The localizing value of ictal EEG in focal epilepsy

Objective: To investigate the lateralization and localization of ictal EEG in focal epilepsy. Methods: A total of 486 ictal EEG of 72 patients with fo cal epilepsy arising from the mesial temporal, neocortical temporal, mesial frontal, dorsolateral frontal, parietal, and occipital regions were analyz ed. Results: Surface ictal EEG was adequately localized in 72% of cases, mo re often in temporal than extratemporal epilepsy. Localized ictal onsets we re seen in 57% of seizures and were most common in mesial temporal lobe epi lepsy (MTLE), lateral frontal lobe epilepsy (LFLE), and parietal lobe epile psy, whereas lateralized onsets predominated in neocortical temporal lobe e pilepsy and generalized onsets in mesial frontal lobe epilepsy (MFLE) and o ccipital lobe epilepsy. Approximately two-thirds of seizures were localized , 22% generalized, 4% lateralized, and 6% mislocalized/lateralized. False l ocalization/lateralization occurred in 28% of occipital and 16% of parietal seizures. Rhythmic temporal theta at ictal onset was seen exclusively in t emporal lobe seizures, whereas localized repetitive epileptiform activity w as highly predictive of LFLE. Seizures arising from the lateral convexity a nd mesial regions were differentiated by a high incidence of repetitive epi leptiform activity at ictal onset in the former and rhythmic theta activity in the latter. Conclusions: With the exception of mesial frontal lobe epil epsy, ictal recordings are very useful in the localization/lateralization o f focal seizures. Some patterns are highly accurate in localizing the epile ptogenic lobe. One limitation of ictal EEG is the potential for false local ization/lateralization in occipital and parietal lobe epilepsies.