Barbiturates are anticonvulsants that can also have proconvulsant actions. Slight structural changes of a barbiturate (e.g., sulfuration or methylation of the 1 position) can convert it from an anticonvulsant to a convulsant.47 Depending on the dose used, some barbiturates can have both proconvulsant and anticonvulsant properties, with low doses associated with seizure activity in epilepsy patients48 and higher doses leading to burst suppression.
Thiopental is used to stop seizures, including those due to overdose of local anesthesia. Thiopental may be a safer induction agent than methohexital for patients with epilepsy.50 The proconvulsant action of thiopental is lower than that of methohexital.49 It is less effective than methohexital in activating existing epileptogenic activity on the electrocorticogram in epilepsy patients.
Other ultra–short-acting thiobarbiturates, such as thiamylal sodium, buthalitone, and thialbarbitone, have the same pharmacologic properties as thiopental.
Methohexital is an ultra short–acting methyl barbiturate that does not cause seizure activity in patients without epilepsy. It can be associated with some excitatory phenomena, however, such as hiccoughing, tremors, and abnormal muscle movements.23
In patients with epilepsy, electrographic and clinical seizures can occur after intravenous (0.5–1.0 mg/kg), intramuscular (10 mg/kg), and rectal (25 mg/kg) administrations.50,51 Low-dose methohexital (less than 0.5 mg/kg) can activate interictal and ictal discharges among epilepsy patients during electrocorticography.49 The activating effect of methohexital may be largely restricted to patients with partial epilepsy, occurring in up to 72% of such patients.52 Methohexital does not appear to cause seizures during anesthetic induction of patients with generalized epilepsy.50,52
At higher doses, methohexital can suppress epileptogenic foci and cause electrical silence. This suppressive effect is used in the methohexital suppression test, which has been recommended as an adjunctive tool to distinguish the primary focus in patients with secondary bilateral synchrony or multifocal discharges.53 This is most valuable in the investigation of temporal lobe epilepsy. Methohexital is administered in incremental doses of 0.5–1.0 mg/kg until EEG silence is obtained or only a single focus remains. If total EEG silence is obtained, it is hoped that a single focus becomes evident as the EEG returns.
Reviewed and revised April 2004 by Steven C. Schachter, MD, epilepsy.com Editorial Board.
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