Consistent EEG focalities detected in subjects with primary generalized epilepsies monitored for two decades

Purpose: To describe the evolution of interictal findings in serial EEGs from patients with primary generalized epilepsy. Methods: A cohort of 89 subjects with various primary generalized epilepsies were reviewed. Thirty-one did not meet a priori criteria. Of the 58 patients analyzed, 12 had only absence seizures, 28 had absence seizures followed by one or more generalized tonic-clonic seizures, 9 had generalized tonic-clonic seizures followed by absence and/or myoclonic seizures, and 9 had juvenile myoclonic epilepsy. Patients were followed for a mean of 16 years. An average of 39 EEGs were obtained on each patient. Results: Thirty-two patients (56%) had focal features present in up to 65% of the EEGs in each of the patients. Accepted focalities were only those that were consistent in lateralization, location and, often, morphology across the span of the study. Focal findings were most often temporal or frontal. Conclusions: Patients with typical primary generalized epilepsies show a high incidence of focal EEG features that cannot be explained on the basis of structural lesions, coincidental factors, or to artifacts of the selection criteria. Although the data do not allow a definitive explanation, possible mechanisms include associated focal cortical pathology such as microdys genesis, and development over time of localized, self-sustaining hyperexcitability in low-threshold cortical structures subjected to repeated generalized spike-wave activity. Either hypothesis implies the participation and interaction of genetic, ontogenic, and environmental factors.