Article In Brief
A research team found that visual processing, as measured by visual evoked potentials, is different in patients with visual snow syndrome (VSS) and control patients with migraine who don't have VSS. Independent experts said the researchers observations offered important insights into the pathophysiology of the disorder but they questioned the use of visual evoked potentials in the study to measure dysfunction in the visual association cortex.
The phenomenon of visual snow, which appears like static in the visual field, appears to be a true neurological symptom that is related to dysfunction in the visual association cortex, and it is distinctly different from migraine, according to a small study published online on November 1, 2018, in Annals of Neurology.
Patients with visual snow syndrome (VSS) experience continuous television-static-like tiny flickering dots in the entire visual field and additional visual symptoms such as palinopsia or photophobia, the authors of the study explained. In patients with VSS, neurological and ophthalmological clinical examinations, as well as brain imaging, are typically normal.
“To the clinician this appears to be a purely subjective syndrome without objective measures confirming the diagnosis,” said the senior study author Christoph Schankin, MD, an attending neurologist at the Bern University Hospital in Switzerland. “Until a few years ago, physicians concluded that VSS is likely malingering or psychogenic. This has always been a source of enormous distress for patients.”
“Most patients with VSS have comorbid migraine with aura,” he added. “Therefore, the few physicians who believed patients concluded that this might be a problem of migraine with aura, something called ‘persistent migraine aura.’ However, treatment with antimigraine medication was often not helpful.”
In the current study, the researchers found that visual processing, as measured by visual evoked potentials, is different in patients with VSS and control patients with migraine who don't have VSS. “This confirms at an objective level that both conditions are different,” Dr. Schankin told Neurology Today. “For our understanding of VSS pathophysiology and for future therapy, it will be important to broaden the focus from migraine to other therapeutic options, such as pharmacological and nonpharmacological modulation of the visual association cortex.”
“Patients with VSS are not malingerers or faking their symptoms,” he said. “They likely have a disorder of visual postprocessing that is related to migraine but differs from migraine and especially from persistent migraine aura. Treatment is difficult but understanding visual postprocessing in the visual association cortex will hopefully result in therapeutic options in the future.”
Experts who reviewed the study for Neurology Today said the key takeaway, that VSS is a real neurologic condition, is important. Research demonstrating that VSS is a true neurological condition is vital not only to reassuring patients but to finding effective treatments, of which there are currently none, they said.
Study Design, Findings
Dr. Schankin and colleagues advertised the study on social media with support from the VSS self-help group Eye on Vision Foundation. Through telephone interviews, the researchers determined if the patients met their enrollment criteria: They had to be 18 years or older and have experienced VSS in accordance with published criteria. Patients were excluded if there was evidence they used illicit drugs prior to VSS onset.
The research team used visual evoked potentials (VEP) to assess the visual pathways through the optic nerves and brain, measuring the latency and amplitude of brain waves specific to vision in three groups: 18 patients with VSS, 18 age-matched migraineurs, and 18 healthy controls.
Participants were seated in a relaxed position in a quiet room with standardized dimmed artificial light conditions. A screen displaying a checkerboard was placed 135 centimeters in front of the eyes with the right eye covered, and standard surface electrodes were attached to the scalp. The evoked potentials were recorded when a black-and-white checkerboard reversal pattern was presented.
A normal VEP response to a pattern-reversal stimulus is a positive midoccipital peak that occurs at a mean latency of 100 milliseconds. In this study the three groups differed significantly in N145 latency (p=0.046). Statistical analysis revealed increased N145 latency and reduced N75-P100 amplitude in VSS patients in comparison with both, migraineurs and healthy controls. [See “By the Numbers” for the measurements.]
These findings support the idea that the primary disturbance in VSS is a dysfunction of the visual association cortex, Dr. Schankin said.
“A bias from comorbid migraine or aura, which might have influenced results of previous case results on this topic, is further unlikely,” the authors of the study wrote. Although VSS is associated with migraine, it is “electrophysiologically different from migraine and typical migraine.”
In a 2014 paper in Headache, Dr. Schankin and colleagues investigated the relationship between migraine and VSS and found that patients with migraine had more severe visual snow, suggesting that migraine worsens the phenomenon. “But that does not prove that it causes VSS,” he emphasized. “In another 2014 paper in Brain, we have shown that neither migraine aura nor migraine attacks are associated with the onset of VSS. So although migraine is very common in patients with VSS we believe it does not cause the condition.”
Experts who reviewed the study for Neurology Today said it provides insights on a condition that is reasonably common — although prevalence studies are rare — and for which there are not good treatments.
“The study by this group and others is demonstrating that the phenomenon of visual snow is a true, neurological condition and one that appears to be distinct from migraine, although there may be some overlap,” said Victoria S. Pelak, MD, FAAN, professor of neurology and ophthalmology and director of the neuro-ophthalmology fellowship at the University of Colorado School of Medicine.
“As with most neurological conditions, there is a spectrum of disability, and some patients do find the phenomenon disabling,” she told Neurology Today. “This is especially true at the onset of the syndrome.”
Dr. Pelak said the frequency of the phenomenon among patients with migraine may be related to overlapping genetic causes, or a common hypersensitivity in an area of the brain.
Grant Liu, MD, FAAN, professor of neurology and ophthalmology at the University of Pennsylvania, agreed. He believes the term “visual snow” is an umbrella term for a condition that encompasses migraine but includes patients who don't have migraine.
Dr. Liu published one of the first papers on the condition in 1995 in Neurology. “At the time we called it ‘persistent positive visual phenomenon,’ and we studied it in migraine patients,” he said. “The characteristics are the same as what people call visual snow: It's everywhere in the visual field, in both eyes, and pretty much constant.
“In my experience, patients don't find it disabling, but they do find it annoying,” he said. “They can read, drive a car, watch television. They can see right through the phenomenon. I think a lot of patients have this and are just reassured to know they aren't imagining it and they aren't going to go blind.”
However, visual evoked potentials are a less than perfect method for localizing functional phenomena in the brain, Dr. Liu and his colleagues said.
“The findings are interesting, but the evoked potentials are not that sensitive or reliable and can be highly dependent on where the leads are placed,” said Kathleen B. Digre, MD, FAAN, distinguished professor of neurology and ophthalmology and chief of the division of neuro-ophthalmology and headache at the University of Utah.
Dr. Digre agreed that the phenomenon does appear to be distinct from migraine. “Migraine aura usually has a beginning, a middle, and an end,” she said. “Visual snow is almost continuous.”
She noted: “The study is interesting, and it is good that people are studying this phenomenon. Even though evoked potentials may not be the most reliable tool for localization, it's good that this study gets publicity. We really need to understand visual snow pathophysiology and also the range of snow that people experience. There may be other causes that we are not aware of.”
The take-home message for clinicians? “Visual snow is a real phenomenon,” Dr. Digre said. “These individuals are not crazy but are experiencing noise in the visual system. Most of the time simple reassurance is all they need. For those who are really bothered, we need to find better treatments.”