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Summary:Purpose: During presurgical evaluation, 14 patients with medically intractable focal epilepsies underwent magnetoencephalographic (MEG) recordings to localize the epileptogenic focus. To increase the number of epileptiform discharges required for MEG analysis, methohexital a shortacting barbiturate that is known to activate epileptiform activity, was used. Additionally, we investigated the spikeprovoking properties of clonidine in comparison to methohexital.Methods: After oral premedication with clonidine, shortlasting anesthesia was provided by intravenously administered methohexital. The number and location of epileptiform MEG discharges were assessed after clonidine premedication and during methohexital anesthesia. Results were compared with baseline MEG recordings.Results: Methohexital increased the frequency of focal epileptiform discharges in eight of 13 patients (one of the 14 patients did not receive methohexital after premedication with clonidine). Additionally, premedication with clonidine was found to increase focal epileptiform discharges in nine of 14 patients. When compared with baseline MEG recordings, recordings after treatment with both clonidine premedication and methohexital anesthesia showed a significant increase in the total number of epileptiform signals and the number of spikes contributing to MEG source localizations.Conclusions: This study confirms the selective proconvulsant effects of methohexital on the epileptogenic focus as suggested previously by EEG and electrocorticogram (ECoG) investigations. Additionally, our data establish for the first time that clonidine increases epileptiform activity in patients with seizure disorders. These results indicate that clonidine is suited as an activating agent for the localization of epileptogenic foci by means of MEG. This effect of clonidine on specific epileptic activity also indicates that clonidine should be used with caution as an antihypertensive drug in patients with seizure disorders. Key Words: Epilepsy-MagnetoencephalographyMethohexital-Clonidine-Focus localization.Localization of the epileptogenic brain area is the most important goal of presurgical evaluation of patients with localization-related epilepsies. Magnetoencephalography (MEG) is a useful noninvasive diagnostic method that often provides important information in presurgical localization of epileptiform activity (1-5). However, epileptiform discharges are not always present during routine MEG recordings because the time is limited by the need for the patient to remain motionless with a fixedhead position. Epileptiform activity can be provoked by use of proconvulsant drugs such as methohexital in addition to standard activation procedures such as hyperventilation, photic stimulation, and sleep.Methohexital is an anesthetic agent of fast onset, short duration of action, and relatively few side effects. Its dose-dependent, seizure-activating properties (evaluated

Summary: Purpose: During presurgical evaluation, 14 patients with medically intractable focal epilepsies underwent magnetoencephalographic (MEG) recordings for focus localization. To increase the number of epileptic discharges required for MEG analysis, we administered methohexital (MHT), a short-acting barbiturate known to provoke epileptic activity. We also investigated the spike-provoking properties of clonidine in comparison with MHT.Methods: Patients were briefly anesthetized with intravenously administered MHT after being premedicated orally with clonidine. Numbers and locations of epileptic MEG discharges were assessed after clonidine premedication as well as during MHT anesthesia. Results were compared with baseline MEG recordings.Results: MHT increased the frequency of focal epileptic discharges in 8 of 13 patients ( I of the 14 patients did not receive MHT after premedication with clonidine). Premedication with clonidine also increased focal epileptic discharges in 9 of 14 patients. The numbers of epileptic signals and numbers of spikes contributing to MEG source localizations were significantly increased in MEG recordings under both treatment conditions (clonidine premedication and MHT anesthesia) as compared with baseline MEG recordings.Conclusions: Our results confirmed the selective proconvulsive effects of MHT on the epileptic focus, as previously suggested by EEG and electrocorticographic (ECoG) investigations. However, our present data establish for the first time that clonidine increases epileptic activity in patients with seizure disorders and indicate that clonidine is suitable as an activating agent for localization of epileptogenic foci by MEG. This effect of clonidine on specific epileptic activity also indicates that specific care must be taken when clonidine is used as an antihypertensive drug in patients with seizure disorders. Key Words: Epilepsy-Magnetoencephalography-Methohexital-Clonidine-Focus localization.The most important goal of presurgical evaluation of patients with focal epilepsies is the exact localization of the primary epileptic focus. Magnetoencephalography (MEG) is a useful noninvasive diagnostic method which often provides important information in presurgical localization of epileptic activity (1-5). Epileptic discharges, however, are not always present during routine MEG recording because the available recording time is limited by the regimen of motionless posture and head fixation of the patient. In addition to standard activation procedures such as hyperventilation, photic stimulation, and sleep, proconvulsive drugs such as methohexital (MHT) can be used to provoke epileptiform activity.MHT is an anesthetic agent of fast onset, short duration of action, and relatively few side effects. Its dosedependent seizure-activating properties (evaluated by

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