Visual impairment
Medical condition
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:
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:
Additional and non-visual symptoms like tinnitus, ear pressure, brain fog, and more might be present. It can also be diagnosed by PET scan.
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.
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.
References.
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