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 duri ng epilepsy surgery there is little evidence that the extent of the di scharging regions is a useful guide to tailoring the resection or that the findings are predictive of outcome or pathology. Patterns of disc harge propagation have, however rarely been considered in assessing th e ECoG. We hypothesize that regions where discharges show earliest pea ks ('leading regions') are located in the epileptogenic zone, whereas sites in which late, secondary propagated activity occurs have less ep ileptogenic potential and do not need to be excised To allow intraoper ative topographic ECoG analysis, a computer program has been developed to identify leading regions and the sites showing greatest rates or a mplitudes of spikes. Their topography has been compared retrospectivel y with pathology and seizure control in 42 consecutive patients follow ing temporal lobe surgery. Leading regions were most often found in th e hippocampus, the subtemporal cortex and the superior temporal gyrus. The most common propagation patterns were from hippocampus to subtemp oral cortex and vice versa There was no association between seizure ou tcome 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 l eading regions other than those in the posterior subtemporal cortex. A ll leading regions (other than posterior subtemporal) were resected in 27 patients of whom 25 had a favourable outcome. Leading regions (oth er than posterior subtemporal) remained in 14 patients of whom only fo ur had a good outcome. One patient had no epileptiform activity in the ECoG and good outcome. Persistent posterior subtemporal leading regio ns 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 intera ction between separate regions, resulting in propagation and recruitme nt of neuronal activity along specific neural pathways; (ii) removal o f all discharging areas appears unnecessary to achieve seizure control provided that lending regions (other than posterior subtemporal) are removed; and (iii) identification of such leading regions could be use d to tailor resections in order to improve seizure control and reduce neurological, neuropsychological and psychiatric post-surgical morbidi ty.