The effect of vagus nerve stimulation on epileptiform activity recorded from hippocampal depth electrodes.

PURPOSE: To assess the effect of vagus nerve stimulation (VNS) on interictal epileptiform activity in the human hippocampus. Clinical studies have established the efficacy of vagus nerve stimulation in patients with epilepsy (VNS Study Group, 1995), although the electrophysiologic effects of VNS on the human hippocampus and mesial temporal lobe structures remain unknown. METHODS: We report a case study in which a patient with an implanted VNS underwent intracranial electrode recording before temporal lobectomy for intractable complex partial seizures. Epileptiform spikes and sharp waves were recorded from a depth electrode placed in the patient's left hippocampus. Spike frequencies and sharp-wave frequencies before and during VNS were compared using both a 5- and a 30-Hz stimulus. Different stimulation rates were tested on different days, and all analyses were performed using a Student's t test. RESULTS: We found no significant differences in spike frequency between baseline periods and stimulation at 5 and 30 Hz. In contrast, stimulation at 30 Hz produced a significant decrease in the occurrence of epileptiform sharp waves compared with the baseline, whereas stimulation at 5 Hz was associated with a significant increase in the occurrence of epileptiform sharp waves. CONCLUSIONS: VNS produces a measurable electrophysiologic effect on epileptiform activity in the human hippocampus. Although a clinical response to VNS did not occur in our patient before surgery, 30-Hz VNS suppressed interictal epileptiform sharp waves that were similar in appearance to those seen during the patient's actual seizures. In contrast, 5-Hz stimulation appeared to increase the appearance of interictal sharp waves.

Postictal EEG suppression and hippocampal atrophy in temporal lobe epilepsy.

Postictal EEG suppression and slowing recorded with scalp electrodes in patients with partial epilepsy is often maximal over the cortical area of ictal onset. The aim of this study was to determine whether a quantitative relationship exists between immediate postictal EEG suppression and hippocampal atrophy. Immediate postictal EEG was analyzed in 31 scalp-recorded seizures obtained from 8 patients who underwent temporal lobectomy with seizure-free outcomes (2 left, 6 right). Quantitative EEG analysis was performed using a temporal power asymmetry index for each frequency band. The hippocampal asymmetry (left-to-right ratio) based on T1- and T2-weighted MR images was determined by hippocampal volumetric analysis. The relationship between the average temporal power asymmetry index and either T1 or T2 hippocampal asymmetry ratio was assessed for each frequency band using Pearson's correlation coefficient. Only correlations of the temporal power asymmetry index with T1 hippocampal asymmetry were significant for the total bands (r = 0.768, P < 0.026) and 8-bands (r = 0.728, P < 0.041). The findings suggest that a quantitative relationship exists between postictal EEG suppression in the 6-frequency band and hippocampal atrophy in temporal lobe epilepsy.

The Louisiana State University Comprehensive Epilepsy Program: procedures and outcomes.

The first comprehensive epilepsy surgery center in Louisiana was established in 1990 at the Louisiana State University Medical Center in New Orleans by the Departments of Neurology and Neurosurgery. The center performs a wide variety of diagnostic tests essential for the medical and surgical treatment of epilepsy including EEG and video monitoring, quantitative hippocampal MRI volumetry, ictal SPECT brain scanning, intracranial evoked potential and subdural stimulation functional mapping, neuropsychological evaluations, and intracarotid amobarbital (Wada) language and memory localization. Surgical interventions include (1) the placement of subdural strip and grid electrodes, depth electrodes, and foramen ovale electrodes, (2) temporal lobectomies, and (3) frontal, temporal, parietal, and occipital lobe resections. From August 1990 through October 1995 41 patients with medically intractable seizures underwent neurosurgical procedures for epilepsy. Thirty-five patients had resective surgery, while six had only intracranial monitoring by subdural or intracerebral electrodes. The surgical outcomes thus far compare favorably with those of other established centers in North America.