INTRAINDIVIDUAL ANALYSIS OF ANTIEPILEPTIC DRUG EFFECTS ON EEG BACKGROUND RHYTHMS

Antiepileptic drug (AED) therapy with either phenytoin or carbamazepin e has been associated with generalized slowing of EEG background rhyth ms. These effects have been seen in groups of patients undergoing AED manipulation, although the background slowing has been highly variable from patient to patient. Background slowing may represent an objectiv e physiologic measure of drug impact on cerebral function and could be useful in monitoring for AED neurotoxicity in individual patients. Th is would require an intraindividual analysis of AED effects on EEG bac kground rhythms. The present study was designed to develop a methodolo gy for intraindividual analysis of EEG background changes and to apply this methodology to patients beginning or ending chronic AED therapy. EEG recordings were obtained under controlled conditions in 31 health y subjects and were repeated after an interval of 12-16 weeks. EEG bac kground rhythms from each record were analyzed using the fast Fourier transform, and test-retest differences for several quantitative measur es were calculated from each subject's paired recordings. EEG recordin gs were also obtained in 6 patients beginning or ending chronic AED th erapy. Test-retest differences for each patient's quantitative EEG mea sures were statistically compared with the distributions of test-retes t measures obtained from the healthy controls. AED therapy was associa ted with an increase in absolute delta and/or theta power and a slowin g of the dominant posterior rhythm; however, these EEG background chan ges varied widely in degree from patient to patient. Intraindividual a nalysis revealed that 5 patients had statistically significant slowing relative to the control group on at least 1 of the 9 target quantitat ive EEG measures, as well as a composite measure. Statistically signif icant slowing was also seen as a group effect. The results suggest tha t among patients undergoing chronic therapy with phenytoin or carbamaz epine there are widely different degrees of EEG background slowing, of ten significantly beyond that expected on the basis of test-retest var iability. Intraindividual analysis of the EEG background may be a prac tical objective measure of a patients unique response to chronic AED t herapy.