# Visual snow syndrome

Visual snow syndrome (VSS) is an uncommon neurological condition in which the primary symptom is that affected individuals see persistent flickering white, black, transparent, or colored dots across the whole visual field.
Other common symptoms are palinopsia, enhanced entoptic phenomena, photophobia, and tension headaches. The condition is typically always present and has no known cure, as viable treatments are still under research. Astigmatism, although not presumed connected to these visual disturbances, is a common comorbidity. Migraines and tinnitus are common comorbidities that are both associated with a more severe presentation of the syndrome.
The cause of the syndrome is unclear. The underlying mechanism is believed to involve excessive excitability of neurons in the right lingual gyrus and left anterior lobe of the cerebellum. Another hypothesis proposes that visual snow syndrome could be a type of thalamocortical dysrhythmia and may involve the thalamic reticular nucleus (TRN). A failure of inhibitory action from the TRN to the thalamus may be the underlying cause for the inability to suppress excitatory sensory information. Research has been limited due to issues of case identification, diagnosis, and the limited size of any studied cohort, though the issue of diagnosis is now largely addressed. Initial functional brain imaging research suggests visual snow is a brain disorder.

## The difference between Visual Snow and Visual Snow Syndrome

Visual snow is a phenomenon where a person perceives visual disturbances, such as fine graininess or "static," in their field of vision. This can occur in low-light conditions, in the dark, or when the visual system amplifies light perception. In these cases, visual snow is a normal reaction of the body, related to the way photoreceptors (rods) and neurons respond to weak or insufficient stimuli.
However, if visual snow becomes persistent and continuous — especially if it is accompanied by other symptoms, such as headaches, difficulty focusing, or intense visual distortions — it may indicate visual snow syndrome. This syndrome is not just a phenomenon, but a neurological disorder, where a person experiences constant visual disturbances, regardless of lighting conditions. Visual snow syndrome is often linked to heightened neural sensitivity in the visual system and may require diagnosis and treatment, as it could be associated with underlying eye conditions or neurological disorders.
In summary:
* Visual snow as a temporary occurrence under certain conditions is normal and doesn't require intervention.
* Visual snow syndrome is a pathological condition, where visual disturbances persist constantly and may be caused by issues in the visual or nervous system, requiring medical attention.

## Signs and symptoms

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.
Visual snow likely represents a clinical continuum, with different degrees of severity. The presence of comorbidities such as migraine and tinnitus is associated with a more severe presentation of visual symptoms.
Non-visual symptoms may include difficulty concentrating, insomnia, frequent migraines, nausea, and vertigo.

### Diagnosis

Visual snow syndrome is usually diagnosed with the following proposed criteria:
* Visual snow: dynamic, continuous, tiny dots observed across the entire visual field at any time of the day, regardless of lighting conditions, persisting for more than three months.
** The dots are usually black/gray on a white background and gray/white on a black background; however, they can also be transparent, white flashing, or colored.
* Presence of at least 2 additional visual symptoms of the 4 following categories:
** i. Palinopsia. At least 1 of the following: afterimages or trailing of moving objects.
** ii. Enhanced entoptic phenomena. At least 1 of the following: excessive floaters in both eyes, excessive blue field entoptic phenomenon, self-light of the eye (phosphenes), or spontaneous photopsia.
** iii. Photophobia.
** iv. Nyctalopia; impaired night vision.
* Symptoms are not consistent with typical migraine aura.
* Symptoms are not better explained by another disorder (ophthalmological, drug abuse).
** Normal ophthalmology tests (best-corrected visual acuity, dilated fundus examination, visual field, and electroretinogram); not caused by previous intake of psychotropic drugs.
Additional and non-visual symptoms like tinnitus, ear pressure, brain fog, and more might be present. It can also be diagnosed by PET scan.

### Common misconceptions

* Perceiving visual static, flickering, or graininess on monochrome colors, in the sky, or in darkness can be a normal phenomenon associated with neural noise, amplified in the absence of bright visual stimuli. This effect is known as the Ganzfeld Effect. In conditions of low illumination, especially in dimly lit environments, this phenomenon is related to how the eyes and the brain process visual information in insufficient lighting. The visual system becomes more sensitive to light and can amplify noise or minor changes in visual signals. For example to the work of different types of photoreceptors in the retina of the eye. In the dark, the rod photoreceptors, which are responsible for the perception of light in low light, are mainly activated, but they are not able to distinguish details or colors. This is why in dim light we often see the world blurred and grainy, without clear contours. While cones (which are responsible for color perception and detail) are activated in brighter light, the rods provide vision in low light, but with a loss of precision. Therefore, the graininess we perceive in the dark is a natural adaptation of our vision to photoreceptor limitations, not a sign of some abnormality. It's important to note that the perception of such phenomena may vary among different individuals due to individual differences in perception and sensitivity.
* When the eyes are closed, visual static may be related to the first level of visual hallucination.
* Eye pathologies or other neurological conditions can also be a cause of visual anomalies, including the appearance of visual static or other changes in perception. Additionally, psychological disorders, such as somatic disorders, could potentially contribute to these perceptual disturbances.

### 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.
Psychological side effects of visual snow can include depersonalization, derealization, depression, photophobia, and heliophobia in the individual affected.
Patients with visual "snow" have normal equivalent input noise levels and contrast sensitivity. In a 2010 study, Raghaven et al. hypothesize that what the patients see as "snow" is eigengrau. This would also explain why many report more visual snow in low light conditions: "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".

## Causes

The causes of VSS are not clear. 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 (scintillating scotoma), but manifests with a large variety of focal neurological symptoms.
Visual snow does not depend on the effect of psychotropic substances on the brain. 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 are disputed. Most of the evidence for both is generally anecdotal and subject to spotlight fallacy. Visual snow has also been correlated with head trauma and infection.

## Timeline

* In May 2015, visual snow was described as a persisting positive visual phenomenon distinct from migraine aura in a study by Schankin and Goadsby.
* In December 2020, a study found local increases in regional cerebral perfusion in patients with visual snow syndrome.
* In September 2021, two studies found white matter alterations in parts of the visual cortex and outside the visual cortex in patients with visual snow syndrome.
* In November 2023, a study revealed that glutamate and serotonin are involved in brain connectivity alterations in areas of the visual, salience, and limbic systems in VSS. Importantly, altered serotonergic connectivity is independent of migraine in VSS, and simultaneously comparable to that of migraine with aura, highlighting a shared biology between the disorders.

## Treatments

It is difficult to resolve visual snow with treatment, but it is possible to reduce symptoms and improve quality of life through treatment, both of the syndrome and its comorbidities. In some studies, lamotrigine as a treatment for visual snow syndrome only showed efficacy in 20% of patients, and in one study, patients using lamotrigine even reported worsening symptoms. Medications that may be used include lamotrigine, acetazolamide, verapamil, clonazepam, propranolol, and sertraline but these do not always result in positive effects. As of 2021, two ongoing clinical trials were using transcranial magnetic stimulation and neurofeedback for visual snow.
A recent study in the British Journal of Ophthalmology has confirmed that common drug treatments are generally ineffective in visual snow syndrome (VSS). Vitamins and benzodiazepines, however, were shown to be beneficial in some patients and can be considered safe for this condition.