motor evoked potentials (Tcemep)

'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)

• Lesion lies outside neural tract being assessed (dorsal column)
• Lesion not at level being monitored
• Pre-existing neurological deficit compromising quality of recordings
• Technical problems (noise) during surgery
• Deficit may have occurred post operatively
• Deficit onset slowly (ischemia may onset over 20-30 minutes) and was not reversible once detected
• Anesthetic effect

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:

• Accidental extubation or loss of lines
• Possible patient recall
• Non-continuous, slow and does not provide early warning (can take up to 15 min to complete)
• Not well suited for repeated tests
• Best for normal, cooperative patients

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.

Transcranial MEPs

Two types of transcranial MEPs have generally been used, depending on stimulation type: Transcranial electrical and transcranial magnetic EPs.
 

• 1980 – Merton and Morton and Marsden – first published report of single phase TceMEP in an awake human
• 1985 – Barker et al. – Pulsed magnetic MEPs in human

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=83
SSEP 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

• Upper and lower motor pathways are usually monitored together
• Additional focal coverage of C5 bowel and bladder and L4, L5 and S1 can be included

Special considerations

• Bite block must be used to prevent tongue injury
• Monitoring anesthesia regimen & paralytics must be tailored
• TceMEPs produce 'snap shot'  pictures of motor function

Contra indications:

• History of seizures
• Selected implants (pacemakers, cranial plates)

Summary of TceMEPs:
Upside

• Instant results – does not require averaging
• Can be done in patients where SSEPs not obtainable
• Directly measures corticospinal pathway
• More sensitive to ischemia than SEP
• Less sensitive to electrical noise

Downside

• Movement
• Anesthetic and paralytic sensitive
• Contraindicated in seizures
• Requires bite guard to prevent tongue bite
• Standardized paradigms for use not fully agreed on

Last Word on TceMEPs:

• SSEP remains the gold standard for spinal tract monitoring in IOM
• Although SSEP has a good record, motor outcomes are only inferred since it does not directly measure motor pathways
• TceMEP provides direct monitoring of motor pathways and immediate non averaged results
• TceMEP is now in general use, although standards for application vary
• Using both modalities together appear to improve sensitivity and specificity of results
   

R. O'Brien MD

Selected References

Adams DC, Emerson RG, Heyer EJ, et al. Monitoring of intraoperative motor-evoked potentials under conditions of controlled neuromuscular blockade. Anesth Analg 1993;77:913-8.

Bose B, Sestokas AK, Schwartz DM. Neurophysiological monitoring of spinal cord function during instrumented anterior cervical fusion. Spine J

2004; 4:202-7.

Calancie B, PhD, Harris W, MSc, Brindle F, MD, Green B, MD and Landy H, MD The Miami Project to Cure Paralysis and the Department of Neurological Surgery and Anesthesiology, University of Miami School of Medicine, Florida. Threshold-level repetitive transcranial electrical stimulation for intraoperative monitoring of central motor conduction. Journal of Neurosurgery (Spine 1) 95: 161-168, 2001

Calancie B, Harris W, Broton JG, Alexeeva N, Green BA. "Threshold-level" multipulse transcranial electrical stimulation of motor cortex for intraoperative monitoring of spinal motor tracts: description of method and comparison to somatosensory evoked potential monitoring. J Neurosurg 1998;88:457-70.

de Haan P, Kalkman CJ. Spinal cord monitoring: somatosensory- and motor-evoked potentials. Anesthesiol Clin North Am 2001;19 (4):923-45.

Delecrin J, Nguyen The Tich S, Passuti N, Pereon Y. [Neurogenic mixed evoked potential monitoring during scoliosis surgery: retrospective analysis of 149 cases]. Rev Chir Orthop Reparatrice Appar Mot 2000; 86:46-53.

Deletis V: Electrical and Magnetic Stimulation of the Brain and Spinal Cord Diveinsky O, Beric A, Dogals M (ed) New YorkIntraoperative monitoring of the functional integrity of the motor pathways: Raven Press, 1993, pp 201-214

Deletis V, Kothbauer K: Spinal Cord Monitoring Stalberg F, Shamma HS, Olsson Y (ed);Intraoperative Neurophsyiology of the Corticospinal Tract.  Springer, 1998, pp 421-444

Delitis, V. Intraoperative Monitoringof the functional integrity of the motor pathways. In "Advances in neurology: Electrical and magnetic stimulation of the brain" (O. Devisky, A. Beric, and M. Dugali, eds.) pp. 201-214, Raven Press, New York.

Guerit JM, Dion RA. State-of-the-art of neuromonitoring for prevention of immediate and delayed paraplegia in thoracic and thoracoabdominal aorta surgery. Ann Thorac Surg 2002;74(5):S1867-9; discussion S1892-8. 

Hilibrand AS, Schwartz DM, Sethuraman V, Vaccaro AR, Albert TJ. Comparison of transcranial electric motor and somatosensory evoked potential monitoring during cervical spine surgery. J Bone Joint Surg Am 2004; 86-A:1248-53.

Isley M, Balzar J, Pearlman R, and Zhang XF, Intraoperative Motor Evoked Potentials., American Journal of ENG Technolgies 41: 266-338, 2001

Iwasaki H, Tamaki T, Yoshida M, Ando M, Yamada H, Tsutsui S, et al. Efficacy and limitations of current methods of intraoperative spinal cord monitoring. J Orthop Sci 2003; 8:635-42.

Kothbauer, Karl MD, MOTOR EVOKED POTENTIAL MONITORING DURING SPINAL CORD SURGERY; Pediatric Neurosurgery, Hyman-Newman Institute for Neurology and Neurosurgery Beth Israel Medical Center, New York City 1998

Langelon, D et al. Transcranial Electrical Motor-Evoked Potential Monitoring During Surgery for Spinal Deformity. Spine Vol 28(10), p 1043-1050

Levy WJ Jr. Clinical experience with motor and cerebellar evoked potential monitoring. Neurosurgery 1987;20:169-82.

Lubitz SE, Keith RW, Crawford AH. Intraoperative experience with neuromotor evoked potentials. A review of 60 consecutive cases. Spine 1999;24(19):2030-4.

Noonan KJ, Walker T, Feinberg JR, Nagel M, Didelot W, Lindseth R. Factors related to false- versus true-positive neuromonitoring changes in adolescent idiopathic scoliosis surgery. Spine 2002; 27:825-30.

Padberg AM, Wilson-Holden TJ, Lenke LG, Bridwell KH. Somatosensory- and motor-evoked potential monitoring without a wake-up test during idiopathic scoliosis surgery. An accepted standard of care. Spine 1998;23:1392-400.

Pelosi L, Lamb J, Grevitt M, Mehdian SM, Webb JK, Blumhardt LD. Combined monitoring of motor and somatosensory evoked potentials in orthopaedic spinal surgery. Clin Neurophysiol 2002;113(7);1082-91.

Stephen JP, Sullivan MR, Hicks RG, Burke DJ, Woodforth IJ, Crawford MR. Cotrel-dubousset instrumentation in children using simultaneous motor and somatosensory evoked potential monitoring. Spine 1996; 21:2450-7.