Electrocorticography and outcome in frontal lobe epilepsy

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Abstract

The prognostic significance of epileptiform activity (EA) recorded at electrocorticography (ECOG) was examined in a group of 60 consecutive non-tumoral patients with intractable frontal lobe epilepsy (FLE). Pre-excision EA was documented as absent, focal (one gyrus), regional (two gyri), lobar (3 gyri) or multilobar (frontal+temporal gyri). Post-excision EA was documented as absent, restricted to the resection border, or recorded distant to the resection border, and was quantitated by spike frequency. Pre-excision EA from ≤2 gyri and absence of post-resection EA correlated with Class I or II (Engel classification) outcome while pre-excision EA from ≥3 gyri and persistent post-resection EA, especially distant to the resection border, correlated with Class III or IV outcome (P<0.001). A significant correlation between poorer outcomes and increased abundance of distant post-resection EA was observed (P<0.001). EA restricted to the resection border was not significantly correlated with outcome. Presence of a circumscribed lesion correlated with Class I outcome (P<0.01) and absence of pathological abnormality correlated with Class IV outcome (P<0.05). Neither side nor extent of surgical excision correlated with outcome. EA recorded at ECOG is of prognostic significance in FLE. A lobar or multilobar distribution of pre-excision EA and persistent post-excision EA distant to the resection border, especially when abundant, are highly unfavorable prognostic indicators. In contrast, a restricted distribution of pre-excision EA and absence of post-resection EA both herald a favorable outcome.

Introduction

The role of intra-operative electrocorticography (ECOG) in the surgical treatment of partial epilepsy is controversial. At many centers, interictal epileptiform activity (EA) recorded at ECOG has served as both a guide to the extent of surgical resection and as a prognostic indicator of post-surgical outcome (Jasper, 1954, Jasper, 1958; Bengzon et al., 1968; Gloor, 1975; So et al., 1989; Fiol et al., 1991; McBride et al., 1991; Ojemann, 1992; Salanova et al., 1994; Tuunainen et al., 1994; Kanner et al., 1995). A number of reports have indicated persistence of post-excision EA to portend an unfavorable outcome, and absence, or relative paucity, of post-excision EA to herald a favorable outcome (Jasper, 1954, Jasper, 1958; Bengzon et al., 1968; Gloor, 1975; So et al., 1989; Fiol et al., 1991; McBride et al., 1991; Salanova et al., 1994; Kanner et al., 1995). However, many other reports have found no significant correlation between post-resection EA and outcome (Falconer, 1958; Walker et al., 1960; Wyllie et al., 1987; Cendes et al., 1993; Tuunainen et al., 1994; Cascino et al., 1995; Tran et al., 1995; Kanazawa et al., 1996; Wennberg et al., 1997a).

With few exceptions (Quesney et al., 1992a; Salanova et al., 1994), the relevant literature has concentrated on temporal lobe epilepsy (TLE) or considered temporal and extra-temporal epilepsy together. It is apparent that, with respect to TLE, the prognostic utility of ECOG is equivocal (Fiol et al., 1991; McBride et al., 1991; Cendes et al., 1993; Tuunainen et al., 1994; Cascino et al., 1995; Tran et al., 1995; Kanazawa et al., 1996; Wennberg et al., 1997a). The utility of ECOG findings to guide the extent of surgical resection in TLE would also appear to be negligible, at least for standard anterior temporal resection (Tran et al., 1995) or selective amygdalo-hippocampectomy (Cendes et al., 1993; Wennberg et al., 1997a), although a different viewpoint has been expressed for smaller `tailored' temporal resections (Kanner et al., 1995).

Outcome following resective surgery for frontal lobe epilepsy (FLE) is less favorable than for TLE (Rasmussen, 1963; Talairach et al., 1974; Hajek and Wieser, 1988; Rasmussen, 1991; Cascino et al., 1992; Quesney et al., 1992b; Talairach et al., 1992; King et al., 1993; Salanova et al., 1993; Salanova et al., 1994; Olivier, 1995; Quesney et al., 1995). The reasons for this are unclear, but are presumably related to the propensity for FLE to be associated with large, or difficult to localize, epileptogenic zones (Quesney et al., 1992a; Salanova et al., 1993; Salanova et al., 1994; Olivier, 1995) as well as to the lack of a frontal equivalent to the highly organized epileptogenic temporal limbic system. To examine whether ECOG could be of prognostic significance specifically in FLE, the results of pre- and post-resection ECOG were compared with clinical outcome in a large group of patients with non-tumoral FLE. Results from this work have been previously published in abstract form (Quesney et al., 1996; Wennberg et al., 1996).

Section snippets

Materials and methods

A total of 60 patients with intractable FLE underwent resective surgery with ECOG between 1970 and 1994. Seven patients underwent re-operation with pre-resection ECOG (3 of whom also had post-resection ECOG) during this period for a total of 67 records (63 with post-resection recording). All patients had undergone extensive clinical, neuropsychological and neuroimaging studies prior to surgery as well as prolonged electroencephalographic (EEG) monitoring with extracranial +/− intracranial

Results

Detailed patient data are given in Table 1. Outcomes were 15 (22.4%), 11 (16.4%), 14 (20.9%) and 27 (40.3%) cases in Classes I, II, III and IV, respectively.

Pre-excision EA recorded from ≤2 gyri was significantly correlated with better (Class I or II) outcome, and pre-excision EA from ≥3 gyri with poorer (Class III or IV) outcome (P<0.001; Table 2; Fig. 1, Fig. 2A and Fig. 3A). Class I or II outcome was 87.5% (14/16) with EA from ≤1 gyrus and 72% (21/29) with EA from ≤2 gyri. In contrast, Class

Discussion

Outcome with respect to seizure control after surgery for FLE is less good than for TLE (Rasmussen, 1963; Talairach et al., 1974; Hajek and Wieser, 1988; Rasmussen, 1991; Cascino et al., 1992; Talairach et al., 1992; Quesney et al., 1992b; King et al., 1993; Salanova et al., 1993; Olivier, 1995; Quesney et al., 1995). Poorer outcome in FLE has been attributed to difficulty in localization of the epileptogenic zone within the broad expanse of the frontal lobe (Quesney et al., 1992a; Quesney et

Acknowledgements

We would like to acknowledge the work of Drs. P. Gloor, E. Andermann and F. Dubeau who performed some of the electrocorticograms used in this study. Dr. Quesney is a recipient of a Killam scholarship.

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