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NEW ORLEANS -- A mysterious and troubling phenomenon called "visual snow" is a distinct syndrome that does not appear to stem from illicit drug use, a researcher said here.

People with visual snow report seeing a mix of swimming dots, flying objects, and trailing shadow images that fill the visual field all the time, explained Christoph Schankin, MD, of the University of California San Francisco, during a platform presentation at the American Academy of Neurology's annual meeting.

Although people with the condition can still see well enough to function, it is upsetting and drives many to seek medical help, or at least an explanation.

Visual snow has been described sporadically in the literature, but never before studied systematically in a large number of patients. Those earlier reports have suggested that the condition is a form of migraine aura or a late effect of hallucinogen intake, Schankin said

But his study of 120 people reporting the syndrome suggests that, although migraine is relatively common in these individuals, it also affects many nonmigraineurs and that a history of illicit drug use is uncommon.

He said the visual-snow phenomenon centers on a disturbance described as similar to watching a grainy TV picture. Patients can still see real objects but with tiny black and white dots throughout the visual field.

Another common aspect of the syndrome is that, when patients look at a clear blue sky, they see small white objects zooming around randomly.

A third feature is that moving objects appear to leave trails behind them.

Reports of visual snow abound on the Internet. An online advocacy group called Eyeonvision.org contacted Schankin's laboratory to request a study and to offer cooperation.

Working through the group, Schankin and colleagues distributed an online survey to 140 people believed to experience visual snow. They then sought to conduct detailed telephone interviews with those having what the researchers considered classic visual snow -- tiny black and white dots filling the visual field.

Schankin and colleagues made initial telephone contact with 120 of the online respondents who had described their symptoms. Their mean age was 31 (SD 12) and were divided about evenly between males and females.

The interviews disclosed substantial variation in the manifestations. About half the respondents (57) described dots as solid, black and white, and not flashing. But the others saw dots that were in color, flashing, and/or transparent.

Detailed phone interviews were conducted with the 57 who saw nonflashing black and white dots.

Most but not all respondents saw the zooming white objects in the sky and the trails behind moving objects. More than half, but not necessarily the same half, reported a variety of other effects. These included halos and starbursts surrounding objects, floaters, poor night vision, photosensitivity, and colored swirls and waves when their eyes were closed.

Overall, of the 57 with the classic visual snow, nearly all reported at least one additional symptom and 93% had at least three.

Most could recall the exact day that the disturbances began and what they were doing at the time, Schankin said. Only three said they had taken illicit drugs -- marijuana, with one reporting psychedelic mushrooms, and another having taken cocaine and amphetamines as well -- within seven days before onset.

But more than half (54%) said they also had migraines, with two-thirds of those accompanied by aura. On the other hand, Schankin said the condition appears distinct from migraine aura even in those individuals.

Commenting after Schankin's presentation, Deborah Friedman, MD, of the University of Texas Southwestern Medical Center in Dallas, said she thought the phenomenon is actually extremely common, just neglected in the scientific literature.

She agreed that drug-taking is not an issue and that the association with migraine is common, though "it has nothing to do with their migraines."

Instead, she suggested, it is probably nothing more than an exaggerated type of normal visual function.

Those with the disturbance "tend to be people who are very observant ... people who are really hyperobservant of their world. There's a real controversy as to how much of this is a heightened awareness of normal phenomena," Friedman said.

She noted the prevalence of so-called entopic phenomena in Schankin's sample -- the floaters, zooming white objects, and images seen even with closed eyes. Such perceptions usually stem from retinal activation.

"When we have entopic firing from our retina ... if you look at the white wall and really focus on it with the right lighting, you can see it," she said. "It's the same thing with afterimages. It's normal to see afterimages."

"Most of us pay no attention to it," Friedman continued. "But if you're a really observant person, you start noticing it, and then you notice it all the time."

Schankin said that many of the patients were indeed "extremely high functioning," but he disagreed that the phenomenon can, at this point, be dismissed as a harmless variation on normal visual perception.

Attempts to portray what visual snow looks like, produced by advocacy groups, can be seen and .

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