In conclusion, our results suggest that VSS is characterised by a complex disturbance in the interaction of multiple brain systems. This dysfunction particularly involves the pre-cortical and cortical visual pathway, the visual motion network, the attentional networks and finally the salience network; further, it does not depend on the activity state of the brain. What we observed suggests that there is a disruption in the filtering and integration of incoming sensory visual stimuli, versus the modulation of internally generated visual information. Future studies will help determine if this fingerprint of altered network dysfunction, indeed represents the main pathophysiological mechanism underlying the symptoms of this complex and disabling condition. In particular, it will be important to distinguish the impact of concomitant migraine, photophobia and aura on these findings, especially with regard to changes in connectivity affecting the visual system.
Given that no widely applicable treatment seems to fully suppress the symptoms of VSS, achieving an improved understanding of its underlying neurobiology is important, as it aids clinicians in explaining the condition to affected patients and also directs future research to more targeted approaches. In particular, the finding that diffuse brain networks are implicated in the genesis of VSS, could have the clinical implication of redirecting treatment from generic pharmacological interventions to a more focused modulation of brain function, possibly through techniques such as neuromodulation or neurofeedback.

onlinelibrary.wiley.com/doi/10.1002/hbm.25343

ConclusionsIn conclusion, patients with VSS present increased activation in a wide network of intrinsic brain areas that are key in the processing of complex sensory and cognitive states. The fact that rCBF increases were independent of the presence of an external visual stimulus, suggests that these abnormalities could be a causal factor of the disorder.
This study expands on previous neuroimaging findings, confirms VSS to be a complex brain problem, and helps to improve our understanding of a condition for which treatment is still lacking.

jnnp.bmj.com/content/early/2021/07/13/jnnp-2020-325881

Results(1) Of 120 patients with “visual snow,” 70 patients also had migraine and 37 had typical migraine aura. Having comorbid migraine was associated with an increased likelihood of having palinopsia (odds ratio [OR] 2.8; P = .04 for “afterimages” and OR 2.6; P = .01 for “trailing”), spontaneous photopsia (OR 2.9; P = .004), photophobia (OR 3.2; P = .005), nyctalopia (OR 2.7; P = .01), and tinnitus (OR 2.9; P = .006). Typical migraine aura was associated with an increased likelihood of spontaneous photopsia (OR 2.4; P = .04). (2) After adjusting for typical migraine aura, comparison of 17 “visual snow” patients with 17 age and gender matched controls showed brain hypermetabolism in the right lingual gyrus (Montreal Neurological Institute coordinates 16-78-5; kE = 101; ZE = 3.41; P < .001) and the left cerebellar anterior lobe adjacent to the left lingual gyrus (Montreal Neurological Institute coordinates -12-62-9; kE = 152; ZE = 3.28; P = .001).
Conclusions—Comorbid migraine aggravates the clinical phenotype of the “visual snow” syndrome by worsening some of the additional visual symptoms and tinnitus. This might bias studies on “visual snow” by migraineurs offering study participation more likely than non-migraineurs due to a more severe clinical presentation. The independence of entoptic phenomena from comorbid migraine indicates “visual snow” is the main determinant. The hypermetabolic lingual gyrus confirms a brain dysfunction in patients with “visual snow.” The metabolic pattern differs from interictal migraine with some similarities to migrainous photophobia. The findings support the view that “visual snow,” migraine, and typical migraine aura are distinct syndromes with shared pathophysiological mechanisms that need to be addressed in order to develop rational treatment strategies for this disabling condition.

headachejournal.onlinelibrary.wiley.com/doi/full/10.1111/head.12378?fbclid=IwAR0I8gU1CZe2nBbdovCQsl1Lxob3GHjCQ7rYnEMKvM-Qeguhmip45XuSsu8

ResultsWe found reduced BOLD responses to the visual stimulus with respect to baseline in VS patients compared to controls, in the left (k = 291; P = 0.025; peak MNI coordinate [-34 12 -6]) and right (k = 100; P = 0.003; peak MNI coordinate [44 14 -2]) anterior insula. Our spectroscopy analysis revealed a significant increase in lactate concentrations in patients with respect to controls (0.66 ± 0.9 mmol/L vs. 0.07 ± 0.2 mmol/L; P < 0.001) in the right lingual gyrus. In this area, there was a significant negative correlation between lactate concentrations and BOLD responses to visual stimulation (P = 0.004; r = −0.42), which was dependent on belonging to the patient group.

onlinelibrary.wiley.com/doi/full/10.1002/acn3.50986?fbclid=IwAR3e5p3JGgl1mfwgbVOFrEe0y63oLpSrqYuQwgaWJnSE3HveG3Ek-CoTK4Q

The prevalence of visual snow syndrome in the UK is
1.4%–3.3%. Visual snow syndrome is associated with
tinnitus and, possibly, to a lesser degree with mood
impairment and headache. It is of note that unprimed
laypeople with visual snow syndrome in the general
population are on average older than those seeking
medical attention and, perhaps, less concerned.
Although prospective follow-up data are needed,
visual snow syndrome is very unlikely to be a progres-
sive disorder. We suggest that reassuring information
can be given to those seeking medical attention for
visual snow syndrome.

onlinelibrary.wiley.com/doi/epdf/10.1111/ene.14150

From the data presented here, almost all patients with ‘visual snow’ have a variety of additional visual symptoms (palinopsia, enhanced entoptic phenomena, photophobia, and nyctalopia), which do not sound like typical migraine aura at all. Visual snow therefore represents a unique clinical syndrome. Our data acknowledge an overlap of migraine and visual snow but do not support the hypothesis that migraine attacks or individual episodes of migraine aura ‘cause’ visual snow. Our data do not support a view the visual snow syndrome is caused by anxiety, depression or the intake of illicit drugs, such as LSD. Remarkably, most patients with visual snow have normal best corrected visual acuity, perimetry and fundoscopy. Any association with visual loss or acute onset of visual symptoms similar to visual snow, especially floaters and photopsia, would therefore require appropriate assessment by a specialist before calling it ‘visual snow’.
We would define the ‘visual snow’ syndrome by the presence of visual snow as the main criterion, with some additional visual criteria, and exclusion of migraine aura, and overlapping diseases, such as ophthalmological pathology or intake of psychotropic drugs (Table 4).

academic.oup.com/brain/article/137/5/1419/334357

Results: The visual snow group demonstrated reduced center-surround contrast suppression (p = 0.03) and elevated luminance increment thresholds in noise (p = 0.02). Groups did not differ on the global form or global motion task.
Conclusion: Our study demonstrates that visual perceptual measures involving the suprathreshold processing of contrast and luminance are abnormal in a group of individuals with visual snow. Our data are consistent with elevated excitability in primary visual cortex; however, further research is required to provide more direct evidence for this proposed mechanism. The ability to measure perceptual differences in visual snow reveals promise for the future development of clinical tests to assist in visual snow diagnosis and possibly a method for quantitatively assaying any benefits of treatments.

n.neurology.org/content/88/13/1243

Results From 2007 to 2018, 58 patients were diagnosed with visual snow. Comorbid migraine was present in 51.7% of patients, lifetime depression in 41.4%, and lifetime anxiety in 44.8%. Lamotrigine was prescribed most frequently (26/58) and resulted in partial remission of symptoms in 5/26 (19.2%). No patients reported complete remission. Adverse events occurred in 13/26 (50.0%) patients. None of the other prescribed drugs (valproate [n = 7], topiramate [n = 4], acetazolamide [n = 2], flunarizine [n = 1]) led to improvement except for topiramate in one patient, who discontinued, however, because of adverse events.

Conclusions Of medication prescribed (lamotrigine, valproate, acetazolamide, flunarizine), only lamotrigine afforded some improvement in a small minority of patients. Migraine, depression, anxiety, and tinnitus were common comorbid diseases.

n.neurology.org/content/93/4/e398

We identified objective and quantifiable measures of visual processing changes in patients with VSS. The absence of any additional switch cost on the AS-PS task in VSS suggests that the PS latency and AS error differences are attributable to a speeded PS response rather than to impaired executive processes more commonly implicated in poorer AS performance. We propose that this combination of latency and error deficits, in conjunction with intact switching performance, will provide a VS behavioral signature that contributes to our understanding of VSS and may assist in determining the efficacy of therapeutic interventions.

n.neurology.org/content/95/13/e1784