'Motor Evoked Responses' are waveforms recorded after stimulation of the motor (movement) pathways of the central nervous system. There are several types of MEPs depending upon where the stimulation occurs and from where the response is recorded. Transcranial MEPs involve stimulation of the brain through the skull, with recording either from the nerves or muscles of the arms and legs. So called 'Neurogenic Motor Evoked Responses' are recorded from the caudal (towards the lower back) spinal cord after stimulation of the cord more rosterally (towards the head). This latter type of recording has since been shown to be primarily from sensory pathways rather than from the the motor pathways and adds no additional protection to traditional SSEP recordings.
The Problem with lone Spinal SSEP IOM
SSEP recordings provide reasonable protection of the spinal cord during surgery, so why are MEPs also used? The basis for using intraoperative SSEP monitoring as a representative index of motor function is the supposition that any mechanical or vascular compromise that may cause motor dysfunction will affect the lateral corticospinal tract (motor) and the dorsal columns (sensory) together. While likely true for most cases, there are multiple reports of false negative outcomes (Levy 1983; Ginsburg et al 1985; Takaki and Okumura 1985; Lesser et al. 1986; Ben-Davis et al 1897; Diaz and Lockhart 1987; Chartrain et al 1988; Zornow and Drumond 1989; Newer et al. 1995, 1998). Dawson et. al. estimated that 28% of neurological complications occurring were not detected by SSEP monitoring (Dawson - et al. 1991 - retrospective, multi-center, 33000 patients with spinal surgery). One of the reasons for this failure likely is due to the separate vascular supplies of the motor and sensory pathways (see figure on right). Other reasons SSEPs may fail are listed below. Reasons SSEP monitoring may fail(Burke and Hicks 1998, Schwartz et al. 1997)
More direct observation of motor function during surgery can be achieved by intermittent wake up tests, but these have several drawbacks as a back up to SSEPs as listed below.
Reasons Stagnara Wake Up Test in not an optimal back up for SSEP:
Motor evoked responses are the logical extension of the wish to back up SSEP monitoring with more direct and timely monitoring of the motor tracts during surgery.
Two types of transcranial MEPs have generally been used, depending on stimulation type: Transcranial electrical and transcranial magnetic EPs.
Outcomes with TceMEPs
Animal studies have shown TcMEPs to be more sensitive to ischemia and cord compression than are SSEPs. Criteria for significant changes in TceMEPs vary between institutions and practitioners, complicating outcome measurement. Many investigators use an 'all or none' paradigm or an 80% drop in amplitude threshold after stimulation with fixed parameters. Others use a 'threshold-level' paradigm (Calancie et al. 1998, 2001) in which the change is measured by the degree of required increase in stimulation to return waveforms to baseline. Most studies suggest a higher sensitivity and variable but higher specificity for TceMEPs to post operative motor deficit than SSEPs alone (Calancie 2001, N=83SSEP Sensitivity 0.87, Specificity 0.90 SSEP relative to post operative deficit, TceMEP Sensitivity 1, Specificity 1 relative to post operative motor deficit). In one study of high risk myleopathic patients monitored with TceMEPs (N=34 ), "as a result of intraoperative TceMEP findings, the surgical plan was altered or otherwise influenced in six patients (roughly 15% of the sample population), possibly limiting the extent of postoperative motor deficit experienced by these patients" (Calancie et. al. 1998). TCeMEPs have been used as a sole monitoring modality in the absence of SSEPs by some authors (Langelon, D et al. 2001) . TceMEPs seem to be fairly specific to body part involved. Intraoperative loss of muscle MEPs indicates some postoperative impairment of voluntary motor control with a specificity of about 90% and sensitivity of 100%For instance, muscle MEPs lost in one leg during the resection means that the patient will postoperatively be unable to move this particular extremity (Kothbauer, Karl MD 1998)
Specific set up and coverage
Summary of TceMEPs:
Last Word on TceMEPs:
R. O'Brien MD
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