Dr. Schankin noted that some of these symptoms map onto the well defined clinical phenomena of palinopsia (trailing and prolonged after-images), photophobia, and impaired night vision.
And others fall into a category of entoptic phenomena, or visual symptoms originating in the eyes themselves, namely, floaters (likely protein aggregations in the vitreous fluid that cast a shadow on photoreceptors); photopsia (bright flashes typically elicited by mechanical stimulation of the eyes); Scheerer’s phenomenon (small moving objects against the sky thought to be due to blood cells moving in the retinal vessels that cast a shadow on the photoreceptors); and self-light of the eye (the colored swirls, clouds, and waves), whose etiology is unknown.
In the second part of the study, the investigators conducted telephone interviews with another 120 patients with visual snow to further explore the nature of symptoms and antecedent events. These patients were 31 years old on average and nearly evenly split between men and women.
Results showed that the textural patterns described for the snow varied considerably. The most common pattern reported was dots alternating from black (on light backgrounds) to white (on dark backgrounds) (48%), while some patients reported flashing dots, transparent dots, or other patterns. "We don’t know what that means – whether that has some pathophysiologic relevance," Dr. Schankin commented.
Analyses restricted to the subset reporting black and white dots showed that 98% had at least one additional visual symptom, and 93% had three or more. In this part of the study, another symptom identified was halos or starbursts, seen in 65% of cases.
Of the 40 patients with onset of visual snow later in life, 54% had a history of migraine. However, when asked about events in the week before the onset of visual snow, only 33% reported headache, and just 10% reported aura symptoms. But none had classic features of visual aura, such as unilaterality, zig-zag lines, or scotoma, during visual snow. Additionally, only 8% had used illicit drugs, mainly marijuana, in the week leading up to the start of visual snow, and none had significant ophthalmologic findings.
Dr. Schankin disclosed no relevant conflicts of interest.