The value of intraoperative electrocorticography in surgical decision making for temporal lobe epilepsy with normal MRI

The role of surgical resection in temporal lobe epilepsy (TLE) is well-established (Wiebe et al., 2001; Jutila et al., 2002; Spencer et al., 2003; Yoon et al., 2003; McIntosh et al., 2004; Spencer et al., 2005). Although historically patients with TLE have been categorized as a single group, recent work has shown subclassifications based on clinical features, imaging, histology, and electroencephalography (EEG) (Du et al., 1993; Burgerman et al., 1995; Gil-Nagel & Risinger, 1997; Bertram & Scott, 2000; Bertram, 2009; Ogren et al., 2009; Stefan et al., 2009). One of the main challenges facing clinicians is the patient with presumed TLE but a normal magnetic resonance imaging (MRI) scan. Unlike patients with hippocampal atrophy, who have a seizure-free rate after surgery approaching 70–80%, TLE with normal MRI has a 40–60% rate of surgical cure (Berkovic et al., 1995; Holmes et al., 2000; Ozkara et al., 2000; Cohen-Gadol et al., 2005; Alarcon et al., 2006; Tatum et al., 2008; Bell et al., 2009). Ambiguities about seizure focus localization pose a challenge to resection, since onsets may be either neocortical or mesial and be clinically indistinguishable (Sylaja et al., 2004; Chapman et al., 2005; Bell et al., 2009). For these patients, chronic intracranial electrocorticography (ECoG) is often employed to acquire ictal data to guide temporal lobe resection (Cascino et al., 1995; Siegel et al., 2001; Kuruvilla & Flink, 2003; Cohen-Gadol et al., 2005; Oliveira et al., 2006). Although relatively safe, electrode implantation significantly increases length of stay and is associated with a 10–26% rate of complications, including hematoma and infection (Van Gompel et al., 2008; Wong et al., 2009). The risks and costs are higher than a single-stage surgery (Hamer et al., 2002). For this reason, techniques to localize seizure foci such as positron emission tomography (PET), ictal single-photon emission computed tomography (SPECT), functional (f)MRI, and magnetoencephalography (MEG) have been pursued (Assaf et al., 2003, 2004; Stephen et al., 2005; Willmann et al., 2007; Boling et al., 2008; Chassagnon et al., 2009; la Fougere et al., 2009; Kobayashi et al., 2009). Clinical history, neuropsychological, and Wada testing can also assist in this distinction (Loring et al., 1994; Sperling et al., 1994; Sylaja et al., 2004). However, none of these adjuncts have high enough sensitivity and specificity to completely obviate the need for ECoG in these patients. Another useful technique for localizing epileptogenic cortex is acute (intraoperative) ECoG. The main limitation of acute ECoG is that usually only interictal data is recorded. Acute ECoG has been proven helpful in defining the limits of neocortical lesional epilepsy surgery; however, its use in TLE is controversial (Schwartz et al., 1997; Holmes et al., 2000; Oliveira et al., 2006). Studies have evaluated the predictive value of postresection spikes or preresection spike-tailored resections (Schwartz et al., 1997; McKhann et al., 2000; Chen et al., 2006; Oliveira et al., 2006), but few have correlated the results of acute ECoG with chronic ECoG. Likewise, there are few data on the utility of intraoperative ECoG in MRI-negative TLE. A recent report showing a strong correlation between the distribution of intracranially recorded interictal spikes and ictal onsets in TLE supports an interest in evaluating acute ECoG efficacy in MRI-negative TLE (Goncharova et al., 2009). The purpose of this study was to determine if acute ECoG could screen patients with MRI-negative TLE to select who would require chronic implantation of electrodes and who could move directly to a standard anteromedial resection (SAMR) (Spencer et al., 1984). We also hypothesized that these patients might have better outcomes with respect to seizure control.