Visual snow, also known as visual static, is a proposed condition in which people see white or black dots in parts or the whole of their visual fields. The problem is typically always present and can last years.
The cause is unclear. Those affected typically also have migraines. The underlying mechanism is believed to involve excessive excitability of neurons within the cortex of the brain.
Some neuro-ophthalmologists believe that visual snow is not a medical condition, but a poorly understood symptom. People report seeing "snow", much like the visual noise on a TV screen after transmission ends. These authors hypothesize that what the patients see as "snow" is their own intrinsic visual noise.
Many report more visual snow in low light conditions. This has a natural explanation. "The intrinsic dark noise of primate cones is equivalent to ~4000 absorbed photons per second at mean light levels below this the cone signals are dominated by intrinsic noise".
In addition to visual snow, many of those affected have other types of visual disturbances such as starbursts, increased afterimages, floaters, trails, and many others.
The cause is unclear. The underlying mechanism is believed to involve excessive excitability of neurons within the cortex of the brain, specifically the right lingual gyrus and left cerebellar anterior lobe of the brain.
Persisting visual snow can feature as a leading addition to a migraine complication called persistent aura without infarction, commonly referred to as persistent migraine aura (PMA). In other clinical sub-forms of migraine headache may be absent and the migraine aura may not take the typical form of the zigzagged fortification spectrum, but manifests with a large variety of focal neurological symptoms.
The role of hallucinogens in visual snow is not clear. Hallucinogen persisting perception disorder (HPPD), a condition caused by hallucinogenic drug use, is sometimes linked to visual snow, but both the connection of visual snow to HPPD and the cause and prevalence of HPPD is disputed. Most of the evidence for both is generally anecdotal, and subject to spotlight fallacy.
Proposed diagnostic criteria for the "visual snow" syndrome:
Dynamic, continuous, tiny dots in the entire visual field.
Additional visual symptoms of at least two of the following four types:
Symptoms are not consistent with typical migraine aura.
Symptoms are not attributed to another disorder (ophthalmological, drug abuse).
Migraine and migraine with aura are common comorbidities. However, comorbid migraine worsens some of the additional visual symptoms and tinnitus seen in "visual snow" syndrome. This might bias research studies by patients with migraine being more likely to offer study participation than those without migraine due to having more severe symptoms. In contrast to migraine, comorbidity of typical migraine aura does not appear to worsen symptoms.
Patients with visual "snow" have normal equivalent input noise levels: Visual snow is a poorly understood symptom. Patients report seeing "snow", much like the visual noise on a TV screen after transmission ends. Some hypothesize that what the patients see as "snow" is their own intrinsic visual noise. Dennis Pelli and others' measurements assess whether visual-snow patients have increased levels of intrinsic visual noise.
There is no established treatment for visual snow. It is difficult to resolve visual snow with treatment, but it is possible to reduce symptoms and improve quality of life through treatment.