Induction of prolonged electrographic seizures in vitro has a defined threshold and is all or none: implications for diagnosis of status epilepticus

Abstract

Purpose: To study whether induction of prolonged (>30-min duration) in vitro electrographic seizure discharges resembling status epilepticus (SE) is graded or all-or-none, and to determine the critical factors mediating SE induction.

Methods: Prolonged electrographic seizure discharges were induced in combined hippocampal-entorhinal cortical (HEC) brain slices by electrical stimulation of the Schaeffer collaterals. Discharges were recorded by using field-potential electrodes in the dentate gyrus, CA3, CA1, and entorhinal cortex. Slices were prepared from rats that were (a). 21- to 30-day-old naive, (b). 60- to 120-day old naive, (c). epileptic, and (d). status post a prior traumatic brain injury.

Results: Induction of SE discharges was dependent on the duration, but not amplitude of the preceding stimulus train-induced afterdischarge in HEC slices from 21- to 30-day-old control, brain-injured, and epileptic animals, but not from 60- to 120-day-old animals. In slices from 21- to 30-day-old control animals, once afterdischarges exceeded 4 min in duration, SE was induced in 50% of slices, and after >or=6 min 37 s seizure activity; SE was induced in 95% of slices. A defined SE threshold also was evident in brain-damaged rats, including rats in which an epileptic condition was induced by pilocarpine injection 4-16 weeks before recording, and rats subjected to a fluid percussive head trauma 1-8 weeks before recording. However, in these brain-damaged animals, mean SE threshold was considerably lower (24 and 44 s, respectively). HEC slices from 60- to 120-day-old controls for the brain-injured and epileptic animals did not develop SE even after 20 stimulations, demonstrating the pronounced effect of brain injury and epilepsy on the development of SE in the HEC slice preparation compared with that in age-matched controls.

Conclusions: In vitro, SE discharges have a defined temporal threshold for initiation. Once a seizure exceeds 6-7 min in duration in control animals, and 30-55 s in brain-damaged animals, the probability of SE induction is greatly increased. This demonstrates that brain injury lowers the afterdischarge duration required to produce SE and suggests that brains injured from trauma or SE are more susceptible to develop status epilepticus.

FIG. 1

Electrographic seizure discharges >30 min in duration fit the definition of status epilepticus (SE). Self-sustaining epileptic activity (SE) triggered by repeated stimulus trains in a hippocampal–entorhinal cortical slice prepared from a 30-day-old animal. Continuous SE was triggered after the eighth stimulus train, which persisted for >1 h. The increasing duration of the afterdischarges can be seen in the earlier stimulus trains. DG, dentate gyrus; EC, entorhinal cortex.

FIG. 2

Afterdischarge duration and not amplitude is critical for induction of status epilepticus (SE) in hippocampal–entorhinal cortical slices from 21- to 30-day but not from 60- to 120-day-old animals. Plots of CA1 afterdischarge amplitude (normalized to maximal amplitude) and afterdischarge duration against the stimulus train number eliciting the afterdischarge for (A) three slices from 21- to 30-day-old animals, and (B) three slices from 60- to 120-day-old animals. Note that the afterdischarge amplitude plateaus several trains before the induction of SE in all slices, whereas afterdischarge duration continuously increases until SE is induced. Open symbols, the afterdischarge duration; solid (black) symbols, the normalized amplitude. The different symbols used in each graph highlight the fact that each plot presents data from an individual animal.

FIG. 3

Quantification of stimulus train–induced electrographic seizure activity in hippocampal–entorhinal cortical slices prepared from 21- to 30-day-old control (A), 60- to 120-day-old control (B), brain-injured (C), and epileptic (D) animals. The graphs present a plot of Schaeffer collateral stimulus train number versus the duration of the afterdischarge elicited by the train. Recordings were stopped after 30 min of continuous electrographic seizure activity was triggered, representing the ceiling of activity duration (1,800 s). The status epilepticus (SE)-like discharges were triggered the earliest in the epileptic slices. The brain-injured slices also elicited prolonged discharges much earlier than the 21- to 30-day-old slices. The age-matched control slices (60–120 days old) for the brain-injured and epileptic animals did not develop SE. This demonstrates the marked effect of pilocarpine-induced SE and brain injury on the susceptibility of developing SE. The data give the means and standard errors of the afterdischarge duration in seconds.

FIG. 4

Histogram quantitating differences in status epilepticus (SE) thresholds. Note the statistically significant differences in SE afterdischarge duration threshold between control, brain-injured, and epileptic hippocampal–entorhinal cortical (HEC) slices. Control slices had significantly higher thresholds than either brain-injured or epileptic HEC slices *(p < 0.0001, t test), and brain-injured slices had SE thresholds significantly longer than epileptic slices *(p < 0.005, t test).