Patients with visual snow syndrome experience occipital cortex hyperexcitability indicated by a loss of reversal visual evoked potential habituation and a decrease in phosphene thresholds when compared with healthy individuals, according to a study published in Headache.

Researchers of this prospective, observational study completed neurophysiological examinations to assess the clinical characteristics and functional properties of patients with visual snow syndrome, and incorporated the use of transcranial magnetic stimulation phosphene detection to evaluate occipital cortex hyperexcitability. This study included patients with visual snow syndrome and migraines, patients with visual snow syndrome without migraines, and healthy controls. All patients completed questionnaires to evaluate demographics, headache characteristics, migraine disability, and visual snow syndrome characteristics. Magnetic resonance images of the brain were evaluated for occipital bending and asymmetry. Neurophysiological assessments of reversal visual evoked potential and transcranial magnetic stimulation for phosphene thresholds were completed on all participants.

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Of the 29 participants included in this study, 10 had visual snow syndrome and migraines, 7 had visual snow syndrome without migraines, and 12 were healthy controls. There were no significant demographic differences between the 3 cohorts. Patients in the visual snow syndrome and migraine cohort more frequently reported photophobia (P <.05) and concentration difficulty (P <.035), while patients in the visual snow syndrome without migraine cohort more frequently reported nyctalopia (P =.593). Patients in the visual snow syndrome and migraine cohort also had significantly higher visual analog scale scores for palinopsia, than patients in the visual snow syndrome without migraine cohort (P =.043).

Researchers found that the photomicrograph of reversal visual evoked potential traces, the healthy control cohort showed habituation, while both cohorts of patients with visual snow syndrome showed a loss of habituation. Post hoc analysis and pairwise comparisons revealed significant difference among the visual snow syndrome cohort vs the healthy control cohort for right eye ratio of habituation measurements (visual snow syndrome and migraine cohort vs healthy control cohort P =.011; visual snow syndrome without migraine cohort vs healthy control cohort P =.008). Additional differences were found among the visual snow syndrome and migraine cohort when compared with the healthy control cohort for left eye ratio of habituation measurements (P =.003). The phosphene threshold was lower in both visual snow syndrome cohorts when compared with the healthy control cohort. Post hoc analysis and pairwise comparisons revealed significant differences among patients in the visual snow syndrome and migraine cohort vs the healthy control cohort for left occipital cortex phosphene threshold (P =.009).

Limitations of this study include small cohorts that could impact statistical significant differences and not including migraine with aura as a cohort.

The researchers concluded “the lack of [reversal visual evoked potential] habituation and decreased [phosphene thresholds] indicate occipital cortex hyperexcitability in both groups of [patients with visual snow syndrome].”

Reference

Yildiz FG, Turkyilmaz U, Unal-Cevik I. The clinical characteristics and neurophysiological assessments of the occipital cortex in visual snow syndrome with or without migraine [published online March 8, 2019]. Headache. doi: 10.1111/head.13494