Visual Snow Syndrome

Visual snow syndrome (VSS) is a vision disorder that is characterised by the presence of numerous tiny dots that disrupt a patient's field of vision. These dots are dynamic, flickering across the patient’s field of view and are usually black and white but can also be coloured. The presence of these dots is often likened to the static observed on an analogue television set or the “snow” that flutters within a snow globe.

Although in some settings, VSS is considered benign and therefore harmless, recent studies show that the associated burden of disease experienced by some patients with VSS is comparable with most chronic diseases. This disparity can be attributed to the fact that there is very little available information or research on VSS, and as a consequence, VSS is often misdiagnosed, and the treatment approaches are not optimised.

Clinical features & pathophysiology

The lack of awareness regarding VSS poses a major challenge to clinicians, and this is often compounded by the fact that VSS shares pathophysiological characteristics and comorbidities with other visual disorders.

VSS, however, can develop in patients who do not have any underlying ophthalmic (relative to the eye) conditions, and having a preceding retinal pathology is not necessarily considered a trigger for VSS.^1,2 In addition to the snow or static vision, other symptoms that occur in people with VSS are photophobia (light hypersensitivity), palinopsia (repeated after-images of a specific object), entoptic phenomena (floating shapes or streaks across one’s vision), and nyctalopia (poor night vision).. To diagnose VSS, at least two of these secondary symptoms should be present.²

Comorbidities

Patients who suffer from migraines are commonly affected by VSS, as a migraine attack can cause transient visual disturbances.⁴ Tinnitus is another condition that co-presents with VSS. It is also considered a characteristic feature of VSS, as approximately 75% of patients report having experienced this particular comorbidity.² Like VSS, tinnitus affects the visual field of both eyes and is continuous and non-pulsatile. This has led to the theory that both conditions could be caused by a dysfunction in visual processing rather than a disruption to anterior visual pathways.³ This hypothesis may help to explain why patients may experience palinopsia, for example, even when there are no external stimuli (detectable chemical or physical changes that can trigger other changes) to cause it.²

Mimics

While comorbidities like migraine and tinnitus illustrate ophthalmic dysfunction, the compounding nature of VSS means that it is very difficult to attribute specific symptoms to one condition or another. One approach to understanding the mechanism of disease in VSS is to explore other known conditions that mimic or exhibit similar behaviours to VSS. Charles Bonnet Syndrome (CBS) and Hallucinogen Persisting Perception Disorder (HPPD) are two examples of such disorders that may provide useful insight that can increase our understanding of VSS.

CBS occurs in patients who develop loss of vision due to glaucoma or age-related macular degeneration. Its symptoms manifest as palinopsia or hallucinations of light as flashes or dots.³ This visual dysfunction is mainly attributed to the destruction of afferent nerves associated with visual processing. The type of hallucination experienced by a patient with CBS depends on the specific area of the brain's deafferented (its afferent connections have been disrupted) cortical region. We do not yet know whether such changes in the brain, particularly within specific areas related to our perception of space, are responsible for the onset of VSS. Further research is still required to determine whether there is a correlation and perhaps an overlap between VSS and CBS disease mechanisms.

HPPD is a visual disorder characterised by recurrent hallucinations in the peripheral vision (the area of your vision that is outside the centre of your object of focus). These hallucinations involve seeing shapes, afterimages or trailing effects.³ Individuals with VSS can experience similar effects. However, an important distinction is the fact that patients with HPPD are usually individuals who have been exposed to recreational drugs like cannabis, psilocybin and ethanol or have undergone anaesthesia.^2,3,5 Another distinction is that people with HPPD do not typically co-present with migraine and tinnitus. In individuals with HPPD, the sensory filtering of external stimuli (external changes detected by the body) is impaired, and our understanding of its disease mechanism may provide insight into how VSS progression occurs.

Epidemiology & demographics

In the UK, approximately 2% of the population live with VSS.³ A similar figure has also been reported in Italy, Australia, Korea, Israel and the United States of America populations.^6,7,9 Amongst this, there is a high prevalence of patients with VSS that co-present with migraines. This ranges between 50% and 80%, with the onset of symptoms reported to be triggered by trauma, particular medication, and the presence of an infection.³ Interestingly, a case of VSS has been reported to have been triggered by a SARS-CoV2 infection.10 There is no known sex prevalence for VSS; therefore, it equally affects both males and females, whereas the average reported age of VSS development in large patient cohort studies is 29.³

Diagnosis & treatment

An article recently published in Current Neurology and Neuroscience Reports proposed several diagnostic standards for VSS:

• The presence of “snow” or “static” in the visual field for at least 3 months.
• The presence of two secondary symptoms: Nyctalopia, photophobia, palinopsia or entoptic phenomena
• Symptoms are distinct from migraine visual aura
• Symptoms cannot be accounted for by any other occurrence, e.g., drug abuse.³

Nonetheless, attaining a correct and accurate diagnosis of VSS still has its own set of challenges in a real-world setting. As mentioned previously, a lack of literature and the shared pathophysiology with other visual disorders means that recognising and correctly diagnosing VSS elevates the difficulties faced by most clinicians working in the fields of neurology and ophthalmology. Ongoing debate and a lack of consensus also serve to further confound this difficulty.1 While there is undoubtedly a long road to cover regarding VSS diagnosis and management, this should not discount the opportunities and possibilities that can be investigated at present. An example of this involves investigating the reasons behind the high comorbidity observed with migraine and tinnitus. The common underlying mechanisms suggest a shared dysfunction in individuals with both conditions. Conducting parallel research on both can provide valuable insights for distinguishing, as well as enhancing our understanding of the unique features associated with each disorder.

There is no known effective treatment for VSS, and as it is nominally considered to be a benign disorder, focus is placed on patient management and reassurance. Although physically benign, the condition can be very distressing for patients, and it is therefore recommended to provide serious consideration towards their mental well-being, as this can significantly affect their overall quality of life.¹¹

Summary

Although considered as not physically harmful, VSS is a clinical disorder that we have very little understanding about. The “static” or “snow” that characteristically appears in a patient's field of view is bilateral (affects both sides of vision). This observation is significant as it can steer the research toward investigating a neurological disruption rather than classifying VSS as retinopathy (disease of the retina). Current and future research on other visual disorders that have some degree of clinical or pathophysiological overlap with VSS, such as migraines, HPPD and CBS, may provide some further insight and increase our understanding of development and progression in patients.

Currently, there is no cure for VSS, and patients are being taught to manage their symptoms. Understandably, living with severe and constant visual interruptions can be highly stressful for an individual and can exert a heavy mental toll. Therefore, significant weighting is given to how both the clinician and the patient approach and manage the patient’s day-to-day life and how they can tackle the challenge of balancing this with their overall well-being.