Clinical significance of EEG abnormalities during photic stimulation in patients with photosensitive epilepsy

Purpose: The continued presence of EEG abnormalities in patients with a history of photosensitive seizures is used to signify the persistence of photosensitive epilepsy. The extent to which this approach places patients at risk of seizures is unclear, however. We describe those EEG abnormalities that may be tolerated with low risk of further seizures, and those that are indicative of poor seizure control. Methods: Fifty patients with EEG evidence of persistent photosensitive epilepsy underwent photosensitivity testing with diffuse and patterned light; 58% of patients continued to experience seizures, and 42% were seizure free. The incidence of EEG abnormalities to diffuse and patterned intermittent photic stimulation (IPS) was analysed as a function of recent seizures. Results: All patients showed EEG abnormalities to patterned IFS; there was no significant association between patterned IFS and poor seizure control. EEG abnormalities to diffuse IFS occurred in 58% of patients, and 76% of these patients had experienced a seizure within the previous year. These patients were more than twice as likely to he poorly controlled than those who showed abnormalities only to patterned IFS. These results were consistent for both medicated and unmedicated patients. Conclusions: EEG abnormalities to patterned IFS can be used to signify the persistence of photosensitive epilepsy, but abnormalities to diffuse IFS are more likely to indicate the patient is poorly controlled and at risk of further seizures.