Visual snow--persistent positive visual phenomenon distinct from migraine aura

Patients with visual snow complain of uncountable flickering tiny dots in the entire visual field similar to the view of a badly tuned analogue TV channel (TV snow). The symptoms are often continuous and can persist over years. This condition is grouped among the persistent visual phenomena in migraine, although it clinically presents a unique entity distinct from persistent migraine aura or migraine aura status. Here, we review the recent literature leading to the identification of the visual snow syndrome. The additional visual and non-visual symptoms are described in detail, and criteria are presented for future studies. Using these criteria, the relationship to migraine and typical migraine aura was recently evaluated. Further, patients with visual snow differ from controls in respect of hypermetabolism in the supplementary visual cortex (lingual gyrus). This provides evidence that visual snow, despite being purely subjective in the individual patient, has a clear biological basis. The area of hypermetabolism overlaps with the functional correlates of photophobia in migraine supporting the close relationship of migraine and visual snow.