Impedance Changes during Epileptic Seizures

Abstract

SUMMARY Impedance measurements and EEG recordings were made in ventral and dorsal hippocampus, amygdala and from the cortex during epileptic seizures induced in cats by injecting tungstic acid gel into the dorsal hippocampus or basolateral amygdala. Consistent changes in impedance occurred during seizures with an increase in the resistive component and a decrease in the capacitive component. Impedance changes provided focal correlates of the presence of epileptic activity. In regions of the brain where discharges did not spread, or spread only weakly, there were no significant impedance changes, but in regions powerfully involved in the seizures, significant shifts in impedance amounting to 10–12% of baseline were found. A regional specificity is indicated by large and consistent impedance changes in hippocampus on the one hand, particularly in the ventral hippocampus, and on the other by very small changes observed in the amygdala. Impedance changes begin after the onset of the seizure and return toward a baseline after termination of the seizure. This is interpreted to indicate that the observed changes in impedance are a consequence of the ictal events. RÉASUMÉA Les variations de l'impeAdance et l'eAlectroenceAphalogramme de l'hippocampe ventral et dorsal, de l'amygdale et du cortex furent enregistreAs sur des chats pendant des crises eApileptiques provoqueAes par injection de gel d'acide tungstique dans l'hippocampe dorsal ou dans la region basolateArale de l'amygdale. Des variations significatives de l'impeAdance ont eAteA observeAes pendant ces crises avec des augmentations de la composante reAsistive et des diminutions de la composante capacitive. Ces changements repreAsentent des manifestations localiseAes de la preAsence d'une activiteA eApileptique. Les reAgions du cerveau peu ou non atteintes par ces deAcharges n'ont montreA aucun changement significatif, par contre des variations de l'ordre de 10—12% furent rencontreAes dans les reAgions ouG la propagation des deAcharges eAtait importante. Une speAcificiteA reAgionale est indiqueAe par de larges variations de l'impeAdance dans l'hippocampe, particulieGrement dans l'hippocampe ventral, et par de petites variations dans l'amygdale. L'impeAdance commence aG varier apreGs le deAbut de la crise et tente de revenir aG son niveau initial apreGs la fin de la crise. Ce dernier point indique que les variations observeAes sont une conseAquence des eAveAnements critiques.

Determination of Optimal Parameters and Feasibility for Imaging of Epileptic Seizures by Electrical Impedance Tomography: A Modelling Study Using a Realistic Finite Element Model of the Head

“…Since cell swelling is less in epilepsy, the variation in the real and imaginary component of the resistivity would be reduced to the same extent in epilepsy with respect to SD; the same conversion was used also at frequencies different from 50 kHz. The predictions from these assumptions were similar to those found experimentally by Elazar et al (Elazar et al, 1966) and Fox et al (Fox et al, 2004). Table 2.…”

“…If successful, this would provide imaging evidence not possible with any other method and would obviate the need in some difficult cases for invasive investigation with intracranial electrodes (Fabrizi et al, 2006). It has already been shown that impedance increases locally in the brain by 3-12% at 1kHz up to 22% at 50 Hz during induced epileptic seizures (Elazar et al, 1966;Fox et al, 2004; VAN HARREVELD and Shade, 1962). Resistance changes of 5.5-7.1% associated with focal and generalized seizures have been imaged with EIT using a ring of electrodes placed on the exposed cortex of rabbits at 51 kHz (Rao, 2000).…”

An Evaluation of the Conductivity Profile in the Somatosensory Barrel Cortex of Wistar Rats

“…For example, brain electrical conductivity measurements have been applied to detect several brain pathologies, such as spreading depression (Holder 1992) and epilepsy (Elazar et al 1966). Abnormal neuronal activity in such patients might cause local disruption to ion homeostasis in the brain.…”

Characterisation and imaging of cortical impedance changes during interictal and ictal activity in the anaesthetised rat

“…EIT has recently entered into clinical use for lung imaging in hospital ICUs ( Becher et al, 2014 ), and has the potential for imaging brain function or pathological conditions ( Boone et al, 1994 , Oh et al, 2011 ). Localisation of the seizure onset zone has been proposed as a possible application of EIT ( Boone et al, 1994 ); it is based on the well-established rise of impedance of cerebral tissue during seizure activity ( Van Harreveld and Schade, 1962 , Elazar et al, 1966 ). This increase in impedance can be attributed to the shrinkage of extracellular space that follows intense neuronal activity, caused by homeostatic mechanisms that redistribute water and excess potassium ions generated during the activity ( Dietzel et al, 1980 , Holthoff and Witte, 2000 , Niermann et al, 2001 ).…”