Visual Snow Syndrome as a Network Disorder: A Systematic Review

Abstract

Aim: By reviewing the existing clinical studies about visual snow (VS) as a symptom or as part of visual snow syndrome (VSS), we aim at improving our understanding of VSS being a network disorder.Background: Patients with VSS suffer from a continuous visual disturbance resembling the view of a badly tuned analog television (i.e., VS) and other visual, as well as non-visual symptoms. These symptoms can persist over years and often strongly impact the quality of life. The exact prevalence is still unknown, but up to 2.2% of the population could be affected. Presently, there is no established treatment, and the underlying pathophysiology is unknown. In recent years, there have been several approaches to identify the brain areas involved and their interplay to explain the complex presentation.Methods: We collected the clinical and paraclinical evidence from the currently published original studies on VS and its syndrome by searching PubMed and Google Scholar for the term visual snow. We included original studies in English or German and excluded all reviews, case reports that did not add new information to the topic of this review, and articles that were not retrievable in PubMed or Google Scholar. We grouped the studies according to the methods that were used.Results: Fifty-three studies were found for this review. In VSS, the clinical spectrum includes additional visual disturbances such as excessive floaters, palinopsia, nyctalopia, photophobia, and entoptic phenomena. There is also an association with other perceptual and affective disorders as well as cognitive symptoms. The studies that have been included in this review demonstrate structural, functional, and metabolic alterations in the primary and/or secondary visual areas of the brain. Beyond that, results indicate a disruption in the pre-cortical visual pathways and large-scale networks including the default mode network and the salience network.Discussion: The combination of the clinical picture and widespread functional and structural alterations in visual and extra-visual areas indicates that the VSS is a network disorder. The involvement of pre-cortical visual structures and attentional networks might result in an impairment of “filtering” and prioritizing stimuli as top-down process with subsequent excessive activation of the visual cortices when exposed to irrelevant external and internal stimuli. Limitations of the existing literature are that not all authors used the ICHD-3 definition of the VSS. Some were referring to the symptom VS, and in many cases, the control groups were not matched for migraine or migraine aura.

Case Report: Transformation of Visual Snow Syndrome From Episodic to Chronic Associated With Acute Cerebellar Infarct

“…Cerebellar dysfunction has been implicated in several pathological brain states, including depression ( 25 ) and autism ( 26 ). It is thus not entirely surprising that a similar network-type disorder such as visual snow syndrome ( 27 ) might be caused by at least a partial involvement of this complex brain region.…”

Visual snow: a neuro-ophthalmic syndrome

“…Consequently, other syndromes that result in sensory misperception, such as migraine, persistent perceptual postural dizziness, and tinnitus, overlap with VSS [15]. Ophthalmologists, as the first line of referral, frequently play a decisive role in the diagnosis of VSS [5,7]. The most common positive visual phenomena are associated with migraine.…”

“…VSS is not a progressive disorder but is chronic with remission and recurrence [5]. Bilateral cases of VSS involving the visual field have been reported [6].…”

“…Patients with VSS tend to first visit ophthalmologists. After migrainerelated and ocular causes are excluded, a neurologist is consulted [5,7]. VSS is not notorious in its pathogenesis, although visual disturbances recur.…”

“…VSS is not notorious in its pathogenesis, although visual disturbances recur. The appearance and persistence of visual phenomena may psychologically disturb the patient [2,5,8]. This frightening condition prompts urgent referral to eye care professionals.…”