‘Motor Evoked Responses’ are waveforms recorded after stimulation of the motor 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. ‘Neurogenic Motor Evoked Responses’ are recorded from the caudal spinal cord after stimulation of the cord more rosterally. This latter type of recording has 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 IONM
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 and the dorsal colum 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; Nuwer et al. 1995, 1998). Dawson et. al. estimated that 28% of neurological complications 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 below). Other reasons SSEPs may fail are listed below.

TceMEP

Sensory and Motor Pathways being monitored are supplied

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 is 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 mins. to complete).
• Not well suited for repeated tests.
• Best for normal, cooperative patients.

Motor evoked responses are the logical extension of the need 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

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