Phenytoin is effective for the treatment of all of the following types of seizures except

• A. Generalized tonic-clonic
• B. Simple partial
• C. Complex partial
• D. Absence

Answer: (D)

Phenytoin works by blocking voltage-gated sodium channels, which reduces neuronal excitability and helps control seizures that begin in focal areas of the brain as well as generalized tonic-clonic seizures. Absence seizures are driven by thalamocortical circuits and rely on T-type calcium channels to sustain their characteristic brief lapses in consciousness. Drugs that target those calcium channels, such as ethosuximide (and often valproate for broader seizure control), are the preferred treatment for absence seizures. Since phenytoin doesn’t address the calcium-channel mechanism central to absence, it doesn’t effectively treat these seizures and can be ineffective or even counterproductive. Thus, absence seizures are not responsive to phenytoin, while generalized tonic-clonic and focal (simple or complex partial) seizures typically are.