Detection of recurrent activation patterns across focal seizures: Application to seizure onset zone identification

https://doi.org/10.1016/j.clinph.2017.03.040Get rights and content

Highlights

  • Consistent recurrent seizures allow to estimate channel activation patterns delineating the seizure onset zone (SOZ).

  • The SOZ is characterized by low activation onset times and large time-average activation values.

  • SOZ discrimination is maximized below the gamma band when the entire seizure period is considered.

Abstract

Objective

We introduce a method that quantifies the consistent involvement of intracranially monitored regions in recurrent focal seizures.

Methods

We evaluated the consistency of two ictal spectral activation patterns (mean power change and power change onset time) in intracranial recordings across focal seizures from seven patients with clinically marked seizure onset zone (SOZ). We examined SOZ discrimination using both patterns in different frequency bands and periods of interest.

Results

Activation patterns were proved to be consistent across more than 80% of recurrent ictal epochs. In all patients, whole-seizure mean activations were significantly higher for SOZ than non-SOZ regions (P<0.05) while activation onset times were significantly lower for SOZ than for non-SOZ regions (P<0.001) in six patients. Alpha-beta bands (8–20 Hz) achieved the highest patient-average effect size on the whole-seizure period while gamma band (20–70 Hz) achieved the highest discrimination values between SOZ and non-SOZ sites near seizure onset (0–5 s).

Conclusions

Consistent spectral activation patterns in focal epilepsies discriminate the SOZ with high effect sizes upon appropriate selection of frequency bands and activation periods.

Significance

The present method may be used to improve epileptogenic identification as well as pinpoint additional regions that are functionally altered during ictal events.

Introduction

The accurate identification of the epileptogenic zone in patients with medically refractory epilepsy is central to plan an efficient surgical intervention. Yet, after decades of surgical treatment experience, the outcome is not completely successful in a significant proportion of patients (Spencer and Huh, 2008) for several causes including complex epileptogenic networks, surgery technical limitations, among others. Nowadays invasive recording techniques such as stereoencephalography (SEEG) (Munari and Bancaud, 1985, Talairach et al., 1974, Guenot et al., 2002, Engel et al., 2005) provide a continuous monitoring tool for pre-surgical diagnosis that becomes specially effective in patients with challenging focal epilepsies. However, those patients exhibiting complex activation patterns across different seizures still represent a diagnostic challenge because they involve time-consuming EEG evaluations that may lead to inconclusive interpretations. Hence, the use of quantitative tools that assess the consistency of ictal patterns across recurrent seizures might contribute to objectively identify the epileptogenic zone, thus improving pre-surgical diagnosis (Engel et al., 2005) and reducing potential failures. In addition, quantitative measures might provide additional insights into the mechanisms of focal ictogenesis (Goodfellow et al., 2016) when correlated with available structural and functional information.

Over the last decade, several studies have proposed biomarkers to evaluate the epileptogenicity of recorded intracranial structures. A great number of them have analyzed spectral features of SEEG signals (Bartolomei et al., 2008, David et al., 2011, Gnatkovsky et al., 2011, Gnatkovsky et al., 2014, Andrzejak et al., 2015), which mimic the patterns that are visually identified as epileptogenic during diagnostic EEG inspection (Gnatkovsky et al., 2014, Lagarde et al., 2016). In these works, epileptogenic biomarkers are typically built around two variables, the activation of signal’s spectral properties (e.g., the relative amount of signal power in a frequency band (Bartolomei et al., 2008)) and the onset time of this activation, which measure respectively the amount of ictal activity and the degree of early participation of such structures during seizures. Epileptogenicity is then defined for each structure by combining both variables into an index that may be averaged across distinct seizures (Bartolomei et al., 2008, David et al., 2011, Andrzejak et al., 2015) to obtain a single biomarker per region. Critically, these mathematical transformations leading to a single value per region might prove inaccurate when averages are performed across seizures with heterogeneous activation patterns.

In the current study, we developed a flexible, robust and visual-friendly computer-aided method to assess the homogeneity of two ictal-driven channel spectral patterns (mean power change and power change onset time) across recurrent focal seizures. We evaluated the method in a group of seven epileptic patients with temporal lobe drug-resistant focal seizures. In all patients, we obtained seizure averages of both activation patterns for each recorded channel that were represented in two-dimensional plots for complementary clinical evaluation. As an example of application, we used each averaged pattern to characterize the seizure onset zone (SOZ) of seven patients with well identified seizure focus and tested the variability of the results obtained against the main method’s parameters. The proposed procedure may be integrated into existing epileptogenic indices by choosing the frequency band of interest and appropriately plugging in each averaged pattern, thus contributing to identify in a robust form central regions in the generation and spread of focal seizures.

Section snippets

Ethics statement

All diagnostic and surgical procedures were approved by The Clinical Ethical Committee of our Hospital.

Patients and recordings selection

A total number of 46 focal seizures from seven patients with pharmacoresistant epilepsy were analyzed. A summary of all patients’ characteristics is given in Table 1. Seizure onset and termination times of each seizure were independently marked by two epileptologists (RR and AP) using standard clinical assessment. For each seizure we analyzed SEEG recordings from the marked ictal epoch

Normalized seizure ensemble (NSE)

In all studied patients, we computed the channels’ nMA following the method described above. The homogeneity of normalized values across recurrent seizures was assessed in six patients (Patient 3 had only one seizure) prior to perform seizure averages. In these cases, we identified a NSE composed by a large proportion of focal seizures (84%±6%, mean ± standard deviation, Table S1), which remained stable across a wide range of channel thresholding conditions (Fig. 3A). Furthermore, we could

Discussion

We proposed a robust methodology to assess recurrent ictal-driven patterns in intracranially recorded signals based on a temporal high-resolution spectral method (Hilbert transform Le Van Quyen et al., 2001, Oweis and Abdulhay, 2011). Our approach relies on finding an ensemble of focal seizures for every patient where these patterns can be compared across seizures and then, single biomarkers per channel can be meaningfully obtained to help define the most epileptogenic regions prior to surgery.

Conclusions

The present study proposes a general method to robustly quantify spectral activation patterns that are consistent across recurrent focal seizures. By exploiting the consistent channel activation profiles that patients exhibit across focal seizures, we propose a normalization procedure that allows to jointly analyze channel activation patterns from different seizures. The two-dimensional analysis of the time-average activation and the activation onset time during seizures provides a good

Disclosure

None of the authors has any conflict of interest to disclose. We confirm that we have read the Journal’s position on issues involved in ethical publication and affirm that this report is consistent with those guidelines.

Acknowledgments

The research was supported by the European Union's Horizon 2020 research and innovation program under Grant Agreement No. 720270. M.V. was supported by the Catalan Research project AL08814 - AGAUR - 2014SGR856. G.D. was supported by the European Research Council Advanced Grant DYSTRUCTURE (Grant 295129) and by the Spanish Research Project AEI/FEDER-PSI2016–75688-P. A.T.C. was supported by the European Research Council Advanced Grant DYSTRUCTURE (Grant 295129) and by the Spanish Research Project

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    1

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