Visual snow is a form of visual hallucination that is characterized by the perception of small, bilateral, simultaneous, diffuse, mobile, asynchronous dots usually throughout the entire visual field, but it can be partial, and it is present in all conditions of illumination, even with the eyes closed. 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. In the pulse type the dots are the same color as their background, black or white, and the noise is monopolar. 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 dots 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. 
Alternate names for Visual Snow are Scotopic Sensitivity Syndrome, Meares-Irlen Syndrome, Persistent Visual Phenomena, Visual Stress, Visual Static, and Asfedia.
Visual snow symptoms commonly appear during the late teenage years and early adulthood. In one study the mean age of onset of symptoms was 21 years old but it can occur at any age, in either gender, and in any race. Another study in the UK found a prevalence of 2.2% of individuals in the study meeting criteria for Visual Snow, with a mean age of 50.6 years which is considerably higher than previously thought.   
Idiopathic. Theorized to be caused by thalamo-cortical dysrhythmia but may be associated with persistent migraine with aura or as a feature of hallucinogenic persisting perception disorder. There are ongoing trials to study, and further confirm, the cortical origin and thalamo-cortico dysrhythmia as cause of visual snow using MEG (magnetic encephalography testing). Subjects included controls, migraineurs and patients with visual snow. Initial results showed that patients with visual snow have both cortical hyperexcitability and loss of inhibition of visual processing in V1 (confirming thalamo-cortico dysrhythmia), different from migraine alone. Ongoing study, Jenny L. Hepschke MD presenter NANOS 2020.
Although typically isolated and idiopathic several clinical disorders have been reported to occur with visual snow including stress, nonspecific anxiety, dyslexia, autism spectrum disorder, migraine with aura, or the use of recreational and prescription hallucinogenic drugs. Visual snow may occur even years after hallucinogenic drug use, and even after only one time use. There may be a family history of visual snow or migraine.  Investigations into scotopic sensitivity syndrome have identified similar features in ADHD, dyslexia, and chronic fatigue syndrome. All of these conditions have been shown to share anomalies in lipid metabolism, particularly with allelic variants of the APOB gene. 
The etiology for visual snow is unknown. Some authors believe that is a form of visual processing error of sensitivity or gain but specific visual pathway lesions or a localized neurotransmitter imbalance in the brain parenchyma have not been proven. No structural lesion had been documented on cranial CT or MRI in visual snow., but a current study has proven changes of alpha and gamma waves in MEG. Ongoing study, Jenny L. Hepschke MD presenter NANOS 2020.
The exact pathophysiology of visual snow is unknown but recent studies are on its way to publication that identify neuronal activity through Magnetic Encephalography testing and are able to differentiate between control, patients with migraine and visual snow . It has been hypothesized that there may be faulty signaling processing in the thalamus, afferent signal of the parietal lobe, or the prefrontal lobe of the cerebral cortex. In addition, hyperexcitation of primary and secondary visual cortices, as well as increased saliency of normally ignored subcortical activity may also contribute to the pathogenesis. BOLD fMRI in patients with visual snow show reduced activations to visual stimuli in the anterior insular cortices, as well as increased lactate concentrations in the right lingual cortex. This finding supports the idea that dysfunction of the insular cortex in Visual Snow could be the cause of a decreased saliency threshold to insignificant stimuli. No structural abnormality has been found in CT and MRI brain studies from patients with visual snow. Visual pathway defects are unlikely since visual snow appears all throughout the visual field and is not confined to a definite axonal pathway. 
There is no proven prevention or treatment for visual snow. As migraine with aura is a relatively common comorbidity to visual snow, early diagnosis and treatment of migraine with aura may help prevent the development of visual snow, although the two are separate disease processes and no study has documented migraine treatment decreasing the occurrence of visual snow.
The diagnosis of visual snow is a clinical one made based on a history, and normal results from the funduscopic exam and a head CT or MRI to rule out other diagnoses Proposed diagnostic criteria for Visual Snow are as follows: dynamic, constant, tiny dots in the visual field for at least three months. The presence of two additional visual symptoms of palinopsia (after image or trailing of moving objects), enhanced ectopic phenomena (excessive floaters, excessive blue field entoptic phenomena, self-light of the eye, or spontaneous photopsia), photophobia, nyctalopia (impaired night vision). The symptoms must not be those of a typical migraine with visual aura and not explained by another disorder 
The table below summarizes the criteria for the definition of visual snow syndrome:
Criteria for the definition of the visual snow syndrome according to Puledda et al 
A. Visual snow: dynamic, continuous, tiny dots in the entire visual field lasting >3 mo.
- The dots are usually black/gray on white background and gray/white on black background; however, they can also be transparent, white flashing, or colored.
B. 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.
- Afterimages should be different from retinal afterimages, which occur only when staring at a high-contrast Image and are in complementary color.
- (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, or spontaneous photopsia.
- Entoptic phenomena arise from the structure of the visual system itself. The blue field entoptic phenomenon is described as uncountable little gray/ white/black dots or rings shooting over the visual field in both eyes when looking at homogeneous bright surfaces such as the blue sky; self-light of the eye is described as colored waves or clouds when closing the eyes in the dark; spontaneous photopsia is characterized by bright flashes of light.
- (iii) Photophobia.
- (iv) Nyctalopia.
C. Symptoms are not consistent with typical migraine visual aura.
D. Symptoms are not better explained by another disorder.
- Normal ophthalmology tests (best corrected visual acuity, dilated fundus examination, visual field, and electroretinogram); not caused by previous intake of psychotropic drugs.
The differential diagnosis for Visual Snow includes: ectopic blue field phenomenon, persistent visual migraine aura, eye floaters, posterior vitreous detachment, retinal detachment, dyslexia. It is important to distinguish visual snow from other diagnoses, particularly from migraine with aura. A detailed history can distinguish between visual snow and migraine with aura, depending on the frequency and description of the visual changes. Visual snow is constant and specifically has tiny, flickering dots in the visual field, where a patient with migraine with aura will have visual changes that are not constant and will vary in description from that of visual snow. A thorough funduscopic examination will be able to determine if the patient has a posterior vitreous or retinal detachment perceived by the patient as “floaters”. Dyslexia patients will also have trouble reading, writing, and concentrating, the visual change they experience will be described differently, most commonly as letter switching. Entoptic blue field phenomenon patients will describe their visual disturbance as black dots with a white tail, seen best against a clear blue sky or another blue background. However, these patients see only a few dots at a time and will not see them over their point of focus, since it is caused by white blood cells infiltrating the capillaries of the retina except the foveola. 
The physical examination in Visual Snow patients is unremarkable
There are no signs associated with Visual Snow.
The symptoms for visual snow include both visual and non visual symptoms. Visual symptoms include uncountable, tiny, flickering dots in the visual field, photophobia, visual distortions, contrast problems, decreased clear visual field, decreased depth perception, and palinopsia (prolonged after images). Nonvisual symptoms include trouble concentrating, headaches, migraines, irritability, lethargy, and tinnitus.
Diagnostic Procedures/Laboratory Tests
Head CT and MRI are commonly ordered to rule out other causes of the visual disturbance, although they are normal appearing in patients affected by visual snow. The theorized changes in brain parenchyma or neurotransmitters are thought to be extremely localized, and thus too small to perceive in any imaging studies. FDG-PET and VBM (voxel based morphometry) may show hypermetabolism and increased cortical volume in the extrastriate visual cortex at the junction of the right lingual and fusiform gyrus. There are also structural and functional alterations in the occipital lobe supporting the idea that the visual symptoms might be associated with a disturbance in the visual association cortex. 
The use of dull colored paper, avoiding bright reading lights, and using a bookmark to decrease line skipping can help aid in reading and writing. Tinted glasses lenses have been reported anecdotally to decrease the effects of visual snow, particularly the FL-41 lenses.  
Lamotrigine, nortryptiline, carbamazepine, naproxen, and sertraline have been reported to decrease symptoms of Visual Snow, although they each carry notable side effects and should be reserved for select patients.  Lamotrigine led to partial remission of symptoms in 19.6% of patients in one study, but gave considerable side effects in over 50% of the same patients.  The use of pain medication, anti-epileptics, and migraine prophylaxis have not been shown to consistently improve the symptoms of visual snow.
There are currently no surgical options for visual snow.
While visual snow is not usually progressive, it is not known to disappear. Affected patients typically have chronic and recurrent symptoms but some spontaneously remit or respond to empiric anti-migraine or anti-seizure treatments.
- Here is a website that describes a patient with multiple co-morbidities, including http://www.hale.ndo.co.uk/index.htm
- View simulations of visual snow in this link https://visionsimulations.com/visual-snow.htm
- Fulton, James T., Processes in Biological Vision, Vision Concepts, Corona Del Mar, CA. USA, Aug 2000. Available on the Internet Vision Concepts
- Puledda, Francesca, et al. “Insular and Occipital Changes in Visual Snow Syndrome: a BOLD FMRI and MRS Study.” Annals of Clinical and Translational Neurology, vol. 7, no. 3, 2020, pp. 296–306., doi:10.1002/acn3.50986.
- Kondziella, D. et al. Prevalence of visual snow syndrome in the UK. Eur. J. Neurol. https://doi.org/ 10.1111/ene.14150 (2020)
- Puledda, Francesca, et al. “Visual Snow Syndrome.” Neurology, vol. 94, no. 6, 2020, doi:10.1212/wnl.0000000000008909.
- Schankin, Christoph J, et al., Visual snow: a disorder distinct from persistent migraine aura. A Journal of Neurology, vol. 137, no. 5, May 2014, pp. 1419-1428.
- Loe, Stephen J., and Kenneth Watson. “A Prospective Genetic Marker of the Visual-Perception Disorder Meares-Irlen Syndrome.” Perceptual and Motor Skills, vol. 114, no. 3, 2012, pp. 870–882., doi:10.2466/184.108.40.206.pms.114.3.870-882.
- Hepschke JL. Is Visual Snow a thalamo-cortical dysrhythmia of the visual processing system – a magnetoencephalogram study. North American Neuro Ophthalmology Society annual meeting. Amelia Island, FL March 2020.
- Schankin, Christoph J. et al., Persistent and Repetitive Visual Disturbances in Migraine, A Review. Headache: The Journal of Head and Face Pain, vol. 57, no. 1, July 2016, p.1–16., doi:10.1111/head.12946
- Hale, Alison. My World is not Your World,Revealing Autism, Dyslexic, Scotopic Sensitivity and Asperger Syndrome, 2017 Link
- Axon Optics. What is FL-41. 2017 https://www.axonoptics.com/what-is-fl41/
- Traber, Ghislaine L., et al. “Visual Snow Syndrome.” Current Opinion in Neurology, vol. 33, no. 1, 2020, pp. 74–78, doi:10.1097/wco.0000000000000768
- Optics, Axon. Visual Snow Guide. Axon Optics, Mar 2017 Visual Snow Guide