Partial seizures are associated with early increases in signal complexity

Abstract

Objectives: Partial seizures are often believed to be associated with EEG signals of low complexity because seizures are associated with increased neural network synchrony. The investigations reported here provide an assessment of the signal complexity of epileptic seizure onsets using newly developed quantitative measures.

Methods: Using the Gabor atom density (GAD) measure of signal complexity, 339 partial seizures in 45 patients with intracranial electrode arrays were analyzed. Segmentation procedures were applied to determine the timing and amplitude of GAD changes relative to the electrographic onset of the seizure.

Results: Three hundred and thirty out of 339 seizures have significant complexity level changes, with 319 (97%) having an increase in complexity. GAD increases occur within seconds of the onset of the partial seizure but are not observed in channels remote from the focus. The complexity increase is similar for seizures from mesial temporal origin, neocortical temporal and extra-temporal origin.

Conclusions: Partial onset seizures are associated with early increases in signal complexity as measured by GAD. This increase is independent of the location of the seizure focus.

Significance: Despite the often predominant rhythmic activity that characterizes onset and early evolution of epileptic seizures, partial seizure onset is associated with an early increase in complexity. These changes are common to partial seizures originating from different brain regions, indicating a similar seizure dynamic.

Figure 1

ICEEG of a seizure onset and GAD segmentation procedure to quantify complexity changes. (A) 30 seconds of the ICEEG for seven representative channels recorded at the onset of a complex partial seizure, three channels from the left frontal grid LFG (green) and four bottom channels from left anterior temporal strips LAT (red). (B & D) Color-coded matching pursuit time-frequency reconstruction of one minute of the ICEEG for focus channel LAT06-07 and remote channel LFG43-44 respectively. (C) Gabor atom density plot for the focus channel LAT06-07 and illustration of segmentation procedure. t_EO is the time of electrographical onset of the epileptic seizure (marked as zero). t₁, t₂, A₀, A₁ and A₂ are determined by the segmentation method. LVF is the GAD level difference between A₁ and A₀. TDF is the time difference between t₁ and t_EO. (D) Gabor atom density plot for the remote channel LFG43-44. Time t₁ and t₂ are same as in (C) and are used to calculate the Ar₀, Ar₁, Ar₂ levels. LVR - the GAD level change on the remote channel - is the difference between Ar₁ and Ar₀.

Figure 2

Color-coded GAD propagation map for the same seizure as illustrated in Figure 1. Propagation maps are composite reconstructions of the color-coded GAD for individual channels. Black lines on left side indicate the grids/strips. LFG: left frontal grid. LFP: left fronto-polar. LPT: left posterior temporal. LMT: left mesial temporal. LAT: left anterior temporal. Zero on the horizontal axis indicates the electrographical onset of the epileptic seizure t_EO. Although this is not a spatial representation of the spread over the brain, propagation maps allow us to identify the regions involved during the spread of ictal activity. Channels are listed consecutively, but on occasion consecutive channels are on opposite sides of the grid. The GAD propagation map demonstrates the focal onset (black arrow) at LAT06-07 contacts over mesial temporal regions. In addition the propagation map reveals the subsequent regional temporal lobe spread and minimal spread to frontal lobe contacts. White arrow indicates the chosen remote contact. The postictal period beginning at 145 seconds is characterized by general low complexity signals.

Figure 3

Plots of the GAD levels over a 3 hour period for the focus and remote electrodes with two complex partial seizures. Arrows indicate the time of the two seizures occurring within that window. The RPT strip was over the right posterior temporal region and the RPG grid over the right parietal region.

Figure 4

Boxplots of the GAD level changes for all 330 seizures measured at the focus (LVF) and at the remote electrode (LVR). Boxes extend from the 25% quartile to the 75% quartile (this defines the inter quartile range IQR). The black bar inside box represents the median, the arrow on the side indicates the mean. Bars above and below represents 1.5 times the IQR. Dots and stars represent mild and extreme outliers (defined as outside the IQR). LVF has one extreme outlier not represented in the graph at 0.63.

Figure 5

Histogram of the time of GAD increase for all 330 seizures.

Figure 6

Comparison of the GAD increase at focus (LVF) by region of onset. MTLE: mesial temporal lobe epilepsy. NeoTE: neocortical temporal epilepsy. NeoXTE: neocortical extra-temporal epilepsy. There is no significant difference between the regions of onset. Similar GAD increase was observed for all regions.