Changes in seizure activity following anticonvulsant drug withdrawal.

We retrospectively studied the effects of changing antiepileptic drug levels on patterns of seizure discharge in 8 patients investigated with intracerebral electrodes during presurgical evaluation. We compared seizures recorded at high levels of medication to seizures recorded at low levels for changes in seizure duration, duration from unilateral onset to contralateral spread, inter- and intrahemispheric coherence, and morphology of EEG discharges. Seizures were more frequent with low medication, as was secondary generalization. Reduction in medication did not affect the morphology of discharges at onset, duration to contralateral spread, and coherence between EEG discharges. Seizures of similar type (eg, complex partial seizures without secondary generalization) were not longer with low than with high medication. Whereas medication clearly affects seizure frequency and generalization, it has little effect on the pattern of early parts of seizures. In particular, we found no evidence that seizure discharges become bilateral or generalized more quickly when medication is reduced.