Visual snow syndrome
Visual snow syndrome (VSS) is a form of visual hallucination that is characterized by the perception of small, flickering dots throughout the entire visual field. It is present in all conditions of illumination. The dots remain individual and do not clump together or change in size. Visual snow exists in one of two forms: the pulse type and the broadband type.
| Visual snow syndrome | |
|---|---|
| Other names | Persistent positive visual phenomenon, visual static, aeropsia |
| Animated example of visual snow-like noise | |
| Specialty | Neurology, Neuro-ophthalmology |
| Symptoms | Static and auras in vision, Palinopsia, Blue field entoptic phenomenon, Nyctalopia, Tinnitus |
| Complications | Poor quality of vision, Photophobia, Heliophobia, Depersonalization and Derealization |
| Usual onset | Visual Snow can appear at any time, but it commonly appears at birth, late teenage years, and early adulthood. |
| Causes | Unknown, hyperexcitability of neurons and processing problems in the visual cortex |
| Risk factors | Migraine sufferer, psychoactive substance use |
| Differential diagnosis | Migraine aura, Persistent aura without infarction, Hallucinogen persisting perception disorder |
| Medication | Anticonvulsants (limited evidence and success) |
| Frequency | Uncommon (understudied) |
In the pulse type, the dots are the same color as their background and tend to flicker individually, sometimes suggestive of raindrops on a car windshield. In black pulse visual snow, the dots are always darker than their background, whereas in white pulse the dots are always lighter than their background.
In the broadband type, the noise is perceived as a coarse texture. The noise is bipolar and occurs in contrast to the background: with a light background the dots will appear dark, and with a dark background the dots will appear light.
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. As well, migraine and tinnitus are common comorbidities which are both associated with a more severe presentation of the syndrome. TMJ may also be a common comorbidity.
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 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 inability to suppress excitatory sensory information. Research has been limited due to 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.