Visual Snow
Visual snow, also known as visual static, is a condition in which people see white or black dots in parts or the whole of their visual fields. The condition is typically always present and can last years.
The cause of visual snow is unclear. Those affected typically also have migraines. The underlying mechanism is believed to involve excessive excitability of neurons in the right lingual gyrus and left cerebellar anterior lobe of the brain.
Research has been limited because of issues of case identification and diagnosis, the latter now largely addressed, and the limited size of any studied cohort. Initial functional brain imaging research suggests visual snow is a brain disorder.
Medications that may be used to treat the condition include lamotrigine, acetazolamide, or verapamil. These do not always result in benefits, however.
Signs and symptoms
People report seeing "snow", much like the visual noise or "static" on a TV screen after transmission ends. In a 2010 study Raghaven et al. hypothesize that what the patients see as "snow" is their own intrinsic eigengrau.
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.
Comorbidities
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 and contrast sensitivity.
Causes
The causes are 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.
Another mechanism is a thalamocortical dysrhythmia of the visual pathway similar to tinnitus which is a thalamocortical dysrhythmia of the auditory pathway.
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 (scintillating scotoma), 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.
Diagnosis
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:
- Palinopsia (visual trailing and afterimages)
- Enhanced entoptic phenomena (floaters, photopsia, blue field entoptic phenomenon, phosphenes)
- Photophobia
- Impaired night vision
- Symptoms are not consistent with typical migraine aura.
- Symptoms are not attributed to another disorder (ophthalmological, drug abuse).
- Additional and non visual symptoms like tinnitus or brain fog and more might be present.
It can also be diagnosed by PET scan.
Treatments
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.
Medications that may be used include lamotrigine, acetazolamide, or verapamil, but these do not always result in benefits.