SEEG in 3D: Interictal Source Localization From Intracerebral Recordings

Abstract

BackgroundStereo-electroencephalography (SEEG) uses a three-dimensional configuration of depth electrodes to localize epileptiform activity, but traditional analysis of SEEG is spatially restricted to the point locations of the electrode contacts. Interpolation of brain activity between contacts might allow for three-dimensional representation of epileptiform activity and avoid pitfalls of SEEG interpretation.ObjectiveThe goal of this study was to validate SEEG-based interictal source localization and assess the ability of this technique to monitor far-field activity in non-implanted brain regions.MethodsInterictal epileptiform discharges were identified on SEEG in 26 patients who underwent resection, ablation, or disconnection of the suspected epileptogenic zone. Dipoles without (free) and with (scan) gray matter restriction, and current density (sLORETA and SWARM methods), were calculated using a finite element head model. Source localization results were compared to the conventional irritative zone (IZ) and the surgical treatment volumes (TV) of seizure-free vs. non-seizure-free patients.ResultsThe median distance from dipole solutions to the nearest contact in the conventional IZ was 7 mm (interquartile range 4–15 mm for free dipoles and 4–14 mm for scan dipoles). The IZ modeled with SWARM predicted contacts within the conventional IZ with 83% (75–100%) sensitivity and 94% (88–100%) specificity. The proportion of current within the TV was greater in seizure-free patients (P = 0.04) and predicted surgical outcome with 45% sensitivity and 93% specificity. Dipole solutions and sLORETA results did not correlate with seizure outcome. Addition of scalp EEG led to more superficial modeled sources (P = 0.03) and negated the ability to predict seizure outcome (P = 0.23). Removal of near-field data from contacts within the TV resulted in smearing of the current distribution (P = 0.007) and precluded prediction of seizure freedom (P = 0.20).ConclusionsSource localization accurately represented interictal discharges from SEEG. The proportion of current within the TV distinguished between seizure-free and non-seizure-free patients when near-field recordings were obtained from the surgical target. The high prevalence of deep sources in this cohort likely obscured any benefit of concurrent scalp EEG. SEEG-based interictal source localization is useful in illustrating and corroborating the epileptogenic zone. Additional techniques are needed to localize far-field epileptiform activity from non-implanted brain regions.

A spatial perturbation framework to validate implantation of the epileptogenic zone

“…This was done by evaluating the implantation, and its ability to record an SOZ which fits certain priors on focality and presence of interictal markers of epileptogenicity. Previous studies have only attempted to refine the localization of the epileptic focus 13 , 28 , or aimed to find spiking regions or SOZ missed by the SEEG 14 , 17 – 19 . Although finding novel spiking regions is a valuable tool to study the interictal network of spikes, they are also known to delineate the irritative zone, which encompasses a larger region than the EZ 15 .…”

“…This might suggest that good EZ coverage is required to observe this flow, but this was not assessed in this work. Other studies have applied electrical source imaging methods on SEEG data to improve the localization of the EZ 17 , or predict surgical outcomes 18 , 19 . They have shown that localizing IEDs or seizures recorded by the SEEG can localize far-field activity not sampled by the SEEG, and using this information, predict surgical outcomes.…”

Semiautomated electric source imaging determines epileptogenicity of encephaloceles in temporal lobe epilepsy

“…We defined the onset of an averaged IED as the first 10% of the entire IED, accordingly: either the earliest deflection in the first 7 ms of its rising flank that met the criteria for nonpolyphasic IEDs or the first peak in polyphasic IEDs if the negative amplitude of the highest amplitude of the averaged IED was preceded by an additional amplitude peak (see also Figure 1). 12,27 Half-rise was defined as the midpoint between the onset of the rising flank and the maximum negativity of the averaged IED. 12,27 IED propagation measures the Euclidean distance between the localization of the source maximum at the IED onset and its maximum at the IED peak of the respective inverse method.…”

“…12,27 Half-rise was defined as the midpoint between the onset of the rising flank and the maximum negativity of the averaged IED. 12,27 IED propagation measures the Euclidean distance between the localization of the source maximum at the IED onset and its maximum at the IED peak of the respective inverse method. 12 Colocalization was referred to as concordance by two distinct inverse methods.…”

Automated seizure onset zone locator from resting-state functional MRI in drug-resistant epilepsy

“…Invasive modalities such as intracranial EEG (iEEG) are considered the gold standard for SOZ identification and have shown excellent accuracy for both adults and pediatric DRE. Stereo-electroencephalography (SEEG) is minimally invasive, uses a three-dimensional configuration of depth electrodes to localize epileptiform activity, and has shown some SOZ identification capability recently (Satzer et al, 2022 ).…”