NORD Gratefully acknowledges Dr. Francesca Puledda and Professor Peter James Goadsby, Headache Group, Department of Basic and Clinical Neuroscience, King’s College London, and NIHR-Wellcome Trust King’s Clinical Research Facility, King’s College Hospital, London, UK, for the preparation of this report.
Visual snow is a neurological disorder characterized by a continuous visual disturbance that occupies the entire visual field and is described as tiny flickering dots that resemble the noise of a detuned analogue television. In addition to the static, or “snow”, affected individuals can experience additional visual symptoms such as visual images that persist or recur after the image has been removed (palinopsia); sensitivity to light (photophobia); visual effects originating from within the eye itself (entoptic phenomena) and impaired night vision (nyctalopia).
The prevalence of visual snow in the general population is currently unknown. The average age of the visual snow population seems to be younger than for many other neurological disorders. This early onset, combined with a general lack of recognition by health care providers, suggest it is an uncommon problem.
Research has been limited because of 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.
Visual snow is a chronic condition, sometimes highly disabling, uncommon condition that is in need of collaborative research and lateral thinking to make progress towards understanding, treatment and cure.
Since its first description (1), the introduction of the term visual snow (2) and its formal clinical definition less than five years ago (3), visual snow is now being recognized by physicians and scientists as a new entity in the neurological world. The first literature reports of visual snow mostly represented isolated clinical descriptions in the context of larger groups of patients affected by persistent visual disturbance, previously defined as “persistent positive visual phenomena” (1).
Visual snow has been misdiagnosed in case series being mixed with persistent migraine aura, which has led to mechanistic confusion, diagnostic imprecision (4) and certainly the use of treatments that have not been useful (5). Visual snow has been considered to be the same condition as hallucinogen persisting perception disorder (HPPD) (6). Although it seems hallucinogens can trigger a similar disturbance (7), it is clear visual snow syndrome can be entirely independent of drug triggers. Lastly, many patients are told they simply are normal. Admixing these issues has delayed recognition of the syndrome.
The main clinical feature of the syndrome described consistently by patients is an unremitting, positive visual phenomena, present in the entire visual field and characterized by uncountable tiny flickering dots interposed between the person’s vision and the background. This ‘static’ is typically black and white but can also be colored, flashing or transparent.
In addition to the static, or snow, patients can experience additional visual symptoms of either direct neurological origin, such as palinopsia (either in the form of preserved images from stationary scenes or as visual trailing), photophobia and nyctalopia, or the entoptic phenomena. The latter constitutes a group of symptoms considered to arise from the optic apparatus, which in the case of visual snow is completely intact and undamaged. Entoptic phenomena that are found (either alone or in combination) in visual snow, are the blue field entoptic phenomenon, floaters (the perception of which is defined as myodesopsia), self-light of the eye and spontaneous photopsia.
Up to 75% of individuals with visual snow report at least three of these four ancillary visual phenomena, which along with the static itself form the “visual snow syndrome” (3). It is important for researchers and clinicians to distinguish visual snow from other phenomena and to recognize the associated symptoms (8). Most of these can in fact be experienced by healthy individuals (especially in the case of floaters or retinal afterimages), or by patients with ophthalmological diseases; the key difference with visual snow is that they manifest in a recurring, debilitating and pervasive manner and in the context of a perfectly functional optic apparatus.
The causes of visual snow syndrome are currently unknown. Some key features of the syndrome however, point to a neurological disorder of visual processing in the brain cortex. This is mostly due to the characteristic of the chief symptom of the syndrome (i.e. the visual static) which is a whole-field visual disturbance; this makes a localization of the problem in the visual pathway or primary visual cortex extremely unlikely. Furthermore, additional symptoms such as palinopsia, which can be considered an inability to suppress the just-seen, and the enhanced entoptic phenomena in the context of normal ophthalmological tests, also point in the same direction of a central neurological disorder of the visual pathway, from causes yet to be determined.
A neuroimaging study using [18F]-FDG PET seems to have confirmed these hypotheses. The study demonstrated, in patients affected by visual snow, a hypermetabolism of the lingual gyrus (9); this is an area of the visual cortex involved in several other conditions such as photophobia. The lingual gyrus is also a key element of complex physiological functions such as visual memory, perception of color and identification of facial expressions.
Visual cortical hyperexcitability (10, 11) and thalamo-cortical dysrhythmia (12) have also been hypothesized as possible causes for the pathophysiology underlying visual snow.
Further studies on larger numbers of patients are needed to confirm these initial hypotheses.
It is currently unknown how many patients suffer from visual snow worldwide. The available data tells us that there is possibly a higher prevalence of the disease in the male population and that the average age of affected subjects is relatively young (13).
The onset of symptoms can be in very early life, with most people presenting symptoms for their entire lifetime. There is also a proportion of subjects who have a sudden and unpredictable onset of the disease; this occasionally, but not necessarily, follows an identifiable cause.
Visual Snow is a clinical diagnosis that comes from the fulfillment of a set of criteria and the exclusion of secondary causes of similar visual disturbances, such as underlying ophthalmological and neurological diseases.
Following the systematic characterization of 78 patients with visual snow (3), these criteria have been delineated as follows:
A. Visual snow: dynamic, continuous, tiny dots in the entire visual field lasting longer than 3 months (the dots are usually black/grey on white background and grey/white on black background; they can also be transparent, white flashing or colored).
B. Presence of at least two additional visual symptoms of the four following categories:
i. Palinopsia. At least one of the following: afterimages or trailing of moving objects (After images 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 one 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 grey/white/black dots or rings shooting over 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 (sensitivity to light).
iv. Nyctalopia (impaired night vision).
C. Symptoms are not consistent with typical migraine visual aura (as defined by the International Headache Society in the International Classification of Headache Disorders- currently (17).
D. Symptoms are not better explained by another disorder (Normal ophthalmology tests; not caused by previous intake of psychotropic drugs).
The lack of knowledge on the basic biology of the visual snow syndrome has caused a general deficiency of effective treatment strategies for most patients. No clinical and systematic trials have been performed to date, and all available data on treatment comes from single patients or case reports. The current evidence seems to show that commonly used medications such as migraine preventives, antidepressants or pain medication do not consistently improve or worsen visual snow. There have been single positive experiences with drugs such as lamotrigine (18), which need to be contemplated within their low level of evidence.
Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. Government funding, and some supported by private industry, are posted on this government web site.
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Visual Snow Initiative: Conference on May 8, 2018. https://visualsnowconference.com Accessed May 23, 2018.
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