The drug used in petit mal seizures and has a narrow spectrum of antiepileptic activity is:

Ethosuximide is paicularly effective against absence seizures but has a very narrow spectrum of clinical activity. Documentation of its effectiveness in human absence seizures was achieved with long-term electroencephalographic recording techniques.Ethosuximide has an impoant effect on Ca2+ currents, reducing the low-threshold (T-type) current. This effect is seen at therapeutically relevant concentrations in thalamic neurons. The T-type Ca2+ currents are thought to provide a pacemaker current in thalamic neurons responsible for generating the rhythmic coical discharge of an absence attack. Inhibition of this current could, therefore, account for the specific therapeutic action of ethosuximide. A recently described effect on inwardly rectifying K + channels may also be significant.Reference: Katzung Pharmacology; 13th edition; Chapter 24; Antiseizure Drugs