Ictal spikes: a marker of specific hippocampal cell loss

Implications of seizure termination location in temporal lobe epilepsy

Where propagating symptomatic seizures terminate has not been studied, but might provide insight into mechanisms of seizure termination as well as localization of epileptogenic tissue. We investigated location of seizure termination in 50 refractory temporal lobe epilepsy (TLE) patients who had intracranial EEG recording of spontaneous seizures and subsequent temporal lobe resection with > 1-year follow-up. Only seizures that had onset in the resected temporal lobe were included. Location of the electrical termination for each seizure in each patient was categorized as diffuse, localized to the onset location, or localized elsewhere. The proportion of all seizures in each patient in each category was analyzed with respect to the outcome of surgery. Outcome was classified as seizure-free or persistent seizures. Diffuse seizure termination was noted equally frequently in both outcome groups. However, the 27 patients without seizures postoperatively had a significantly greater proportion of seizures with termination in the onset location (67%) than did the 23 patients with persistent seizures (36%, p < 0.01). The seizure-free patients also had a significantly lower proportion of seizures with localized termination elsewhere than the onset site (13%) than did patients with persistent seizures (45%, p < 0.005). Localization of the site of termination of seizures of focal origin to cortical regions other than the onset is associated with a poorer surgical prognosis. This observation raises the possibility of additional abnormal epileptogenic cortical regions with impaired seizure-terminating capabilities.

Depth electrode studies and intracellular dentate granule cell recordings in temporal lobe epilepsy

Hippocampal depth electrodes are often used to localize seizure onset in patients who may have temporal lobe epilepsy (TLE). A number of features of the spontaneous seizures and of their ictal onset patterns can be analyzed from these recordings. We compared a number of the typical electroencephalographic (EEG) changes at seizure onset with several cellular parameters recorded in dentate granule cells from the same 14 patients diagnosed with medial temporal sclerosis (MTS) to examine the pathophysiological correlates of this spontaneous EEG activity in this form of TLE. The intracellularly recorded parameters include the propensity to fire evoked epileptiform bursts, the absence of evoked inhibitory potentials, the presence of polysynaptic excitatory postsynaptic potentials, and the presence of spontaneous excitatory activity. We noted several correlations between the EEG data and the intracellular recordings. The absence of synaptically evoked bursts was correlated with the presence of low-voltage fast activity at seizure onset. In addition, the loss of inhibitory postsynaptic potentials was correlated with the presence of periodic spiking pre-ictally. Several other correlations were also noted. These data indicate that EEG findings may be predictive of anatomical and cellular pathological changes and provide clues to the physiological mechanisms involved in this form of epilepsy.

Frequency of the electroencephalographic discharge in seizures of focal and widespread onset in intracerebral recordings

The information provided by the EEG during an epileptic seizure is of critical importance in the localization of an epileptic focus. Localizing information is usually provided by the region of onset, by the predominance of the discharge during the seizure, and by postictal slow waves. In patients with intracerebral electrodes, we investigated another aspect of the discharge: its highest frequency. We divided seizures into those with a focal onset and those with a regional onset. Results indicated clearly that fast frequencies (15-30 Hz) were much more frequent during seizures of focal onset than during seizures of widespread onset. Highest frequencies were not necessarily observed at the onset of focal seizures but could be observed at any time during a seizure of focal onset. High frequencies appear to characterize small epileptogenic zones (EZ). Our results may be helpful in the interpretation of intracerebral EEGs when large brain areas remain unexplored and one is often uncertain of the extent of an EZ.

Entorhinal-hippocampal interactions in medial temporal lobe epilepsy

Experimental studies suggest important interactions between hippocampus and entorhinal cerebral cortex in generation of temporal lobe seizure activity. We studied electrical expression of spontaneous temporal lobe ictal activity in hippocampus and entorhinal cortex in 9 medically refractory epileptic patients who had intracranial depth and subdural electrodes implanted during surgical evaluation. All 9 patients subsequently underwent anteromedial temporal lobectomy with hippocampectomy, all had > 50% decrease in neuronal cell density in hippocampal CA1 and CA3, and all had good to excellent seizure outcome after operation. Two to 10 spontaneous seizures were analyzed per patient (total 41 seizures). Nine patients had variable onset of seizure activity recorded in hippocampus, entorhinal cortex, or both simultaneously. Low-voltage fast activity was observed in either location and varied among seizures in an individual patient. Periodic preictal spikes, when present, were often synchronous in both locations, but were noted independently only in hippocampus. Our data suggest that preictal spikes and low-voltage fast seizure discharges have anatomically distinct origins, and that some syndromes of medial temporal lobe epilepsy involve interactions between entorhinal and hippocampal regions that act together to produce and propagate the seizures in such patients.