Visual Snow

From EyeWiki


Disease Entity

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.1

Disease

Alternate names for Visual Snow are Scotopic Sensitivity Syndrome, Meares-Irlen Syndrome, Persistent Visual Phenomena, Visual Stress, Visual Static, and Asfedia.

Epidemiology

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.1

Etiology

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. 5


Risk Factors

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. There may be a family history of visual snow or migraine.5

General Pathology

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 has been documented on cranial CT or MRI in visual snow.1

Pathophysiology

The exact pathophysiology of visual snow is unknown. 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. 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. 1

Primary prevention

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.5


Diagnosis

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 diagnoses1 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 entopic 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.4 There is however no gold standard test for visual snow.

Differential Diagnosis

The differential diagnosis for Visual Snow includes: ectopic blue field phenomenon, persistent visual migraine aura, eye floaters, posterior vitreous detachment, retinal detachment, dyslexia.1 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. 1


Physical examination

The physical examination in Visual Snow patients is unremarkable.4

Signs

There are no signs associated with Visual Snow.

Symptoms

The symptoms for visual snow include uncountable, tiny, flickering dots in the visual field, photophobia, visual distortions, contrast problems, decreased clear visual field, decreased depth perception, trouble concentrating, headaches, migraines, palinopsia (prolonged after images), and tinnitus.5

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 may show hypermetabolism in the supplemental visual cortex (lingual gyrus).4


General treatment

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.2, 6

Medical therapy

Lamotrigine, nortryptiline, carbamazepine, naproxen, and sertraline have been reported to decrease symptoms of Visual Snow.4 The use of pain medication, anti-epileptics, and migraine prophylaxis have not been shown to consistently improve the symptoms of visual snow.5

Surgery

There are currently no surgical options for visual snow.

Prognosis

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.1

Additional Resources

Here is a website that describes a patient with multiple co-morbidities, including [// www.hale.ndo.co.uk/index.htm] View simulations of visual snow in this link [https:// visionsimulations.com/visual-snow.htm]

References

1. Fulton, James T., Processes in Biological Vision, Vision Concepts, Corona Del Mar, CA. USA, Aug 2000. Available on the Internet [// www.neuronresearch.net Vision Concepts]

2. Hale, Alison. My World is not Your World,Revealing Autism, Dyslexic, Scotopic Sensitivity and Asperger Syndrome, 2017 [http:// www.hale.ndo.co.uk/index.htm Link]

3. Optics, Axon. Visual Snow Guide. Axon Optics, Mar 2017 [https:// www.axonoptics.com/2017/03/visual-snow-guide/ Visual Snow Guide]

4. 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, pp. 1–16., doi:10.1111/head.12946.

5. 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.

6. Axon Optics. What is FL-41. 2017 https:// www.axonoptics.com/what-is-fl41/