Magnetic Source Imaging in Posterior Cortex Epilepsies

Abstract

Posterior cortex epilepsies (PCE) are characterized by less satisfying postoperative results than temporal lobe epilepsies and are thus challenging for non-invasive presurgical investigations. The objective of this study was to evaluate the performance of magnetic source imaging (MSI) in PCE, validating the results by the SEEG (stereoelectroencephalography) definition of irritative and epileptogenic zones (IZ and EZ). Fourteen PCE surgery candidates were investigated using MSI and SEEG. LCMV (Linearly Constrained Minimum Variance) and MUSIC algorithms were used. IZ was quantified using a semi-automatic detection of interictal spikes. EZ was quantified using the epileptogenicity index (EI) method that accounts for both the propensity of a brain area to generate rapid discharges and the time for this area to get involved in the seizure. EI values range from 0 (no epileptogenicity) to 1 (maximal epileptogenicity). Levels of concordance between MSI and IZ, MSI and EZ were determined as follows: A = localized on MSI and SEEG for the site of value 1 (IZ and EZ quantification), B = localized on MSI and SEEG for a part of the IZ or a structure involved in the EZ (without the maximal value 1), C = localized on MSI and not SEEG, D = localized on SEEG and not MSI, E = localized on MSI and SEEG, discordant for site. Five PCE cases were characterized by focal IZ, nine by distributed IZ between several distant brain areas. MSI allowed to determinate IZ in 4/5 focal IZ cases. In case of distributed IZ, levels of concordance were A (2 cases), B (4 cases) and D (3 cases). In most distributed cases, MSI allowed to localize only a part of the IZ. Medial temporal involvement in the IZ was frequent (9/12 cases) and not evidenced by MSI. The brain area that demonstrated the maximal value of EI was shown by MSI in four out of five (80 %) focal IZ cases, in two out of nine (22 %) distributed cases. MSI results depend on IZ characteristics. A distributed IZ organization presents difficulties for MSI and highlights the need for further methodological approaches.

Defining epileptogenic networks: Contribution of SEEG and signal analysis

“…This kind of network distribution of spiking activity has also been observed in extratemporal epilepsies (Fig. ) …”

“…S1). 111 A study using the mean phase coherence algorithm revealed that areas of elevated local synchrony overlap with the EZ, suggesting that high local synchrony may be a marker for epileptogenic cortex. 112 Furthermore, this study suggested correlation between the resection of area of high local synchrony and good surgical outcome.…”

Intracranial EEG potentials estimated from MEG sources: A new approach to correlate MEG and iEEG data in epilepsy

“…Therefore, iEEG is only an incomplete gold standard as iEEG/MEG correlations is only valid within a close neighbourhood of each electrode contact. Correlations between MEG sources and invasive recordings has been mainly investigated using ECoG data [Agirre‐Arrizubieta et al, ; de Gooijer‐van de Groep et al, ; Fujiwara et al, ; Knowlton et al, ; Leijten et al, ; Mikuni et al, ; Minassian et al, ; Oishi et al, ; Tanaka et al, ], whereas some studies reported the comparison between MEG sources and iEEG recordings [Badier et al, ; Bouet et al, ; Gavaret et al, ; Jung et al, ; Schwartz et al, ], including our previous study reported in Heers et al []. Most of them consisted in qualitative description of the results, i.e., inference of sublobar accuracy between the two techniques.…”

“…Most of them consisted in qualitative description of the results, i.e., inference of sublobar accuracy between the two techniques. Whereas quantitative detection of the irritative and epileptogenic zone from iEEG data was provided in Badier et al [], the correlation with MEG sources remained qualitative and at a sublobar level. In this study and as a continuation of our previous qualitative evaluation [Heers et al, ], we proposed a quantitative approach to compare iEEG/MEG data taking into account the limited spatial sampling of iEEG data.…”

Focal epilepsies and focal disorders

“…In this case, the irritative zone is larger than the seizure onset zone (Talairach and Bancaud, 1966) and may form a distinct network independent from the seizure onset zone (Bourien et al, 2005). These regions represent the 'secondary irritative zone' and are generally regions affected by seizure propagation (Badier & Chauvel 1995;Badier et al 2014;Bettus et al 2010). Finally, even in intracranial EEG, concordance between the areas generating interictal spikes and those generating seizures is good for only about 60 % of patients (75% with focal cortical dysplasia) (Bartolomei et al, 2016).…”