Agents That Block T-Calcium Channels
It is now generally accepted that the specific antiepilep-tic action of ethosuximide (and the older agent tri-methadione, no longer employed) against absence epilepsy is its ability to reduce the low-threshold cal-cium current (LTCC) or T (transient) current. These currents underlie the 3-Hz spike wave discharges that are characteristic of absence epilepsy. A blockade of T-calcium current is likely also to be a mechanism used by valproic acid.
The only clinical use for ethosuximide (Zarontin) is in the treatment of absence epilepsy. If absence attacks are the only seizure disorder present, ethosuximide alone is effective. If other types of epilepsy are present, ethosuximide can be readily combined with other agents.
For the most part, ethosuximide is a safe drug. Most of the side effects are dose related and consist of nau-sea, gastrointestinal irritation, drowsiness, and anorexia. A variety of blood dyscrasias have been reported, but serious blood disorders are quite rare.