Interictal scalp EEG

In the presence of a neocortical lesion EEG often is unhelpful. The spatial distribution of an EEG focus coincides with lesion localization in less than one-third of patients, and can

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indeed be widely discrepant (Figures 5.13 and 5.14). This is due to the rapid and wide propagation of seizure discharges in neocortical regions. Bilateral changes occur in many patients with unilateral hemispheric lesions, and indeed even large lesions may sometimes be associated with scalp EEG changes that predominate over the contralateral side. The nature of the lesion is important. In cortical dysplasia the interictal EEG is often widely distributed. In tumours and infective lesions it has more specificity but still often lacks reliability. In tumours or other lesional epilepsy there are few data to indicate which neurophysiological factors should influence the extent of resection. The site of the lesion also influences the interictal EEG. In lesional parietal or occipital lobe epilepsy, for example, only a minority of lesions show interictal spikes well correlated to the site of the lesion. Lesions in the temporal neocortex are more often associated with concordant EEG data. The EEG in lesional frontal lobe epilepsy often shows either no inter-ictal spiking or apparently widespread or bilateral epilepti-form discharges.

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