Causes
Visual snow can occur in a variety of ophthalmic disorders that can be diagnosed by the presence of additional clinical signs and symptoms. Persisting visual snow can feature as a leading symptom of a migraine complication called persistent aura without infarction, commonly referred to as persistent migraine aura (PMA). It is important to keep in mind that there exist many clinical sub-forms of migraine where headache may be absent and where the migraine aura may not take the typical form of the zigzagged fortification spectrum, but manifests with a large variety of focal neurological symptoms.
A condition that sometimes produces visual snow is optic neuritis (inflammation of the optic nerve), caused by multiple sclerosis (MS). Moreover, a variety of illnesses (e.g. Lyme disease, auto-immune disease) or noxious events (e.g. prolonged use of a VDU, dehydration, over-acidification) have been blamed by sufferers in self-help internet forums as causes of persisting visual snow, but none of these claims have been confirmed by scientific study. Some patients fail to find any apparent causative illness or event in their lives, instead saying the snow came out of nowhere or has been with them for their whole life.
Hallucinogen persisting perception disorder (HPPD) is another condition which has resulted in the onset of the visual snow following the use of hallucinogenic psychedelic drugs. In HPPD, the symptom of the visual disturbances has been described as as aeropsia (literally "seeing the air"). HPPD very rarely occurs after just a single dose of a hallucinogenic drug and with a considerable latency between last drug intake and onset of persistent perception disorder, so taking a thorough life-time drug history is mandatory in the diagnostic work-up of visual snow. In many cases, the neurological action for HPPD is not known, and the majority of evidence surrounding it is anecdotal and difficult to isolate.
Read more about this topic: Visual Snow