring of sacral root innervation to the anal and urethral sphincters can be performed with either evoked potential monitoring or by manometric recordings. In cases of tethered cord or tumor resection, a comprehensive strategy for monitoring has been proposed, which provides coverage from L2 to S4 [16]. This system uses a combination of SSEP monitoring from tibial nerve and nerve root stimulation with electromyographic (EMG) recordings of muscle from the sphincters and relevant leg musculature. The proposed benefit is an ability to differentiate functional neural tissue from non-functional or fibrous tissue. Despite the successful application of these various monitoring techniques, there has been no controlled study documenting improved neurological outcomes in these cases, and the circumstances in which lumbosacral spinal cord monitoring is efficacious have not been well defined.

Motor Evoked Potentials

Identification of the primary motor cortex and the specifics of the motor homunculus can be accomplished via the use of cortical electrical or magnetic stimulation, using concomitant EMG recording to assess a response to the evoked potential in the periphery. Electrical stimulation is performed with single surface electrodes or electrode grids that are placed in direct contact with the cortex. Transcranial electrical stimulation is remarkably painful, due to current flow across the scalp. Therefore, non-anesthetized recordings are not feasible. Furthermore, tran-scranial electrical stimulation is contraindicated in patients with a history of seizure or an EEG suggestive of seizure tendency. Magnetically induced motor evoked potentials (MEPs) are generated by passing a changing current through a coil held perpendicular to the cortical surface, which induces a magnetic force perpendicular to the electrical field. Transcranial magnetic stimulation (TMS) of the cortex is painless, may be obtained both pre- and intra-operatively, and does not require averaging for analytical purposes. However, this method is cumbersome, expensive and non-specific with regard to the cortex it stimulates.

MEPs are exquisitely sensitive to the effects of anesthetics. It has been conclusively demonstrated that isoflurane will abolish MEPs generated by either electrical or magnetic stimulation of the cortex. Barbiturates, propofol and benzo-diazepines exert a strong depressive effect on MEPs; etomidate causes a milder depression that eventually returns to baseline. Anesthetics that have been shown to have little or no effect on MEP are halothane, fentanyl and ketamine. MEPs generated by stimulation of the spinal cord avoid the cortical effects of these anesthetics and will remain intact. Cortical MEPs may be difficult to generate in young children, in whom the motor cortex is relatively inexcitable.

Enhanced patient outcome has not been clearly documented with the use of MEPs in controlled trials. A retrospective study reviewed the resections of 130 intramedullary tumors performed with the assistance of MEP recordings. The results suggested that gross total resection of these tumors was more likely when MEP monitoring was used; however, no clear reduction in morbidity or improvement in patient outcome related to monitoring was demonstrated [17].

In addition to assessing cortical elements of the motor system, MEP may prove to be a valuable technique in the assessment of intraoperative risk to the motor pathways of the brainstem and spinal cord. Considering that sensory impulses travel in the posterior tracts of the spinal cord and the lateral aspects of the brain-stem, isolated monitoring of SSEP is incomplete for assessing the integrity of all spinal pathways.

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