The 'Photosensitivity Model' uses a standardized stimulation protocol of repeated intermittent photic stimulation (IPS) over a three-day period, with administration of a single dose of an investigational antiepileptic drug (AED) after a baseline IPS day in photosensitive patients, followed by a third IPS day to determine duration of effect. This 'Photosensitivity Model' has shown its value in the development of new AEDs. Levetiracetam (LEV), currently a first-line AED in new-onset focal epilepsies, was not effective in classical animal models, but showed dose-dependent efficacy in the human 'Photosensitivity Model'. Nevertheless, concerns have been expressed that AEDs selectively suppressing focal seizures might not suppress generalized photoparoxysmal EEG responses (PPR), the pharmacodynamic outcome measure in the Model. Herein, the following questions have been addressed: I. Can patients with generalized epileptiform discharges, evoked by IPS, so-called PPR, have focal epilepsy (focal seizures)? II. Are the photosensitive patients with focal epilepsy, who have participated in the photosensitivity trials, non-responsive to a new AED under investigation, as compared to those with generalized epilepsies? III. Are "focal epilepsy" AEDs effective both in the 'Photosensitivity Model' and in real life in photosensitive patients? We performed a systematic literature review of PPR in focal seizures and focal epilepsy and we analyzed data (published and unpublished) from 20 different potential AEDs studied prospectively in the 'Photosensitivity Model'. Finally, the PPR effects of Na+ channel-blocking AEDs (considered as the most typical AEDs for focal epilepsy) are discussed with unequivocal examples given of the focal nature of a patient's PPR. Based on the entire data evidence, we conclude that: 1. PPRs certainly exist in focal epilepsy (17% on average); 2. Clinical signs and symptoms of PPRs can be focal and 3. PPRs can definitely be used to identify or to prove efficacy of new AEDs for patients with focal epilepsy.
Keywords: AEDs; EEG; Epilepsy; PPR; Partial; Pathophysiology.
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