Abstracts

Short-Term Maximum Lyapunov Exponent (STLmax) is a measure of the order of a dynamical system. Smaller STLmax values indicate that the signal is more ordered. In temporal lobe epilepsy, the dynamical properties of the preictal, ictal and postictal states are distinctly different and can be defined quantitatively. STLmax has been observed to be the lowest during a seizure, intermediate in the preictal period and highest in the postictal state. Antiepileptic medications are known to reduce seizure frequency in most epilepsy patients. We postulate that high levels of antiepileptic medications are associated with higher STLmax. The objectives of this study are to compare STLmax values from intracranial EEG recordings when patients have high, medium and low levels of antiepileptic medications. Two patients with intracranial subdural grids were recruited: patient 1 had right fronto-temporal and patient 2 had left fronto-central grids. Patients were tapered off their antiepileptic medications to record seizures. Patient 1 was on phenytoin, levetiracetam, carbamazepine and zonisamide and Patient 2 was levetiracetam and topiramate. Medication dosages were collected daily for all antiepileptic drugs during the entire monitoring period and medication levels were obtained for the older drugs. For each patient, three distinct time segments were determined based on their medication dosages and levels. Day 1: high medication levels, Day 2: medium levels and Day 3: low levels. All three segments were chosen to be at least 6 hours prior or after a seizure. For each patient, a total of 8 channels were selected for analysis. Four channels were within the epileptic focus and four outside the epileptic focus. For each analyzed electrode, STLmax was calculated for each non-overlapping 10.24 second segment (4096 EEG points in 400 Hz recording). A total of 100 mean STLmax values were randomly sampled from time intervals in each of the three medication levels. One-way ANOVA test was applied to test the significance of the effect among medication levels. If the effect was found significant, multiple comparison tests were utilized to test the significance of pairwise differences. In both patients, within both epileptic and non-epileptic areas, EEG signals recorded during high medication level (day 1) exhibited significantly higher (p [lt] 0.01) STLmax values (less ordered) than during medium (day 2) and low (day 3) medication levels. Antiepileptic medications have an influence on STLmax. High levels are associated with increased STLmax value. This effect may be related to the anticonvulsant properties of these medications.