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It has been known for more than a century that flickering sunlight can provoke epileptic seizures in susceptible patients 1. However, the modern technologic environment has led to a dramatic increase in exposure to potential trigger stimuli. Outbreaks of visually induced seizures were reported after the introduction of the television set 2, of video games 3, and of television commercials and programs 4, 5. Other provocative visual stimuli in the contemporary environment include discotheque lighting, rolling escalators (a moving, striped pattern), and rotating helicopter blades 6-9. Such epidemics of visually induced seizures have been a source of great concern to the general public 10. Although much research has been undertaken since the 1950s 4, 8, 11-14, many questions concerning underlying basic mechanisms and clinical semiology remain unanswered, and the terminology of clinical and EEG phenomena is not yet standardized. An initial step has been taken through the publication of proposed international standards for intermittent photic stimulation (IPS) 15. We present a proposal for the terminology and classification of clinical and neurophysiologic phenomena relating to visual sensitivity. It aims to standardize the use of clinical terms and definitions. This proposal is divided into four main areas: Clinical symptoms of visual sensitivity. Classification of the EEG responses to IPS. Classification of electroclinical phenomena. Syndromic classification. Commonly used terms such as photosensitive, photogenic, and photoconvulsive, which have different and inconsistently used connotations, have not been used to avoid any misunderstanding. A distinction has been made between the epileptiform EEG responses to IPS so-called photoparoxysmal responses (PPRs) and clinical signs and symptoms, evoked either by IPS or by visual stimuli in daily life. If a PPR is found, the patient is considered IPS sensitive. Visual sensitivity is defined as the susceptibility toward experiencing seizures, which are triggered by the physical characteristics of visual stimuli and not by their perceptual properties (i.e., reflex seizures induced by the cognitive effects of visual stimuli). A combination of these data with a clinical seizure history and imaging results will assist identification of the various syndromes, thus leading to an assessment of risk factors and prognosis in the various patient groups. Visual-induced epilepsy has a strong genetic component. Siblings of children with generalized PPRs are much more likely to show a similar abnormality than are siblings of control subjects (19.3 vs. 3.4%) 16. A PPR is also significantly more common in 5- to 10-year-old siblings of proband offspring of a parent with a PPR (50%) than in siblings of PPR-positive children of parents without a PPR (14%) 17. Studies performed in humans and in experimental animal models, especially the Papio papio baboons, which respond to IPS at 25 Hz with epileptic seizures resembling those of photosensitive patients, indicate that the cerebral cortex plays a primary role in the genesis of electroclinical manifestations of visual-sensitive epilepsy 18. The frontorolandic cortex and the occipital cortex seem to be the most involved in generating the abnormal response in both species. Unitary recording in the baboon shows that the photoparoxysmal EEG response originates from the frontorolandic cortex 19. Generalized seizures appear to result from spread of seizure activity, which is initiated by IPS in the frontorolandic cortex 18. Blockade of frontorolandic discharges by local γ-aminobutyric acid (GABA) infusion also blocks IPS-induced grand mal seizures 20. However, visual afferents to the frontorolandic cortex are controlled by the occipital cortex, which can generate epileptic activity on its own if made hyperexcitable by a decrease in the level of GABA by alloglycine injections 21. Depressants of photosensitivity in the baboon such as valproic acid (VPA) also are active in humans. Despite many striking similarities, the relationship between the IPS-sensitive baboon and patients with photic-induced seizures has been questioned 22, 23. Neurophysiologic studies in patients with photic reflex myoclonus 24-26 show that the contralateral occipital cortex is activated first and that impulses spread to the primary motor cortex to produce myoclonic jerks. In addition, numerous reports indicate that in a considerable number of individuals with visually induced seizures, ictal activity originates from the occipital cortex 13, 27, 28. Subsequent spread to the suprasylvian cortex often results in generalized tonic–clonic seizures (GTCSs), whereas infrasylvian spread produces complex partial seizures (see also partial seizure section). Studies in pattern-sensitive epilepsy revealed that the two hemispheres can have a different threshold; that a critical area of the visual cortex should be stimulated, and that synchronization of neural activity is necessary to elicit a PPR 29. The corpus callosum is critical for interhemispheric synchronization and generalization of EEG discharges 30. Visually evoked potentials (VEPs) of different contrast show that for stimuli of low to medium frequency, the contrast dependence of VEP amplitude and latency is remarkably abnormal for luminance-contrast, but not so for chromatic-contrast stimuli 14. These data indicate that patients with visually induced seizures lack the normal mechanisms of cortical gain control for pattern stimuli of low temporal frequency and high luminance. Suppression of contrast gain control may be experimentally induced in the cat by local application of bicuculline 31, indicating that reduced GABAergic transmission plays a role in visual cortex hyperexcitability. In patients with progressive myoclonus epilepsy, the mechanism of visual sensitivity has been related to deficit in dopaminergic transmission, because apomorphine, a dopaminergic agonist, abolished the PPRs 32. Many antiepileptic drugs (AEDs), developed for partial and generalized seizures, have reduced or abolished PPRs in humans, suggesting that a variety of neurotransmitters and channel blockers could be involved 33, 34. Some individuals may complain of subjective symptoms when they are exposed to photic stimuli, especially IPS 35. Some of these symptoms are normal phenomena, due to the effect of intense light stimuli. They consist of seeing zigzag lines or colors not actually present in the stimuli. Other symptoms include dizziness, eye pain, dragging sensations in one eye, epigastric discomfort, nausea, or simple visual hallucinations. These manifestations may be unrelated to epileptic activity or they may result from an ictal discharge arising in the occipital cortex 27, 36 or in the mesial temporal structures 37. An accurate clinical diagnosis regarding the nature of mild subjective symptoms can be very difficult if the duration is short and their occurrence infrequent. Some of these subjective manifestations may be definitely ictal but still remain isolated symptoms, which is sustained as long as the triggering stimulus is sustained. On other occasions, they may be part of a more complex ictal episode, if a self-sustained ictal discharge arises and spread occurs (see Partial seizures section). Frontopolar, recruiting, photomyogenic, and photooculoclonic responses are synonymous with orbitofrontal photomyoclonus. The triggering stimulus is IPS. The frequency range of flashes effective in triggering this response is usually between 8 and 20 Hz. It is rarely seen in children, but constitutes a normal finding in adults and, in particular, in the elderly. Patients have rapid myoclonic jerking of the periorbital muscles, which produces eyelid fluttering and blinking, synchronous with the flashes. There may be vertical oscillations of the eyeballs. Amplitude of the response increases progressively during the first flashes, reaching a maximum within a few seconds. The maximal amount of muscle activity is initially observed in the inferior orbicularis oculi muscles, with subsequent irradiation to other facial muscles, the frontal and occipital areas, and the neck 38. Further spread may be seen if IPS stimulation continues. The likelihood of eliciting this response is increased by muscular tension, for instance, if one were to instruct the subject to screw up the eyes and clench the jaws. This response is bilateral and time-locked to the stimulus. Latency between each flash and the corresponding muscle contraction is ∼50–60 ms. Response is blocked when the eyes are opened, and it stops immediately when the stimulation is terminated. Although the physiology and significance of this response have been disputed for a number of years, our current understanding indicates it to be an expression of cortical response 39 within the spectrum of photic cortical reflex myoclonus 25, 26. Eyelid myoclonus may occur either as a very short event lasting ∼1–2 s without any detectable impairment of consciousness or as in Absences with eyelid myoclonus, be prolonged and accompany an absence seizure 40. Eyelid myoclonus must be differentiated from the orbitofrontal photomyoclonus (OPM, see earlier), because its clinical and electrographic relation to epilepsy is obvious. The delay between the stimulus and ensuing eyelid jerking is longer than that seen in the photomyogenic response and is more variable 41. In some patients, myoclonic jerking of the eyelids appears in the context of a complex repetitive self-stimulation habit with deliberate fluttering of the eyes and hyperextension of the head in front of any bright light source, including IPS. Under these circumstances, attempting to draw any distinction between eyelid myoclonus and self-inducing behavior may be particularly difficult. In exceptional cases, focal myoclonic (FM) jerks (e.g., in one hand, arm, or hemiface) can be evoked by IPS 35, 42. Consciousness is retained. Generalized myoclonic jerks are usually symmetric and predominate in the upper limbs. In most cases, they are mild, producing only nodding of the head and slight arm abduction. More generalized jerks, involving the face, trunk, and legs, may occasionally cause the patient to fall. The relationship of myoclonic jerks to the stimulus is complex. Sometimes there is no definite time relationship. On other occasions, the jerks may be repeated rhythmically with the same frequency as the stimulus or at one of its subharmonics 11. Isolated myoclonic jerks occur without impairment of consciousness. However, generalized jerks may be repeated, especially if the stimulus continues. In this situation, consciousness may be impaired, and a GTCS may follow. On rare occasions IPS has been shown to produce version of the eyes and the head toward one side. The versive posture (TVP) may be sustained as long as the triggering stimulus is continued 43, representing a stimulus-dependent localized ictal phenomenon. It may also outlast the stimulus as a feature of a simple partial seizure that may then evolve to complex partial or to a GTCS. In this case, it indicates that focal seizure activity precedes seizure generalization. A small subgroup of patients has loss of awareness as the only symptom. When stimulation is performed with the eyes being held closed, the absences may be manifested only by opening the eyes. Mean age at onset is ∼12 years 4. Absences may outlast the stimulus. A mild myoclonic component and evolution into a GTCS are possible. These are usually, but not always, triggered after sustained exposure to photic stimuli. They may follow an absence, a myoclonic jerk, a series of jerks, or a partial seizure, but can occur without any preceding phenomenon. Secondary generalization may be slow or very fast, after mild clinical signs such as head deviation or visual symptoms, which could possibly indicate generalization of an initially focal, possibly occipital seizure. In up to 65% of patients with photic-induced seizures, focal ictal onset, usually in the occipital neocortex, is clinically demonstrable 27, 28. Photically induced PS is often characterized by a sequence of visual and vegetative symptoms, sometimes accompanied by headache 36, 44. These seizures can be mistaken for migraine, especially if motor manifestations are not recognizable. Clinical seizure semiology is similar to that of spontaneous occipital-onset seizures. Spread may be rapid, but it must be stressed that it can also be remarkably slow, occurring after many minutes of ictal activity limited to the occipital lobe 13, 18, 45, 46. Most patients experiencing subjective symptoms describe visual phenomena as the initial ictal manifestation. These are usually reported as bright, multicolored, or occasionally manifesting dark rings, spots, or simple geometric forms, which are continuous or flashing. Location is usually, but not necessarily, in the periphery of the visual field, crossing to the opposite side while rotating or moving slowly 36, 44, 47. 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Trenité et al. (Sat,) studied this question.