Effects of flash frequency and repetition of intermittent photic stimulation on photoparoxysmal responses.

The protocol used for intermittent photic stimulation (IPS) may determine the likelihood of evoking a photoparoxysmal response (PPR). One-hundred and thirty-five electroencephalograms (EEGs) presenting PPRs, from 125 patients were studied in order to identify the most effective stimulation frequency to evoke a PPR and the effects of repetition of IPS on the occurrence of a PPR. Two stimulation protocols were used: protocol I (starting at 18 Hz and then testing at 2, 6, 8, 10, 15, 20, 30, 40, 50, 60 Hz) and protocol II (stimulating at 2, 6, 8, 10, 15, 18, 20, 30, 40, 50, 60 Hz). Protocol I was used for patients not known to be photosensitive whereas protocol II was used for patients known to be photosensitive before recording. Both latency and PPR grade for frequencies which evoked PPR were measured in all records. The most epileptogenic frequencies (those evoking grade 4 PPRs at the shortest latency) were within the range 15-18 Hz for both protocols. In the records where the IPS was repeated at the same frequency, the PPR latency and grade seen during the first and second stimulation trial were studied in order to establish habituation or potentiation of responses. Repetition of IPS at the same frequency induced habituation more often than potentiation, but only if trials were repeated consecutively which suggests that habituation is frequency specific and trials repeated during EEG recordings to confirm photosensitivity to a particular frequency should be separated in time or be non-consecutive. Five patients studied with protocol I (10.6%) showed a grade 4 PPR only during the initial trial at 18 Hz. Thus, as a general screening procedure for testing for photosensitivity commencing stimulation at 18 flashes/s appears to be justified. The combination of two different protocols delivered to patients with and without a history of photosensitivity appears to achieve a sensible compromise having a high likelihood of demonstrating photosensitivity with a minimum risk of precipitating seizures.