When corticospinal tracts are stimulated, the stimulation descends the spinal cord and Motor Evoked Responses (TCMEPs) are detectable. TCMEPs may take the form of transcranial MEPs, in which electrical impulses evoked by cranial stimulation are recorded in nerves or extremity musculature. The latter mode monitors anterior cord integrity.
This modality also may take the form of Neurogenic Motor Evoked Responses. The latter are detected in the lower spinal cord following upper spinal cord stimulation. Cork-screw electrodes that stimulate the corticospinal tracts often are employed to apply electrical stimulation over the motor cortex area.
TCMEP comprises intermittent rather than continuous monitoring and each muscle recording contains Compound Muscle Action Potentials (CMAPs). This modality often supports SSEP monitoring of the spinal cord by providing slightly faster and more direct motor tract evaluation as well as prompt data with no averaging needed. TCMEP also presents an IONM option for patients in whom SSEP isn’t effective. The indicative value of this modality depends heavily on paralytic agent levels in use during surgery.