Modalities – TcMEP

TcMEPs- Motor Evoked Potentials

TcMEPs evaluate the functional integrity of the motor pathways. Most commonly the primary motor cortex is stimulated and recorded electrical responses are measured in the corresponding muscles. This is a very accurate indicator of the intactness of the motor tracts through the brain, to the brainstem, through the spinal cord, out the nerve roots, and finally to the muscles.

Somatosensory evoked potentials (SSEPs) have been referenced as the “gold standard” for spinal monitoring, so why is there a need to use TcMEPs? SSEPs are only effective representing motor function if the compromise that causes motor dysfunction will affect the motor tract and the dorsal columns (sensory) simultaneously. In 1991, Dawson reported that 28% of neurologic complications occurring were not detected by SSEP monitoring (Dawson-et al. 1991-retrospective, multicenter, 33000 patients with spinal surgery). The motor and sensory pathways often have different vascular supplies, which probably accounts for Dawson’s findings.

Multi-modality equipment (equipment able to perform EMG, SSEP, TcMEP, ABR, etc. test simultaneously) is a technology of the new millennium. When TcMEPs were not available and SSEPs alone were not sufficient (scoliosis correction surgery), it was common for the surgery team to perform Stagnara’s Wake Up Test. Patients were awakened and motor function evaluated. There are several reasons why wake-up test may not be suitable for a back-up to SSEPs:

Only suited for compliant, cooperative patients
Patients can have recall incidents
Accidental extubation
Tests too slow, can take up to 15 minutes
Non-continuous, and does not provide early warnings
Repeated testing very difficult

TcMEPs are the logical and sensible back-up to SSEPs in complex surgery or where SSEPs are not adequately reliable. With TcMEPs you have direct and timely monitoring of the motor tracts throughout the surgical procedure.