Visual snow
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 on dark backgrounds.
Visual snow is non-specific as regards etiology (cause of illness). Therefore, each subject with a leading complaint of visual snow needs a full diagnostic work-up including ophthalmic, neurological and psychiatric examinations as well as an MRI scan of the brain.

# 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 , first described  under the designation prolonged migraine aura status. 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.
Another possible cause of visual snow is hallucinogen persisting perception disorder (HPPD) following use of LSD, MDMA, psilocybin or other hallucinogens. In HPPD, the symptom of visual snow has been described  as aeropsia (literally "seeing the air"). It is noteworthy that HPPD can occur after a single dose of a hallucinogen 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.
Moreover, a variety of illnesses (e.g. Lyme disease, auto-immune disease) or nocious 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 supported by evidence-based medicine. 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.

# Related symptoms
In addition to visual snow, patients suffering from persistent perception disorder frequently have other types of visual disturbances such as starbursts, increased afterimages, floaters, trails, palinopsia and many others . Non-visual symptoms such as tinnitus or depersonalisation-derealisation are also frequently encountered. All of these additional symptoms have been described as manifestations of both migraine aura and HPPD, emphasizing the major importance of these two diagnoses for an explanation of the visual snow condition. Secondary psychiatric sequelae such as anxiety, panic attacks or depression may develop and necessitate appropriate treatment.

# What visual snow is not
According to the notion of hallucinatory form constants , visual snow can be conceived as a variety of visual hallucinations of random form dimension. As such, the phenomenon should not be confused with normal entoptic phenomena such as Haidinger's brush, which almost never have sufficient intensity to gain clinical significance as a source of suffering or functional impairment.

# Treatments
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 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 . However, with very little scientific research on the condition taking place, for the time being the effectiveness of such treatments remains based solely on anecdotal evidence. Beyond pharmacological approaches, appropriate counselling and cognitive behavioral interventions that focus on coping with the condition may be of huge importance.