Photoparoxysmal responses in children: Their characteristics and clinical correlates

Correspondence Photoparoxysmal Responses in Children: Their Characteristics and Clinical Correlates To the Editor: With great interest we have read the recent article “Photoparoxysmal Responses in Children: Their Characteristics and Clinical Correlates” by Nagarajan et al. [1]. This study adds much to the discussions on both the clinical relevance of finding a photoparoxysmal response (PPR) in an electroencephalogram (EEG), and especially the methodology of standardized intermittent photic stimulation (IPS). Both for detection and follow-up of this EEG trait PPR, we need a reliable method, including a photic stimulator of high quality. Nagarajan et al. used a Nihon Kohden photic stimulator, which has important consequences for interpreting some of the results. In most published studies, a Grass PS22 (or its modern version, Grass PS33plus, Grass-Telefactor, West Warwich, RI) has been used. There are, however, several differences between these two stimulators: the Nihon Kohden has a rectangular shape compared with the Grass’s round shape, a lower intensity per flash (0.64 Joule compared with the 1 Joule used on average with the Grass setting 8), a longer pulse duration, an increase in pulse duration at higher frequencies, and most importantly, the Nihon Kohden shows a clear decline in intensity per flash with increasing frequencies (0.5-33 Hz). The Grass stimulator is more stable in this respect and reaches higher frequencies (1-60 Hz). Thus the Grass stimulator produces a stronger overall stimulus with a correspondingly higher likelihood of finding an abnormal response to IPS. This was shown in a comparative technological study by the physicist M.P. Jonker, which was presented at the 1998 European consensus meeting and in a comparison of electrencephalogram results in the same patients [2]. Nagarajan et al.’s finding, that 10 Hz flashes were clearly the most provocative, whereas this level was found to be 20 Hz in the other studies, illustrates the difference in the two stimulators [3]. It could possibly partly explain the very low prevalence of partial epilepsy in this study (6%); much higher frequencies of partial epilepsies (15% and 29%, respectively) were reported from studies at a general hospital by Jayakar and Chiappa in 1990 [4] and a related publication by Gilliam and Chiappa in 1995 [5], and by Kasteleijn-Nolst Trenité in an epilepsy center in 1989 [6]. Another remarkable finding reported by Nagarajan et al. is the highest prevalence of PPRs during “eyes closed” rather than at “eye closure,” which has been found by most other authors [3]. Nagarajan et al. do not describe their IPS procedure in detail, but by combining their information on 15 seconds continuous stimulation with 5 seconds rest per flash frequency, and their tests on both eye closure and opening, I assume the following: the child first had its eyes closed, the IPS began, and after 5 seconds the child was asked to open its eyes and then to close them again after another 5 seconds. The sequence of eye conditions and the decisions made by the EEG technician and interpreting neurophysiologist could markedly influence the test outcome. For example, if my assumption on their method is correct, what was the cut-off time point after eye opening/closure for deciding that a PPR was related to that eye condition and when was it observed as related to the subsequent eyes open/closed states? To substantiate Nagarajan et al.’s conclusion, that a minimal of 10 seconds stimulation yields more PPRs than a 5-second

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stimulation, a controlled study is required with IPS at the same eye condition and for short and long stimulations in the same population. Dorotheé G.A. Kasteleijn-Nolst Trenité, MD, PhD, MPH Dept. of Biomedical Genetics University Medical Centre Utrecht The Netherlands Henk Spekreijse, MSc Department of Visual System Analysis University of Amsterdam The Netherlands References [1] Nagarajan L, Kulkarni A, Palumbo-Clark L, et al. Photoparoxysmal responses in children: their characteristics and clinical correlates. Pediatr Neurol 2003;29:222-6. [2] Kasteleijn-Nolst Trenité DGA, Binnie CD, Harding A. Wilkins Photic Stimulation: Standardization of Screening Methods. Epilepsia 1999;40:75-9. [3] Harding GFA, Jeavons PM. Photosensitive epilepsy. London: MacKeith Press, 1994. [4] Jayakar P, Chiappa KH. Clinical correlations of photoparoxysmal responses. Electroencephalogr Clin Neurophysiol. 1990;75:251-4. [5] Gilliam FG, Chiappa KH. Significance of spontaneous epileptiform abnormalities associated with a photoparoxysmal response. Neurology 1995;45:453-6. [6] Kasteleijn-Nolst Trenité DGA. Photosensitivity in epilepsy: Electrophysiological and clinical correlates. Acta Neurol Scand. 1989;125:80.

Response: We thank Dr. Dorothee G.A. Kasteleijn-Nolst Trenité for the comments. We acknowledge the limitations of our study—it was undertaken as a pilot study in a clinical setting and we hope the main findings (increasing the duration of intermittent photic stimulus [IPS] train to 10 seconds or more will increase the diagnostic yield and that self-limited photoparoxysmal responses [PPRs] likely have greater significance than previously attributed to them) will contribute to management in the clinical context. We certainly agree that the technique of IPS testing, the stimulus parameters, the type of stimulator, the state of background illumination and the cognitive state of the patient are important and can influence photic driving and the PPRs. Some of the PPRs reported in the literature as being due to “eyes closed” or “eyes open” may have been due to the state of “eye closure or opening” and most studies do not define the time period of the state of eye closure. This may have contributed to the varied results regarding the frequency of PPRs in the different eye conditions [1]. In our study, if a PPR occurred during the eye movement artifact of “eye closing” or “opening” or within 100 miliseconds of that we decided to attribute it to the condition of eye closure or opening. We chose 100 miliseconds after the artifact, as that time frame has been used to define self limiting or outlasting of the PPR to IPS [2,3]. In our study, the IPS train began at random in the eyes closed or open state and then the child was requested to either open their eyes for a few (⬃5) seconds and then close them (or vice versa, as appointed). Children did not always respond promptly to the request and the speed at which the manoeuvre was undertaken was also variable.

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