Teaching NeuroImage: Subharmonic Frequency Occipital Spikes

A 15-year-old female patient with a history of self-limited epilepsy with centrotemporal spikes (SeLECTS) on valproic acid underwent a routine EEG for consideration of weaning off antiseizure medication.Her prior EEG captured centroparietal spikes.An updated EEG captured 7.5 Hz occipital spikes provoked by photic stimulation at 15 Hz (Figure 1), representing a subharmonic frequency and a normal photic driving response.Importantly, this finding does not represent a photoparoxysmal response.A photoparoxysmal response is defined as epileptiform discharges in response to light that are (1) not confined to the occipital regions and ( 2) not phaselocked to the flash. 1 Given the spikes remained localized to the posterior region without evolution, this patient's EEG finding met criteria for category 2 of the Jeavons and Harding classification system 2 and thus did not indicate a decreased threshold for seizures.The patient discontinued valproic acid and remained seizure-free (Figure 2).