Abstracts

Rationale: Absence epilepsy is a frequent seizure type of school age children. Children have periods of impaired attention coincident with the 3 hertz (Hz) generalized spike and slow wave on electroencephalogram (EEG). The phenomena of the 3 Hz spike and wave is poorly understood. Animal models provide insights, but fail to reproduce the clinical condition seen in children. There has been an evolution of the hypotheses to explain the electrical generation of the spike and wave pattern. Dynamic systems modeling of EEG may provide insights into the mechanisms. Phase space plots are the starting point of non-linear system analysis. With phase space plots, there is a recurrent pattern that reflects the behavior of the system analyzed. The spatial distribution of the trajectory is plotted against time. If the plot returns to a specific region, then an attractor is said to exist. Methods: The EEG of 12 children, meeting the ILEA criteria for childhood absence epilepsy (CAE), were examined. All data were acquired using Nicolet BMSI hardware to record a 24 channel, up to 1 hour, EEG using the international 10/20 of Electrode placement. EEG records were reviewed using 15 mV per millimeter sensitivity with high voltage filter at 15 Hz and low frequency filter set at 1.0 Hz. The F7 channel of the EEG was selected for actual analysis. The EEG seizure data was converted to a .cvs file. The .cvs file was read by a MatLab macro for phase space plotting. Each point in time was contained in the .cvs file. Curvilinear modeling was performed with the values of voltage (t) plotted as a factor of time against its first derivative (1/t). Phase space plots were generated from clinical and electrographic absence seizure for the length of the seizure plus 1 second on either end of the seizure, and of the baseline EEG. Results: There were 12 children with EEG’s meeting the criteria of CAE on EEG from 3/07 to 7/08. There were 10 girls and 2 boys with an average age of 8.26 years. Average seizure length was 15.3 seconds [range 8-36]. Twenty-two seizures were analyzed. Phase space plots were generated of the absence seizures and compared to the baseline phase space plots. With seizure onset, there is an abrupt expansion of recurrent loops around the attractor until the seizure terminates (Figure 1a). The baseline plots reveal stable, small loops around the attractor (Figure 1b). Plots are also made as the phase space plot is generated over the entire seizure, and in one to five second steps. Conclusions: 1. Consistent phase space plots are generated in seizures of children with CAE and common patterns are seen, but several subgroups may exist in this sample. 2. Baseline EEG phase space plots show spatially restricted attractors. 3. Further studies of the phase space plots in a larger population would help confirm that several patterns of phase space plots exist and could be correlated with clinical covariates. 4. Further analysis with non-parametric modeling could provide further insights, since the patterns of phase space plots are most consistent with a nonlinear, chaotic or a limit torus attractor.