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Visual snow syndrome (VSS) -- a disorder in which patients experience persistent tiny flickering dots that resemble analog television static in their visual field -- likely represents a clinical continuum with different degrees of severity, a large online survey showed.

In a web-based study of 1,174 people with self-reported symptoms, 90% of respondents had visual static that lasted more than 3 months with accompanying visual disturbances like floaters, after-images, and photophobia; these people were classified as having VSS, reported Francesca Puledda, MD, of King's College in London, England, and co-authors, in .

A total of 43 survey respondents were classified as having VS without the syndrome. In addition, the researchers identified a group of 70 people who had hallucinogen-persisting perception disorder (HPPD), based on their history of recreational drug exposure.

"This paper describes the largest cohort of subjects with visual snow to date," Puledda said. "It allowed us to confirm the usefulness and validity of the available criteria for this syndrome, which is still highly unrecognized in clinical and scientific settings."

The survey also confirms "that visual snow is completely independent of the use of hallucinogens and, most importantly, that it represents a spectrum-type disorder," she told 番茄社区app. "In the clinical continuum of visual snow, the more severe cases are more likely to also be affected by migraine or tinnitus, which are its most frequent comorbidities."

VS was and is now included in the as a complication of migraine. The included only patients with continuous VS that lasted weeks or months, but recent reports have shown that some headache patients may have associated with migraine attacks.

While VS is considered a rare phenomenon, it's not clear "whether the perceived rarity of this syndrome reflects the prevalence of patient experience or lack of clinical recognition," noted Fiona Costello, MD, of the University of Calgary in Canada, and co-authors in an .

"Until the last decade, references to this clinical syndrome have been sporadic," they continued. VSS, in fact, was not fully characterized until around 2015, when researchers proposed from the following four categories to establish a VSS diagnosis: (1) entoptic phenomena, (2) palinopsia, (3) photophobia, and (4) nyctalopia (poor vision in dim light).

In their study, Puledda and co-authors built on previous research to better characterize VSS, recruiting patients for their survey through the online advocacy group Eye On Vision. ( for this study are on the group's web site.) Data was collected from April 2016 to March 2018.

Respondents who had dynamic, continuous, tiny dots in the entire visual field that lasted more than 3 months and also had two or more additional visual symptoms were classified as having VSS. Respondents with no additional visual disturbances or symptoms in only one category were considered to have VS only, without the syndrome. People who reported onset of VS symptoms within 12 months of exposure to recreational drugs were excluded from the VSS and VS groups.

The VS population had an average age of 29 and about half were men. The disorder usually started in early life (average age of 13 for people with VSS), and about 40% of participants said they had symptoms for as long as they could remember.

The most commonly experienced visual static was black and white. Floaters, afterimages, and photophobia were the most reported additional visual symptoms.

Tinnitus and migraine had a high prevalence: 75% of VSS participants reported tinnitus, 72% reported migraine, and 37% reported migraine with aura. These comorbidities were independently associated with a more severe presentation of the syndrome. Female sex also was associated with severity.

In contrast, most participants with HPPD were male (71%) and had sudden onset of symptoms (81%) that occurred significantly later in life (age 21). By definition, all these participants had symptoms that started within a year of using recreational drugs.

"A major strength of the study is its size; it represents one of the largest and most comprehensive studies of visual snow to date, which automatically positions it as an important note in the literature," the editorialists noted. "Distinguishing visual snow rigorously from HPPD further de-stigmatizes VSS and elucidates the syndrome as an idiopathic brain disorder."

However, its relative lack of migraine data is a limitation of the study, Costello and co-authors observed. "Migraine and, in particular, migraine with aura, are overrepresented in patients with visual snow and represent important comorbid conditions. The temporal sequence of their co-occurrence is still not clearly known and requires further study."

The study also was limited by its high likelihood of recruitment bias through Eye On Vision, which may have led to a younger sample or people with more severe cases of VSS. There was also no objective measure of symptom severity.

Still, surveys like this can help people become more aware of the syndrome, Puledda noted. "It is our hope that these kinds of studies will aid the recognition of visual snow, and bring under the spotlight a disorder which is widely distributed around the world but for which treatment is still lacking."

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