Abstracts

Rationale: For precise functional brain mapping and the better understanding of seizure propagation, knowledge of the normal cortical function and network is important. Cortico-cortical evoked potential (CCEP), which can be induced by single pulse electrical stimulation to human cortex and recorded in the neighboring or remote cortices, has been widely used to probe brain network. Typically CCEP waveform consists of two negative potentials, N1 (peak: 10-50 ms) and N2 (peak: 100-500 ms). We, however, occasionally encounter atypical responses, such as those with a prolonged N1 peak latency. We propose CCEP waveform classification and attempt to clarify these atypical patterns and their anatomical distribution. Methods: e retrospectively reviewed CCEP responses in five patients with intractable focal epilepsy. CCEPs were obtained by applying single pulse electrical stimulation (1 Hz) to all the possible pairs of implanted subdural electrodes and averaging electrocorticogram time-lock to the stimuli (2 × 30 trials) (IRB#443/C1212). We classified CCEP into 6 groups based upon the N1 and N2 peak latencies; Ia: N1 (10-50 ms), N2 (100-500 ms), Ib: N1 (10-50 ms), N2: (50-100 ms), Ic: N1 (10-50 ms) without definite N2, IIa: N1 (50-100 ms), N2: (100-500 ms), IIb: N1 (50-100 ms) without definite N2, III: N1 (100-500 ms) without definite N2, IV: mirror CCEP response with positive peaks, V: oscillatory response with similar three peaks or more. CCEPs within 3 cm from the stimulated site, CCEPs obtained with stimulation to the electrodes on seizure onset zone (SOZ) or CCEPs observed in the SOZ electrodes were excluded in this study. Results: A total of 231stimulus pairs and 19903 recording electrodes were evaluated. Normal CCEP responses in terms of N1 latency and polarity (type Ia, Ib, and Ic) accounted for 48.5% among all the responses. Atypical CCEP responses (type IIa, IIb, III, IV, and V) accounted for 12.1%, 17.4%, 15.4%, 6.1%, and 0.4% among all the responses, respectively. With regards to anatomical preference, type IIa responses were more frequently observed in the interhemispheric fissure, type IIb responses in the lateral prefrontal/premotor area and in the interhemispheric part of superior parietal lobule, type III and IV responses in the temporal base, and type V responses in the postcentral gyrus and temporal base area. Conclusions: Among the atypical CCEP responses, type IIa, IIb and III responses with a relatively prolonged N1 latency were not rare phenomena. Their preferential anatomical distribution is likely due to the difference of cytoarchitectures, sulcal patterns or connectivity patterns. Further case accumulation is warranted to verify the results, which helps understand the significance of CCEP for presurgical evaluation. Funding: This study have been partly supported by KAKENHI 26282218, 15H01664, 15H05874, 15K10361 from the Japan Ministry of Education, Culture, Sports, Science and Technology (MEXT), the Research Grants from the Japan Epilepsy Research Foundation.