Abstract
Electrical Impedance Tomography (EIT) is an emerging medical imaging technique which can produce tomographic images of internal impedance changes within an object using non-penetrating surface electrodes. It has previously been used to image impedance changes due to neuronal depolarisation during evoked potentials in the rat somatosensory cortex with a resolution of 2 ms and <200 μm, using an epicortical electrode array. The purpose of this work was to use this technique to elucidate the intracortical spatiotemporal trajectory of ictal spike-and-wave discharges (SWDs), induced by electrical stimulation in an acute rat model of epilepsy, throughout the cerebral cortex. Seizures lasting 16.5 ± 5.3 s with repetitive 2–5 Hz SWDs were induced in five rats anaesthetised with fentanyl-isoflurane. Transfer impedance measurements were obtained during each seizure with a 57-electrode epicortical array by applying 50 μA current at 1.7 kHz to two electrodes and recording voltages from all remaining electrodes. Images were reconstructed from averaged SWD-related impedance traces obtained from EIT measurements in successive seizures. We report the occurrence of reproducible impedance changes during the initial spike phase, which had an early onset in the whisker barrel cortex and spread posteriorly, laterally and ventrally over 20 ms (p < 0.03125, N = 5). These findings, which confirm and extend knowledge of SWD initiation and expression, suggest that EIT is a valuable neuroimaging tool for improving understanding of neural circuits implicated in epileptic phenomena.
Highlights
Averaged dZ responses to ictal spike-and-wave discharges (SWDs) were characterised by a consistent impedance decrease of −0.31 ± 0.06%, lasting ~20 ms, in phase with the spike component in the ECoG and a more variable increase of 0.16 ± 0.19% associated with the wave component, lasting > 100 ms (n = 168 seizures, 5 rats, Fig. 3)
Reproducible tomographic images of fast electrical activity during the spike phase of ictal SWDs are presented for the first time; these were generated from dZ recordings with a spatial and temporal resolution of 300 μm and ≤ 2 ms, respectively
The current work has shown that Electrical Impedance Tomography (EIT) can be employed for imaging fast electrical activity associated with epileptic discharges at a resolution of 300 μm and ≤2 ms in the rat cerebral cortex using non-penetrating surface electrodes
Summary
In many cases, such as non-lesional or extratemporal lobe epilepsy, scalp EEG recording is not adequate for localising epileptogenic foci and additional invasive telemetry methods are necessary (Duncan, 2011) This may be accomplished with implantation of subdural electrode mats on the exposed surface of the brain (electrocorticography (ECoG)) or insertion of depth electrodes into a lesion or the brain parenchyma (stereoencephalography) (Podkorytova et al, 2016). SWDs are seen in many focal and generalized epilepsies, and are common in typical childhood absence seizures, where they occur bilaterally at a frequency of 3–4 Hz (Panayiotopoulos, 2008) Of those individuals who undergo resective epilepsy surgery, under half remain seizure free ten years after surgery (de Tisi et al, 2011); postoperative seizure relapse can often be attributed to inaccuracies in localising foci (Spencer et al, 2005). Limitations of ECoG include the fact that epileptic discharges may not have electroencephalographic correlates if they originate from sources that are: located in deeper subcortical structures, oriented tangentially to the scalp, or extend over sulci or gyri with opposing source orientations, leading to selective cancellation of signals (Lüders, 2008; Schomer and Lopes da Silva, NeuroImage: Clinical 20 (2018) 674–684
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