Introducing the Concept of Error Vectors to Improve Source Localization of Epileptic Discharges

Objective: To optimal estimate, the source location of epileptic discharges (ED) by improving the source localization of ED recorded on simultaneous stereo-EEG/scalp EEG.

Methods: Localization error of electrical source imaging (ESI) was evaluated in comparison to known sources of stimulation potentials (SP) by recording simultaneous stereo-EEG and 37-electrode scalp EEG. Error vectors were defined as the directed offset of the ESI-dipole of the SP to the stereo-EEG contacts used for stimulation. Error vectors were applied reversely to the ESI-dipole of interictal ED (IED-dipole).

Results: Seven interictal ED (IED) populations (5 temporal from non-lesional patients, 2 prefrontal from patients with suspected FCD) were evaluated (the latter, ictal ED were recorded at the same stereo-EEG contacts). Corrected IED-dipoles were located closer to IED onset contacts on stereo-EEG than uncorrected IED-dipoles (median (IQR): 7.8(2.5) vs 18.7(10.7) mm, P=0.02). For high skull conductivities (skull: skin conductivity ratio of 1:8 & 1:16), all corrected IED-dipoles were located partly in cortex and/or adjacent to focal cortical dysplasia whereas uncorrected dip-ED were located in white matter or CSF (P=0.03). The cortical extent of IED generators, which estimated from corrected IED-dipole was 16.5 (IQR 10.4) cm². The relationship of stereo-EEG IED amplitude (a) drop with increasing distance (d) from corrected IED-dipole could be modeled as a negative power equation a(d)∝ 1/d^b (R2=0.87, P<0.01), b ranging from -0.79 to -2.3, median:-1.57.

Conclusions: Application of error vectors moves the ESI-dipole of ED to an anatomically and physiologically plausible location.