Proposed diagnostic criteria for the "visual snow" syndrome:
Dynamic, continuous, tiny dots in the entire visual field.
At least one additional symptom:
Palinopsia (visual trailing and afterimages)
Enhanced entoptic phenomena (floaters, photopsia, blue field entoptic phenomenon, self-light of the eye)
Impaired night vision
Symptoms are not consistent with typical migraine aura.
Symptoms are not attributed to another disorder (ophthalmological, drug abuse)
Schankin, Christoph; Maniyar, Farooq; Hoffmann, Jan; Chou, Denise; Goadsby, Peter (February 2013). "Clinical characterization of "visual snow" (Positive Persistent Visual Disturbance)". The Journal of Headache and Pain.