|Animated example of visual snow-like noise|
|Classification and external resources|
Visual snow, visual static, or persistent visual snow is a transitory or persisting medical disorder in which sufferers see snow or television-like static in parts or the whole of their visual fields, constantly in all light conditions, even visible in daylight, darkness and with closed eyelids. The severity or density of the "snow" differs from one person to the next; in some circumstances, the condition greatly affects daily life, making it difficult or impossible to read, drive, perform routine tasks, see in detail (even in bright daylight) or focus correctly because of afterimages and numerous other visual and non-visual co-morbidities, as discussed below. Indeed, medical literature refers to the condition as "disabling" and "debilitating". The use of computer screens can exacerbate symptoms considerably in many patients. Also see Closed eye hallucination: Level 1, which makes people aware that they start to notice it even in normal situations, because what-ever is visible in the dark will be visible on any color plain surface.
Little is known about this rare condition, and it has conventionally been regarded as a variant of migraine aura—though recent research shows this is not the case. It is commonly confused with floaters by opticians and doctors alike when patients describe their symptoms, leading to misdiagnosis as well as underdiagnosis of visual snow. At times, if it is visible on certain surfaces it can be because our eyes sees vitreous fluid present inside our eyes (entopic phenomenon). In some cases, it can be vitreous degeneration.
Recent research has confirmed a brain hypermetabolism in persons with visual snow, located principally in the right lingual gyrus.[unreliable medical source?] Before this, no other cause for visual snow had been identified. Insofar as sufferers of visual snow had undergone ophthalmic, neurological and psychiatric examinations, no systematic problems besides the visual snow were found. The recent research that indicates this disorder occurs in the brain has important ramifications for the possibility of potential treatment. However, standard treatment protocols have yet to be established.
Signs and symptoms
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 May 2014 the results from the first major research trial into visual snow were reported. The study described strong evidence from positron emission tomography scans that the disorder is associated with hypermetabolism in the right lingual gyrus and left cerebellar anterior lobe of the brain. The researchers stated that pinpointing visual snow (and its related symptoms such as afterimages) to a functional problem in a specific brain area may open up possibilities for targeted treatment and that treatment trials will follow.
Visual snow can occur in a variety of ophthalmic conditions that can be diagnosed by the presence of additional clinical signs and experiences.
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). 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). Moreover, a variety of illnesses (e.g., Lyme disease, auto-immune disease) or noxious events (dehydration) 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.[original research?] However, Lyme Disease has been shown to cause optic neuritis. 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 lives.
The role of hallucinogens in the etiology of visual snow is not entirely obvious. 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 etiology 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.
- At least one additional symptom:
- 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, though pharmacological treatment is the preferred option. It is difficult to resolve visual snow with treatment, but it is possible to reduce symptoms and improve quality of life through treatment.
In HPPD, clonazepam has been recommended as medication of first choice in patients seeking medical help. 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.
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