Understanding visual perception in visual snow syndrome: a battery of psychophysical tests plus the 30-day clinical diary.

Patients with visual snow syndrome (VSS) experience uncountable flickering tiny dots in the entire visual field. Symptoms often persist over the years. Very little is known about altered perception in VSS. VSS is diagnosed based on subjective reports because there is no manual with objective measures. In this study, 20 patients with VSS and 17 healthy controls performed a battery of tests assessing visual acuity, contrast sensitivity, illusion perception, spatial-temporal vision, motion perception, visual attention, and selective attention. Surprisingly, except for one test, which is the honeycomb illusion, patients performed at the same level as controls. Patients reporting black and white visual snow performed better in the Stroop test compared to patients reporting other visual snow colours. In addition to a clinical visit, the 30-day clinical diary was administered to patients to broadly measure their symptom severity. We found that better performance in the tests, in particular in the contrast and coherent motion tests, was correlated with lower VSS symptoms, weaker VS characteristics (e.g. density and size) and lower VS severity. Our results suggest that, even if visual abilities are not deteriorated by VSS, they can determine how severe symptoms are, and show that VSS is an heterogenous disorder where symptoms and visual abilities vary between patients, for instance depending on the VS colour. The study was primarily designed to identify tests where performance differs between controls and patients. In addition, exploratory analyses were conducted to initiate an understanding of the overall pattern of relationships between patients' visual abilities and symptoms, which is of clinical relevance. Future studies with more power are necessary to validate our findings.