||This article may contain original research. (July 2008)|
Visual snow is a transitory or persisting visual symptom where people see snow or television-like static in parts or the whole of their visual fields, especially against dark backgrounds. It is much like camera noise in low light conditions.
The severity or density of the "snow" differs from one person to the next; in some circumstances, it can inhibit a person's daily life, making it difficult to read, see in detail and focus correctly. The "snow" is more generalized than the "blue-sky sprites" or "worms" seen in the blue field entoptic phenomenon.
No etiology for visual snow has been identified, and anecdotal reports point to a multitude of associated conditions, possibly rendering it a non-specific symptom. Insofar as sufferers of visual snow have undergone ophthalmic, neurological and psychiatric examinations, no systematic problems besides the visual snow have been identified. Pending recognition of the condition, little medical research is taking place to possibly identify more subtle deviations.
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 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.
Related symptoms 
Non-visual symptoms such as tinnitus, depersonalization-derealization, fatigue, speech difficulties and cognitive dysfunction (brain fog) are frequently encountered. Secondary psychiatric sequelae such as anxiety, panic attacks or depression may develop and necessitate appropriate treatment.
There currently is no established treatment for visual snow. In HPPD, clonazepam has been recommended as medication of first choice in patients seeking medical help. Furthermore, drug abstinence is sometimes said to be of major therapeutic importance in HPPD. In persistent aura without infarction, the evidence so far suggests that acetazolamide may be the premier drug for patients with the repetitive form of aura status and that valproate, lamotrigine, or topiramate should be first choices for patients with the continuous form. When these oral drugs are ineffective, an intravenous injection or injections of furosemide should be tried.
For the time being the effectiveness of such treatments remains based solely on anecdotal evidence. Beyond pharmacological approaches, appropriate counseling and cognitive behavioral interventions that focus on coping with the condition may be of huge importance.
See also 
- International Headache Society. The International Classification of Headache Disorders, 2nd edition. Cephalalgia 2004; 24 (suppl. 1): 1-160.
- Abraham HD. Visual phenomenology of the LSD flashback. Arch Gen Psychiatry 1983; 40: 884-889.
- Podoll K, Dahlem M, Greene S. Persistent migraine aura symptoms aka visual snow.
- Lerner AG, Kladman I, Kodesh A, Sigal M, Shufman E. LSD-induced Hallucinogen Persisting Perception Disorder treated with clonazepam: two case reports. Isr J Psychiatry Relat Sci 2001; 38: 133-136.
- Haan J, Sluis P, Sluis LH, Ferrari MD. Acetazolamide treatment for migraine aura status. Neurology 2000; 55: 1588-1589.
- Rothrock JF. Successful treatment of persistent migraine aura with divalproex sodium. Neurology 1997; 48: 261-262.
- Chen WT, Fuh JL, Lu SR, Wang SJ. Persistent migrainous visual phenomena might be responsive to lamotrigine. Headache 2001; 41: 823-825.
- Podoll K, Dahlem M, Haas DC. Persistent migraine aura without infarction - a detailed description
- Rozen TD. Treatment of a prolonged migrainous aura with intravenous furosemide. Neurology 2000; 55: 732-733.
- Yuku visual snow forum
- Facebook discussion group for people suffering with Visual Snow Syndrome
- What is Visual Snow Syndrome
- The effects of Visual Snow Syndrome]
- Internet Resources For Visual Snow Patients
- Persistent aura without infarction
- Page for raising awareness for Visual Snow
- Eye on Vision Foundation Raises Money for Visual Snow Research
- YouTube video showing how many people see with Visual Snow
- Simulation of Visual Snow