Abstracts

Rationale: We used transcranial magnetic stimulation (TMS) to investigate the effect of seizures on the cortico-motor pathways in patients with new onset seizures to determine whether the immediate pre- or post-ictal state produces changes in cortical excitability.Methods: As part of a large study of TMS in 79 drug naïve patients, we identified 22 patients (10 IGE, 12 focal epilepsy) who were examined serendipitously within 24 hours of a seizure (13 pre-ictally, 9 post-ictally). We measured motor threshold (MT) at rest and performed a recovery curve analysis using paired pulse stimulation at short (2-15 ms) and long (50-400 ms) interstimulus intervals (ISI). A second (baseline) study was performed at least 14 days either pre or post the seizure, no further seizures having occurred in the intervening time and with no anti-epileptic or other medication exposure.Results: Preictal Changes: Compared to baseline measurements, both hemispheres in patients with IGE and the hemisphere ipsilateral to the seizure focus in those with focal epilepsy showed an increase in intracortical facilitation (ICF), (p< 0.01), and decreased short (SICI) and long intracortical inhibition (LICI) (p< 0.01) in the 1-24 hours preceding a seizure. This effect was most marked in patients with IGE with maximum effect sizes of 2.4 at the 15 ms ISI (ICF) and 1.9 at the 250 ms ISI (LICI). There were complex changes within the hemisphere contralateral to the seizure focus in focal epilepsy, with an increase in ICF similar to the ipsilateral hemisphere (p< 0.05) but in contrast to the ipsilateral hemisphere, SICI and LICI were increased (p< 0.05). There was also a significant increase in MT (p<0.05) observed in this hemisphere, whilst there was no change in MT in the ipsilateral hemisphere. Postictal Changes: 1-24 hours postictally, there was an increase in MT (p< 0.05; maximum effect size 0.8 in IGE). An increase in SICI and LICI (p< 0.01) and a decrease in ICF (p< 0.01) was observed in both hemispheres in patients with IGE and in the ipsilateral hemisphere of those with focal epilepsy. These changes were again more marked in patients with IGE with maximum effect sizes of 1.8 at the 15 ms ISI (ICF) and 2.7 at the 250 ms ISI (LICI). The contralateral hemisphere in focal epilepsy showed a similar increase in MT (p<0.05, effect size 0.9) but there were no changes in SICI, LICI or ICF. Conclusions: This study shows that in the 24 hours prior to a seizure there is a net increase in measures of cortical excitability using TMS compared to baseline whereas post-ictally there is a net inhibitory effect on these measures. The effects were most marked in patients with IGE, were also seen in the ipsilateral hemisphere in focal epilepsy, whereas the side contralateral to the focus showed more complex changes, particularly pre-ictally.