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Visual snow or visual static is transitory or persisting visual symptom where people see snow or television-like static in parts or the whole of their visual fields, constantly in all light conditions even visible in day-light. The severity or density of the "snow" differs from one person to the next; in some circumstances, it can inhibit a person's daily life, making it difficult to read, drive, perform routine tasks, see in detail (even in bright daylight) or focus correctly because of afterimages and other visual and non-visual symptoms. Little is known about this rare condition, and it has conventionally been regarded as a variant of migraine aura—though recent research shows this is not the case. Visual snow is now regarded as a unique syndrome—usually presenting with other symptoms, such as persistent afterimages, photophobia, enhanced blue field entoptic phenomenon and tinnitus.〔Schankin, CJ, Maniyar, FH, Digre, KB, Goadsby, PJ (2014) ‘Visual snow’ – a disorder distinct from persistent migraine aura, ''Brain'' 〕 Newly published research has confirmed a “brain dysfunction in patients with visual snow”, located principally in the right lingual gyrus. Before this, no other etiology for visual snow had been identified. Insofar as sufferers of visual snow had undergone ophthalmic, neurological and psychiatric examinations, no systematic problems besides the visual snow were found. The recent research that indicates this disorder occurs in the brain has important ramifications for the possibility of potential treatment. However, standard treatment protocols have yet to be established. ==Causes== In May 2014 the results from the first major research trial into visual snow were reported.〔Schankin, CJ, Maniyar, FH, Sprenger, T, Chou, DE, Eller, M, Goadsby, PJ, 2014, The Relation Between Migraine, Typical Migraine Aura and "Visual Snow", Headache, 〕 The study described strong evidence from positron emission tomography scans that the disease is associated with hypermetabolism in the right lingual gyrus and left cerebellar anterior lobe of the brain. The researchers stated that pinpointing visual snow (and its related symptoms such as afterimages) to a functional problem in a specific brain area may open up possibilities for targeted treatment and that treatment trials will follow. Visual snow can occur in a variety of ophthalmic conditions that can be diagnosed by the presence of additional clinical signs and experiences. Persisting visual snow can feature as a leading addition to a migraine complication called persistent aura without infarction,〔International Headache Society. The International Classification of Headache Disorders, 2nd edition" ''Cephalalgia'' 2004; 24 (suppl. 1): 1-160.〕 commonly referred to as persistent migraine aura (PMA). It is important to keep in mind that there exist many clinical sub-forms of migraine where headache may be absent and where the migraine aura may not take the typical form of the zigzagged fortification spectrum, but manifests with a large variety of focal neurological symptoms. A condition that sometimes produces visual snow is optic neuritis (inflammation of the optic nerve). Moreover, a variety of illnesses (e.g., Lyme disease, auto-immune disease) or noxious events (dehydration) have been blamed by sufferers in self-help internet forums as causes of persisting visual snow, but none of these claims have been confirmed by scientific study. Some patients fail to find any apparent causative illness or event in their lives, instead saying the snow came out of nowhere or has been with them for their whole lives. The role of hallucinogens in the etiology of visual snow is not entirely obvious. Hallucinogen persisting perception disorder (HPPD), a condition caused by hallucinogenic drug use, is sometimes linked to visual snow, but both the connection of visual snow to HPPD and the etiology and prevalence of HPPD is disputed. Most of the evidence for both is generally anecdotal, and subject to spotlight fallacy.〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Visual snow」の詳細全文を読む スポンサード リンク
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