A clinical study of motor evoked potentials using a triple stimulation technique

Abstract

Amplitudes of motor evoked potentials (MEPs) are usually much smaller than those of motor responses to maximal peripheral nerve stimulation, and show marked variation between normal subjects and from one stimulus to another. Consequently, amplitude measurements have low sensitivity to detect central motor conduction failures due to the broad range of normal values. Since these characteristics are mostly due to varying desynchronization of the descending action potentials, causing different degrees of phase cancellation, we applied the recently developed triple stimulation technique (TST) to study corticospinal conduction to 489 abductor digiti minimi muscles of 271 unselected patients referred for possible corticospinal dysfunction. The TST allows resynchronization of the MEP, and thereby a quantification of the proportion of motor units activated by the transcranial stimulus. TST results were compared with those of conventional MEPs. In 212 of 489 sides, abnormal TST responses suggested conduction failure of various degrees. By contrast, conventional MEPs detected conduction failures in only 77 of 489 sides. The TST was therefore 2.75 times more sensitive than conventional MEPs in disclosing corticospinal conduction failures. When the results of the TST and conventional MEPs were combined, 225 sides were abnormal: 145 sides showed central conduction failure, 13 sides central conduction slowing and 67 sides both conduction failure and slowing. It is concluded that the TST is a valuable addition to the study of MEPs, since it improves detection and gives quantitative information on central conduction failure, an abnormality which appears to be much more frequent than conduction slowing. This new technique will be useful in following the natural course and the benefit of treatments in disorders affecting central motor conduction.

Abnormal Corticospinal Excitability in Patients with Disorders of Consciousness

“…In agreement with previous reports in critically ill patients [34], presence of a symmetric sensory and motor axonal neuropathy is common in DOC patients in the subacute period following brain injury, while myopathic involvement is rarer and most probably is associated with neuropathies [35]. For the detection of the failure of corticospinal tract conduction, the triple stimulation technique, which consists of one transcranial magnetic and two peripheral electric stimuli (plexus and nerve) delivered along the motor pathways, might have higher sensitivity than conventional MEP or CMCT testing [36]. We observed higher RMTs in VS/UWS patients compared to healthy controls, while the difference between MCS patients and healthy subjects was not significant.…”

Repetitive spinal motor neuron discharges following single transcranial magnetic stimulation: relation to dexterity

“…The TST is a collision method using a sequence of three stimuli, to the brain, the ulnar nerve at the wrist, and the brachial plexus at Erb's point (Magistris et al 1998(Magistris et al , 1999. The TST test response is calibrated by a TST control response, for which the brain stimulus is replaced by a stimulus at Erb's point (succession of stimuli: Erb-wrist-Erb).…”

Corticospinal excitability in patients with anoxic, traumatic, and non-traumatic diffuse brain injury

“…This might indicate the selective involvement of the small pyramidal tract neurons with a relative sparing of the large fast conducting corticospinal fibres that are responsible for MEP [63]. For the detection of the failure of corticospinal tract conduction, the triple stimulation technique, which consists of one transcranial magnetic and two peripheral electric stimuli (plexus and nerve) delivered along the motor pathways, might have higher sensitivity than conventional MEP or CMCT testing [64]. In contrast to these results, a higher RMT is reported in patients with moderate focal TBI, while no difference was found between patients with minor or moderate diffuse TBI [13,19].…”