REPLY

Photoparoxysmal Response and the Assessment of Seizure Control To the Editor: The article by Fylan et al. (1) has stimulated my interest. 1 wish to add a few comments. Gastau et al. (2) reported that the photoconvulsive response (PCR) occurred mostly in patients with primary subcortical epilepsy, in 40% of cases with petit mal, and in 20% of patients with grand mal. Stevens (3) , on the other hand, found that PCR occurred in 53% of the patients with nonfocal seizures and in 3% of the patients with focal seizures. Niedermeyer (4) states that generalized PCR is highly associated with primary generalized epilepsy. Hence, a systematic classification using International League Against Epilepsy (ILAE) criteria ( 5 ) would give a better understanding of the epileptic syndrome vis-avis photic stimulation. Furthermore, Doose et al. (6) has shown that PCR is most frequently noted between 6 and 15 years of age, affecting mostly females. In adults, the incidence of photic driving evoked by flickering dot pattern is 2.4 times more in females than males (7) and, more often seen in people with epilepsy-24% (8). It’s quite possible that the patient cohort experiencing seizures in the authors’ series might have a gender effect. This would be important when correlating abnormalities to diffuse intermittent photic stimulation (IPS) with patients who experienced recent seizures. In the discussion, the authors conclude that abnormalities to diffuse IPS suggest that the patient is poorly controlled and at twice the risk for experiencing further seiLures. In this context, several factors are known to alter the PCR. PCR most often occurs immediately after eye closure. But, when stimuli are presented with eyes open, a patterned field of vision has been found more effective than a blank field or a flicker of white light in evoking PCR (9). Hence, the 100% of photosensitive epilepsy patients showing PCR to flashed-on pattern seems appropriate. Evidence has shown that the greater part of visual evoked potential (VEP) to pattern is generated by the central 10” of vision (fovea). The relative cortical representations of central areas of retina map to a larger cortical area than the peripheral ones. The central 10” of the visual fields project to the occipital poles and the peripheral field areas project to medially placed cortex. Therefore, stimulus fields that include peripheral visual areas will produce field distributions that are different from stimulus fields restricted to central vision (10). This is corroborated by Takahashi et al. (1 1) who maintained that if the field of view is sufficiently large enough to stimulate the peripheral visual field, an increased incidence of PCR can be obtained. This could be one of the possible reasons for the cohort of photosensitive epilepsy patients with persistent seizures showing a differential response to diffuse IPS (76%), when compared to seizure-free group (33%). Perhaps this indicates that diffuse IPS (flash) induces a greater degree of coherence on the epileptogenic aggregate as compared to patterned IPS.