Abstracts

The pattern of ictal and interictal activity following an epileptogenic insult has not been characterized in detail. For example, it is not clear whether interictal activity precedes or follows ictal activity, or whether the appearance of interictal activity is highly correlated with eventual ictal activity.
EEG data was accumulated from 4 weeks of continuous recordings from epidural electrodes implanted in rats, transmitted via radiotelemetry (Data Sciences International), and digitized via routines written in Visual Basic (Microsoft). Status epilepticus was induced with repeated low-dose (5 mg/kg) injections of kainic acid (intraperitoneal) 1 week following onset of telemetry. Ictal and interictal activity was detected based on spectral characteristics.
Interictal activity preceded ictal activity in all animals (N=4). The frequency of interictal activity was highly correlated with the number of minutes of seizures per day in each rat, but the number of rats was too small to determine if absolute interictal frequency was a reliable indicator of the onset of ictal activity. Interictal activity appeared in the first week following kainate treatment, and morphology of interictal activity varied considerably in the first 4 weeks following kainate treatment. The amount of ictal activity increased steadily at a rate of 0.02-0.3 seizure-minutes/day in the first 4 weeks following kainate treatment.
Ictal and interictal activity following an epileptogenic insult increases gradually over weeks. We did not see evidence for a sudden step from a nonepileptic to epileptic state. The latent period following an epileptogenic insult appears to be a function of the resolution of the techniques used to monitor epileptic activity, and a more accurate hypothesis might be that seizure probability increases steadily though gradually following an epileptogenic insult.
[Supported by: NS 034360 and NS 045144]