Objective: Visual snow syndrome (VS) is mainly characterized by flickering, little dots in both visual fields. The recognition of the clinical entity of VS has been increasing recently. Diagnosis is based on patient reports and not better accounted for by another diagnosis.
Background: The exact pathophysiology of this syndrome is still unknown. In this study, our aim was to investigate the role of neurophysiological assessments of the occipital cortex in VS patients with (VSm ) or without migraine (VSwom ) and the healthy control (HC).
Methods: To assess the occipital cortex hyperexcitability, we conducted a prospective, observational study to investigate the habituation/potentiation response by repetitive pattern reversal visual evoked potentials (rVEP) and examined the phosphene thresholds (PT) by transcranial magnetic stimulation in VS patients with or without migraine who were admitted to our tertiary headache clinic and the healthy control.
Results: Twenty-nine volunteers were recruited. The VSm (n = 10), the VSwom (n = 7), and the HC group (n = 12) did not differ demographically. Flickering and floaters were reported in all VS patients and flickering in the dark was the most distressing symptomatology in both VS groups. Higher VAS scores for palinopsia (trailing), photophobia, and concentration difficulty were more frequently self-reported by VSm patients. The HC demonstrated habituation; however, there was loss of habituation response and decreased PTs in both groups of VS patients. The N1P1 VEP amplitude ratios of the 10th/1st block from right and left eye stimulation disclosed higher values (lack of habituation) in VSm (1.04 ± 0.2 and 1.06 ± 0.2) and the VSwom (1.05 ± 0.2 and 0.96 ± 0.08) patients compared to the healthy control (0.75 ± 0.1 and 0.79 ± 0.1), P = .002 from right eye and P = .003 from left eye. In the post hoc analysis the VS patients did not differ according to the presence of migraine from right or left eye stimulations (both P > .999). The left occipital cortex PTs were lower in VSm (58.00 ± 6.60) and VSwom (62.14 ± 11.53) and higher in the HC (71.33 ± 5.56) P = .009. In the post hoc analysis the VS patients did not differ according to the presence of migraine (P > .999). The right occipital cortex PTs were lower in VSm (60.30 ± 8.15) and VSwom (62.00 ± 10.95), higher in the HC (69.67 ± 8.04); however, statistically, groups did not differ (P = .087).
Conclusions: The loss of habituation and lower threshold for occipital cortex excitability were demonstrated electrophysiologically in VS patients. While statistically significant loss of habituation was seen in both VS patients (with or without migraine) in the right eye, statistically significant loss of habituation in the left eye and decreased threshold of left occipital cortex excitability was seen in visual snow with migraine patients. These findings may provide new insights on "visual snow" pathophysiology and serve as an objective and quantitative assessment tool in VS patients.
Keywords: TMS; habituation; occipital hyperexcitability; phosphene; repetitive pattern reversal VEP.
© 2019 American Headache Society.