A review of existing clinical studies about Visual Snow (VS) as a symptom or as part of Visual Snow Syndrome (VSS).
In this study they aimed at improving the understanding of VSS being a network disorder.
The research team collected the clinical and paraclinical evidence from the currently published original studies on VS and its syndrome by searching via PubMed and Google Scholar for the term visual snow. They 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. They grouped the studies according to the methods that were used.
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.
The combination of the clinical picture and widespread functional and structural alterations in visual and extra-visual areas indicates that 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 a 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.