Prevalence of bilateral partial seizure foci and implications for electroencephalographic telemetry monitoring and epilepsy surgery.

Most patients with partial seizures of temporolimbic onset have predominantly unilateral seizure foci, which partly accounts for the success of unilateral temporal lobectomy. The prevalence of bilateral foci is not known. "Discordant" seizures are those which arise from the contralateral side from the usual focus. "Concordance" is the fraction of seizures which arise from the majority side, ranging from 0.5 to 1.0. From observations of seizures recorded in an epilepsy monitoring unit, a model of the patient population can be determined by non-linear least squares methods. Six hundred and five seizures from 57 consecutive patients with non-lesional temporolimbic epilepsy were studied. In the model population, 15% of patients have concordance of less than 70%, and 80% have concordance > 90%. Using Bayes' theorem, the observation of 5 concordant seizures implies a 95% chance of the patient having a concordance of at least 90%. If one discordant seizure is recorded, then to reach the 95% confidence level of 90% concordance requires a total of 11 concordant seizures. For the smaller group of patients with strictly unilateral interictal spikes, only 4 concordant seizures need to be recorded to achieve the same level of confidence of unilaterality. Interpretation of telemetry studies requires knowledge of the patient population.