Generation of scalp discharges in temporal lobe epilepsy as suggested by intraoperative electrocorticographic recordings.

OBJECTIVES: To study the variability, topography, polarity, duration, and incidence of interictal epileptiform discharges (EDs) in the scalp EEG and electrocorticogram (ECoG) from 16 patients with temporal lobe epilepsy who underwent surgical treatment. METHODS: Preoperative scalp EEGs during quinalbarbitone induced sleep were compared with preresection ECoGs obtained under general anaesthesia. The analysis was based on the initial ECoG record obtained before activation by intravenous thiopentone, and the EEG during stages I and II of sleep. RESULTS: On the scalp, 15 patients had a single discharge pattern, spikes were predominantly negative, EDs were of largest amplitude at the anterior temporal electrode in 13 patients and mean discharge incidence was 4.0 (SD 4.2) discharges/min. In ECoG recordings, nine patients had two independent ECoG patterns, the polarity of spikes was negative, positive-negative, or positive, the site of maximal amplitude varied greatly between subjects, discharge incidence was 7.3 (SD 3.9) discharges/min. There was no relation between the topography of the largest spikes on the scalp and in the ECoG. In 14 patients, scalp spikes showed statistically significant longer duration on the scalp than in the ECoG. In seven patients who had frequent widespread ECoG discharges, averaging spikes across ECoG channels generated spiky patterns of duration similar to that of scalp spikes. CONCLUSION: It seems that, in temporal lobe epilepsy, scalp discharges originate from widespread ECoG discharges and tend to produce a stereotyped pattern on the scalp with largest amplitudes at the anterior temporal electrodes. This is probably due to local anatomical peculiarities in the brain coverings, such as skull discontinuities, rather than to the location of neuronal generators within the temporal lobe. Due to spatiotemporal averaging, widespread cortical discharges which become asynchronous during propagation appear with increased duration and blunted waveform in the EEG, whereas sharply localised phenomena such as positive focal spikes are not recorded from the scalp.

Origin and propagation of interictal discharges in the acute electrocorticogram. Implications for pathophysiology and surgical treatment of temporal lobe epilepsy.

Although acute electrocorticography (ECoG) is routinely performed during epilepsy surgery there is little evidence that the extent of the discharging regions is a useful guide to tailoring the resection or that the findings are predictive of outcome or pathology. Patterns of discharge propagation have, however, rarely been considered in assessing the ECoG. We hypothesize that regions where discharges show earliest peaks ('leading regions') are located in the epileptogenic zone, whereas sites in which late, secondary, propagated activity occurs have less epileptogenic potential and do not need to be excised. To allow intraoperative topographic ECoG analysis, a computer program has been developed to identify leading regions and the sites showing greatest rates or amplitudes of spikes. Their topography has been compared retrospectively with pathology and seizure control in 42 consecutive patients following temporal lobe surgery. Leading regions were most often found in the hippocampus, the subtemporal cortex and the superior temporal gyrus. The most common propagation patterns were from hippocampus to subtemporal cortex and vice versa. There was no association between seizure outcome and the location of regions with greatest incidence or amplitude of spikes or location of leading regions. There was, however, a strong and significant association between poor outcome and non-removal of leading regions other than those in the posterior subtemporal cortex. All leading regions (other than posterior subtemporal) were resected in 27 patients of whom 25 had a favourable outcome. Leading regions (other than posterior subtemporal) remained in 14 patients of whom only four had a good outcome. One patient had no epileptiform activity in the ECoG and good outcome. Persistent posterior subtemporal leading regions remained in nine subjects; all had favourable outcome (Grades I or II) but only three were seizure free. These results suggest that: (i) interictal epileptiform discharges may originate from a complex interaction between separate regions, resulting in propagation and recruitment of neuronal activity along specific neural pathways; (ii) removal of all discharging areas appears unnecessary to achieve seizure control provided that leading regions (other than posterior subtemporal) are removed; and (iii) identification of such leading regions could be used to tailor resections in order to improve seizure control and reduce neurological, neuropsychological and psychiatric post-surgical morbidity.