Stability of Synchronization Clusters and Seizurability in Temporal Lobe Epilepsy

<div><h3>Purpose</h3><p>Identification of critical areas in presurgical evaluations of patients with temporal lobe epilepsy is the most important step prior to resection. According to the “epileptic focus model”, localization of seizure onset zones is the main task to be accomplished. Nevertheless, a significant minority of epileptic patients continue to experience seizures after surgery (even when the focus is correctly located), an observation that is difficult to explain under this approach. However, if attention is shifted from a specific cortical location toward the network properties themselves, then the epileptic network model does allow us to explain unsuccessful surgical outcomes.</p> <h3>Methods</h3><p>The intraoperative electrocorticography records of 20 patients with temporal lobe epilepsy were analyzed in search of interictal synchronization clusters. Synchronization was analyzed, and the stability of highly synchronized areas was quantified. Surrogate data were constructed and used to statistically validate the results. Our results show the existence of highly localized and stable synchronization areas in both the lateral and the mesial areas of the temporal lobe ipsilateral to the clinical seizures. Synchronization areas seem to play a central role in the capacity of the epileptic network to generate clinical seizures. Resection of stable synchronization areas is associated with elimination of seizures; nonresection of synchronization clusters is associated with the persistence of seizures after surgery.</p> <h3>Discussion</h3><p>We suggest that synchronization clusters and their stability play a central role in the epileptic network, favoring seizure onset and propagation. We further speculate that the stability distribution of these synchronization areas would differentiate normal from pathologic cases.</p> </div

Localization of CV₍₁₎ to CV₍₅₎ and resected tissue in each patient (Pearson correlation coefficient).

<p>(A) Cortical grid electrodes: Gray areas represent the approximate lateral cortical tissue resected during surgery. Superimposed, CV minima are displayed according to the key bar (right): CV₍₁₎ = 1st, CV₍₂₎ = 2nd, CV₍₃₎ = 3rd, CV₍₄₎ = 4th, and CV₍₅₎ = 5th. Patients with postoperative seizures are highlighted in blue. G1 is always at the bottom left and G20 at the top right position. (B) Mesial strip electrodes: Same schematic diagram as in panel (A).</p

CV, µ and σ relations.

<p>(A) Three-dimensional plot showing the relations between mean (µ), deviation (σ) of LS, and CV. The first three lower CVs for each record are coloured in green. (B) Projection of the three-dimensional plot of (A) in the µ-σ plane.</p

Surrogate distributions and CV_(i) correlations.

<p>(A) Density distributions of correlation coefficients for surrogate data. The upper panel corresponds to the case of a single random location. The lower panel corresponds to the case of combinations of two, three, four, and five. See text for explanation. (B) Correlation coefficient between seizure success sequence (2) and CV_(i) resection, for 1 = 1, …28.</p

Correlation matrix for the full set of electrodes.

<p>Electrodes labeled G1 through G20 belong to the lateral temporal grid; electrodes labeled S1 through S8 belong to the mesial strip. representation corresponds to a single temporal window of patient “G” (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0041799#pone-0041799-t001" target="_blank">Table 1</a>).</p

Temporal evolution of LS for each electrode in a specific patient.

<p>Each temporal window lasts 10.24 seconds. Representation corresponds to patient “G” (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0041799#pone-0041799-t001" target="_blank">Table 1</a>).</p

Estimates, p-value and Z-scores in the four methods of synchronization between the resection of minima and seizure success.

<p>Estimates, p-value and Z-scores in the four methods of synchronization between the resection of minima and seizure success.</p

CV for the evolution of LS throughout the recording for each electrode.

<p>Data correspond to the activity displayed in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0041799#pone-0041799-g002" target="_blank">Figure 2</a> corresponding to patient “G”.</p

LS and CV distributions. (A) Density and (B) cumulative distributions of CV for every electrode from every patient.

<p>A lognormal distribution is superimposed (meanlog  = −1.9081476, sdlog  = 0.5620142). (C) Density and (D) cumulative distributions of mean LS values for all the electrode of each patient. A normal distribution is superimposed (mean  = 0.5228, standard deviation  = 0.1507480).</p