The first cases of episodic visual snow syndrome, which appears to occur only during a migraine attack, have been reported. The cases provide more understanding of the mechanism behind the syndrome, which could lead to new avenues for treatment. Visual snow syndrome is a condition in which patients visualize hundreds or thousands of small flickering dots across the whole of their visual field. The syndrome was named for the snowy picture that was sometimes seen on old analog televisions when they were not tuned in properly. The syndrome has been recognized for several years. Patients also often have other visual symptoms, including photophobia and an abundance of "floaters" ― small shapes that float across the field of vision. The prevalence of visual snow syndrome is unknown. It is thought to go unrecognized in many patients because their eye exam results are normal. It has been thought that visual snow is a state that is present continuously, but now three cases of the syndrome have been described as occurring episodically, only at the time of migraine attacks. "It was thought previously that visual snow syndrome had a link with migraine, as around 60% of those affected also suffer from migraine, but whereas migraines occur occasionally, the visual snow is normally present continuously," senior author Christoph Schankin, MD, Bern University Hospital, Switzerland, explained to Medscape Medical News. "We think this episodic visual snow is a new symptom of migraine and strengthens the link between visual snow syndrome and migraine," he said. Of the three episodic cases, two patients reported experiencing the onset of visual snow at the time of the migraine attack. In those cases, the visual snow disappeared when the migraine subsided. The third patient reported experiencing visual snow at 2-minute intervals but only during a migraine attack, Schankin noted. The cases are reported in a research letter published online in JAMA Neurology on November 25. Not to Be Confused With Migraine Aura "Originally, this may have been thought to be part of migraine aura, but we are quite sure now that it isn't an aura symptom," Schankin said. "Migraine aura is characterized by a cortical spreading depression causing flashes and patterns to move slowly across the visual field, whereas in patients with visual snow, the flickering dots are seen across the whole visual field with no movement, signifying no cortical spreading." "In clinical practice, a detailed history in patients reporting visual flickering is therefore necessary to differentiate aura from episodic visual snow," the authors write. "This is important because the diagnosis of aura might have implications for patient guidance on contraception or timing of triptan intake." They suggest that visual snow, like migraine, is a disorder of sensory processing in which the brain's filtering mechanisms become dysfunctional. "In migraine, patients are normal most of the time, and the brain filters out stimuli such as brightness of light, loudness of sound, and GI signals that may cause nausea, but during an attack, this filtering mechanism stops working, causing photophobia and phonophobia, nausea, and headache via trigeminal nerve stimulation," Schankin explained. "Normally in visual snow syndrome, patients are photophobic and experience the flickering dots all the time, and we believe they have a permanent disorder of visual processing," he noted. "They often also have 'after imaging,' in which the brain keeps visualizing images previously seen, whereas a normal brain erases previous images. We believe that in visual snow syndrome, the brain is picking up the noise of the optic system which in a normal brain is suppressed." The authors hypothesize that visual snow is a problem of the visual association cortex. "Visual information from the thalamus goes to the visual cortex, which filters out what is not necessary to see, and this filtering out mechanism is dysfunctional in visual snow syndrome," Schankin suggested. He believes there is a spectrum of sensory processing dysfunction with healthy people at one end, migraineurs with intermittent dysfunction in the center, and patients with visual snow syndrome with permanent visual sensory processing dysfunction at the other end. "Now somewhere in between the migraineurs and the visual snow syndrome patients, we have these patients who only experience visual snow at the time of migraine," he said. Treatment of visual snow syndrome is difficult ― there are no very effective treatments, although the antiseizure drug lamotrigine can reduce the severity in a minority of patients, Schankin said. Migraine medications such as triptans do not seem to be effective for visual snow syndrome, but neuromodulation techniques, such as transcranial magnetic stimulation, may be a new approach to investigate, because this works on a different mechanism, he suggested. The work was supported by Deutsche Migräne-und Kopfschmerzgesellschaft, the Eye on Vision Foundation, and the Baasch-Medicus Foundation. Schankin reports receiving grants from the Swiss Heart Foundation outside the submitted work; personal fees from Novartis, Eli Lilly and Company, Almirall, and Allergan outside the submitted work; and personal fees and nonfinancial support from Teva Pharmaceuticals and Amgen outside the submitted work. JAMA Neurol. Published online November 25, 2019. Abstract Source
