Dravet Syndrome SCN1A

A 14-month-old girl is referred to pediatric neurology for refractory epilepsy. She experienced her first seizure at 6 months of age—a prolonged hemiclonic seizure triggered by fever following routine vaccination. Since then, she has had recurrent prolonged seizures triggered by fever, hot baths, photic stimulation, and minor temperature elevations. Her EEG now shows the pattern marked **A** in the diagram: generalized polyspike-and-wave discharges at 3–4 Hz with a striking photoparoxysmal response to intermittent photic stimulation. Genetic testing reveals a heterozygous loss-of-function mutation in SCN1A. Which of the following best explains why sodium channel blockers (carbamazepine, oxcarbazepine, phenytoin) are contraindicated in this patient?