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Summary: Purpose:We reviewed 1,360 EEG reports for all patients studied in two different neurophysiology laboratories during 1 calendar year to determine whether epileptiform discharges have a hemispheric dominance.Methods: Both inpatients and outpatients, with or without epilepsy, were included.Results: Ninety-four records (6.9%) demonstrated generalized epileptiform activity. Of 95 EEG reports indicating spikes solely from one hemisphere, spikes arose from the left in 61 and from the right in 34. Among 50 other records with bilateral independent spikes with lateralization, 40 were left hemisphere 'dominant and 10 were right hemisphere dominant.Conclusions: These findings raise the possibility that the left cerebral hemisphere may generate focal epilepsy more frequently than the right. Key Words: Electroencephalography-Cerebral lateralization-Epilepsy.Reports in the literature have suggested that epileptiform discharges originate more frequently from the left cerebral hemisphere (l), but no larger systematic study has specifically addressed this question. In this study, retrospectively analyzed EEG records from two independent EEG laboratories demonstrated that the left cerebral hemisphere is more likely to generate epileptiform activity than the right. METHODSWe retrospectively reviewed EEG reports for all patients studied in the neurophysiology laboratory at New York Hospital (NYH) during the 1992 calendar year. The referral base included both inpatients and outpatients from a large urban hospital, with or without a known diagnosis of epilepsy. All EEGs were read by one interpreter (G. S.). We also reviewed a comparable number of consecutive EEG reports read by another interpreter (C. H.) at Metropolitan Hos- unaware of the research question at the time of EEG interpretation. We tabulated only the results from the first EEG performed for each patient. Therefore, each patient contributed only one data point. Repeat EEGs were not included. All EEGs were scalp surface recordings with electrodes placed according to the International 10-20 system. All EEGs were recorded with 16-channel machines, and both bipolar and referential montages were used. Each EEG was classified according to the presence or absence of generalized or focal epileptiform activity. When asymmetric spikes were noted, we tabulated whether they arose from the left or right hemisphere, either unilaterally or as asymmetric bilateral activity predominantly on one side. The determination of asymmetric bilateral spikes was made based on the asymmetry of the amplitude and repetition rate of the epileptiform discharges. In general, we considered bilateral independent spikes asymmetrical if the amplitude consistently was twice as great on one side as the other or spikes occurred twice as frequently on one side as the other. RESULTSIn all, 768 studies from NYH were reviewed and classified as shown in Table 1. Most were normal or showed only generalized slowing without epilepti- 503

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