Photosensitivity and Epilepsy
Intermittent photic stimulation (IPS) is a routine provocation method with stroboscopic light flashing between 2 and 60 Hz to diagnose epilepsy—or the genetic trait—and to evaluate (non)pharmacological treatment. Eye closure at the onset of the IPS stimul
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Dorothee Kasteleijn-Nolst Trenite
29.1 Introduction Intermittent photic stimulation (IPS) is a routine provocation method with stroboscopic light flashing between 2 and 60 Hz to diagnose epilepsy. It can easily be repeated in a standardized manner to evaluate the seizure threshold in an individual patient over time and with different AEDs. With IPS or any other provocative visual stimulus (striped patterns, computer screen, etc.), the EEG may show regular or irregular polyspike or spike-and-wave activity starting or maximum over the temporo-parieto-occipital areas with or without generalization: these visually evoked epileptiform EEG reactions are called photoparoxysmal responses or PPR [1]. About 3–5% of epilepsy patients will show PPR; in adolescents PPR can even be found up to 10% [2], and females are more affected than males (greater prevalence and sensitivity to a wider range of flash frequencies) [3]. About 10% of PPR-positive patients were found also recently in first- seizure patients (children 14% and adults 5%) and predominant with generalized epileptiform discharges [4]. A specific subgroup are the patients with Dravet syndrome: 40% will show PPR as early as the age of 1 year, and there is no sex difference [5]. IPS is most effective with an intensity of 1 J/flash and a flash frequency between 10 and 30 Hz [6]. Deep red monochromatic flashing light (~600 nm) as is used in cartoons has proven to be very provocative [7]. Striped patterns with high contrast and a spatial frequency of 4 cycli per degree give PPRs in about 30% of IPS-sensitive patients [8]. Only in specialized laboratories these patterns are tested in photosensitive patients [9]. It is however a read-
D. Kasteleijn-Nolst Trenite (*) Department of Neurosurgery and Epilepsy, University Medical Center Utrecht, Utrecht, The Netherlands Faculty of Medicine and Psychology, Nesmos Department, Sapienza University, Rome, Italy e-mail: [email protected]; [email protected]; [email protected]
ily recognized subtype of photosensitivity; patients suffer differently from environmental stimuli [10]. See for further details the EU guidelines of 2012, published at the ILAE website under guidelines for diagnosis [11]. Below, important issues related to visual sensitivity will be highlighted and examples given.
29.2 Important Issues 29.2.1 Why Do We Use Intermittent Photic Stimulation? How Was It Discovered to Be a Valuable Tool in Epilepsy Diagnostics and Research? After the finding of Hans Berger [12] in the late 1920s of spontaneous occipital rhythm—alpha waves—being blocked by eye opening as well as by light from a torch, the British Adrian and Matthews started experimenting with flickering light in 1934: hereto they used an illuminated opal glass bowl with a rotating sectored disc to produce flickering light of up to 25 flashes/s [13]. Photic driving was shown with harmonics and subharmonics. It took more than 10 years though before Walter et al. in 1946 described that flickering lights could evoke epileptiform dischar
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