Transcranial magnetic stimulation reveals an interhemispheric asymmetry of cortical inhibition in focal epilepsy

Abstract

Transcranial magnetic stimulation (TCS) was applied to both hemispheres of 16 patients affected by criptogenic focal epilepsy to evaluate the interhemispheric symmetry of the motor cortex excitability. The amplitude of the motor evoked potentials (MEPs) and the duration of the post-MEP silent period (SP) were measured at threshold (THR) and at increasing TCS stimulation intensities. The THR was significantly higher in patients than in 16 age-matched control subjects (p < 0.01). No interhemispheric differences were found in MEP amplitude. In controls, the correlation between SP duration and increasing TCS stimulus intensity was linear with a symmetrical progression of the SP duration over the two hemispheres. In patients this linear SP progression was lacking on the 'epileptic' hemisphere: the SP duration did not increase following TCS > 40% above THR, indicating abnormal interhemispheric asymmetry. This finding suggests a selective dysfunction of inhibition in the epileptic hemisphere as signaled by an abnormal SP duration in response to progressively higher TCS intensities.

Changes in interhemispheric inhibition following successful epilepsy surgery: a TMS study

“…We could confirm the findings of most groups that TMS seems to be safe for investigational purpose in epilepsy patients [6,26,[33][34][35][38][39][40].…”

“…Although this may appear less plausible, complex interactions between extended brain areas in patients with epilepsy have been found using other methodological approaches. Effects concerning a pronounced change of intracortical inhibition and facilitation in the non-focal hemisphere have recently been reported [1,21] Other studies in patients with focal epilepsy found bilateral changes in electrophysiological parameters [1,[6][7][8]13]. There is evidence that cortical dynamics are altered by epileptic rhythms even if the focus is located far away [2].…”

Does the region of epileptogenicity influence the pattern of change in cortical excitability?

“…It is known that AEDs have a prominent effect on TMS measure (Ziemann, 2004) and there are many reports of increased MT in patients with different forms of epilepsy on multiple AEDs, an effect which seems to increase as a function of the number of AEDs used (Hamer et al, 2005;Cantello et al, 2000;Caramia et al, 1996;Akgun et al, 2009;Cicinelli et al, 2000;Di Lazzaro et al, 2004;Molnar et al, 2006). While this is not a universal finding and there are reports of no change in MT in treated patients with chronic epilepsy compared to non-epilepsy controls (Manganotti et al, 2000(Manganotti et al, , 2004(Manganotti et al, , 2006Caramia et al, 1996;Badawy et al, 2010b), multiple AED use may explain the increase in MT observed in both the refractory and seizure free focal epilepsy groups compared to non-epilepsy controls.…”

Mobile Phone Emissions Modulate Brain Excitability in Patients with Focal Epilepsy

“…Several studies have demonstrated that the excitation/inhibition imbalance not only affects the seizure focus, but may also involve more distant areas, including motor cortex [20]. In this regard, TMS has been already used in epileptic patients to evaluate excitatory and inhibitory processes underlying different epileptic disorders, but with results somewhat conflicting [21,22].…”