Abstracts

Rationale: Epilepsy is a disease affecting cortical excitation and inhibition networks in the brain. Transcranial Magnetic Stimulation (TMS), a noninvasive tool used for presurgical evaluation of patients considering brain surgery due to epilepsy or tumors can also be used to examine the cortical excitability as its principles are firmly grounded in motor physiology, and its ease of application. Resting motor threshold (rMT) is a measure of cortical excitability (defined as the minimum intensity of stimulation needed to evoke a motor response 50% of the time) that is routinely gathered in both brain hemispheres of patients undergoing motor mapping by TMS. We investigated if rMT determined by TMS is a reliable marker of the presence of lesion in a hemisphere so that in cases where lesional hemisphere cannot be readily identified by MRI or EEG, hemispheric differences in rMT could aid in determining the hemisphere with the lesion. Methods: 162 patients [87 female, average age 16.63 y, range 1.91-52.39 y] had their rMT measured as part of the clinical TMS motor mapping procedure. Electromyography was used to measure motor evoked potentials (MEPs) in the patients' contralateral hand muscles. MRI guided TMS was applied to the patients' precentral gyrus and the rMT was determined through an algorithm implemented in TMS system software (www.nexstim.com). The rMT expressed as percent of maximum machine output (%MO) and electric-field at the site of stimulation (EF) were recorded for both hemispheres. The hemispheres were classified as ipsilesional and contralesional based on the site of lesion noted on the MRI or epileptogenic foci localized using magnetoencephalography or electroencephalography. Results: The average rMT across both hemispheres was 55% MO 19 (EF 12054 V/m). In patients with lesions affecting only one hemisphere (n=125, 77with lesions in left hemisphere) there was no significant difference between the ipsilesional and contralesional hemispheres (%MO p=0.57, EF p=0.58) (fig 1). For patients with bilateral or no lesions no significant difference between the two hemispheres was noted (%MO p=0.89, EF p=0.13). Age of patient, location of lesion, and age of lesion onset did not have any significant effect on hemispheric difference in rMT. Conclusions: Within the patient population examined here hemispheric difference in rMTs does not have any significant predictive value in determining the presence of epileptogenic or other lesions. Even in a subset of 18 patients who had rMT differences more than two standard deviations from the mean, 17 had epilepsy, there was no determined pattern of seizure type, onset or cortical location. While the ease of acquiring rMT makes it attractive as a diagnostic measure, it may ultimately be too nonspecific of a measure to fully capture the complexities of cortical excitation/inhibition interactions. Specifically, this population of pts at a tertiary epilepsy care center generally with refractory epilepsy and on multiple anti-epileptic medications, may have too many variable influences on their cortical excitability to be captured with rMT. Additional measures including intensity/MEP amplitude curves and intracortical inhibition may be better suited in investigating the pathophysiology of cortical excitatory and inhibitory network abnormalities in epilepsy. Funding: Neuroscience Institute, Le Bonheur Children's Hospital