Quantitative Evaluation of Hemiparesis with Corticomyographic Motor Evoked Potential Recorded by Transcranial Magnetic Stimulation

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Abstract
Corticomyographic motor evoked potentials (MEP) activated by transcranial magnetic stimulation of the motor cortex provide clinicians with an opportunity to evaluate corticospinal motor systems quantitatively and noninvasively. Threshold, amplitude, and latency of the corticomyographic MEP, however, are variable between subjects mainly because current directions and intensities induced by magnetic stimulation cannot be determined precisely due to anatomical variations of subjects. The variability of corticomyographic MEPs has limited the use of corticomyographic MEP for evaluating mild changes in corticospinal motor function. In the present study, we used an internal standard to assess hemiplegia, expressing relative amplitude, latency, and threshold of responses on the paretic side as a function of responses elicited from the intact side (%MEP). Neurological function of paretic muscles, as determined by a muscle maneuver test (MMT), clearly correlated to %MEP threshold, amplitude, and latency. Since corticomyographic MEP are similar when recorded from symmetrical sites on two extremities of normal subjects, %MEP provided a sensitive measure of mild hemiparesis. The %MEP approach revealed abnormal MMT scores of 3 or 4 more frequently than did standard MEP approaches. %MEP amplitude was more sensitive to mild hemiparesis than %MEP latency or %MEP threshold. Since magnetic stimulation with a safe intensity range cannot reliably produce corticomyographic MEP in severely paretic muscles with MMT scores of 2 or less, the MEP appears to be most useful for evaluating mild hemiparesis. This technique should expand significantly the clinical usefulness of corticomyographic MEP in neurosurgical practice.