White matter abnormalities in a patient with visual snow syndrome: New evidence from a diffusion tensor imaging study

Visual snow syndrome and migraine: a review

Visual snow syndrome is a neurological condition characterized by ongoing prominent phenomena described consistently as tiny dots moving across the entire visual field, often associated with complex visual symptoms. These can take the form of afterimages, entoptic phenomena, nyctalopia and light sensitivity. Although some of these symptoms can be benign, they can nonetheless become significantly impactful for many who experience them, particularly in cases that have a sudden and abrupt start. As visual snow syndrome becomes increasingly recognized in clinical practice we begin to learn about its typical presentation and underlying pathophysiology. Treatment of visual snow, however, still proves quite challenging, and efforts need to be focused on unravelling the biological mechanisms of the syndrome. This endeavour has characterized the most recent research on visual snow, mostly involving neuroimaging, neurophysiological and neurobehavioral studies aimed at understanding its underlying neural signature. Another important aspect of the syndrome, which will likely prove critical in deepening our understanding of visual snow, is represented by the intricate biological and historical connexion with migraine. This narrative review focused on visual snow syndrome will explore its clinical, pathophysiological and treatment aspects in detail.

Visual Snow: A Review on Pathophysiology and Treatment

OBJECTIVE

Visual snow (VS) is a rare clinical entity in neuro-ophthalmology. It is described as the presence of flickering dots affecting the whole visual field, often compared by patients to snow or pixelated television static. Importantly, it can be an alarming symptom for many patients, lowering their quality of life. Our purpose is to increase awareness of this disease, because many healthcare professionals have difficulty identifying symptoms as the nature of the condition is subjective. In this review, we aimed to describe the updates in the etiology and treatment of visual snow. We searched for articles in English, presenting original data and published after December 2019. Different studies show inconsistent data. Neuroimaging studies found, among other things, hypermetabolism of the lingual gyrus, increased gray matter in different brain areas, and altered connectivity in visual pathways. However, these findings were not present in all patients. According to the literature, among the most effective drugs is lamotrigine. Unfortunately, it also carries a risk of worsening the symptoms. It is crucial to remember that VS can be worsened or induced by alcohol, recreational drugs, and certain medication. In terms of treatment, nonpharmacological approaches such as color filters and repetitive Transcranial Magnetic Stimulation were also used.

INTERPRETATION

Further studies are needed to understand the nature of VS fully. Even though the pathophysiology and effective treatment of the condition remains unknown, expanding the knowledge about visual snow can impact the comfort of patients.

Visual Snow: Updates on Pathology

Purpose of Review

Until the last 5 years, there was very little in the literature about the phenomenon now known as visual snow syndrome. This review will examine the current thinking on the pathology of visual snow and how that thinking has evolved.

Recent Findings

While migraine is a common comorbidity to visual snow syndrome, evidence points to these conditions being distinct clinical entities, with some overlapping pathophysiological processes. There is increasing structural and functional evidence that visual snow syndrome is due to a widespread cortical dysfunction. Cortical hyperexcitability coupled with changes in thalamocortical pathways and higher-level salience network controls have all shown differences in patients with visual snow syndrome compared to controls.

Summary

Further work is needed to clarify the exact mechanisms of visual snow syndrome. Until that time, treatment options will remain limited. Clinicians having a clearer understanding of the basis for visual snow syndrome can appropriately discuss the diagnosis with their patients and steer them towards appropriate management options.

Cognitive Improvement via Left Angular Gyrus-Navigated Repetitive Transcranial Magnetic Stimulation Inducing the Neuroplasticity of Thalamic System in Amnesic Mild Cognitive Impairment Patients

Background: Stimulating superficial brain regions highly associated with the hippocampus by repetitive transcranial magnetic stimulation (rTMS) may improve memory of Alzheimer’s disease (AD) spectrum patients. Objective: We recruited 16 amnesic mild cognitive impairment (aMCI) and 6 AD patients in the study. All the patients were stimulated to the left angular gyrus, which was confirmed a strong link to the hippocampus through neuroimaging studies, by the neuro-navigated rTMS for four weeks. Methods: Automated fiber quantification using diffusion tensor imaging metrics and graph theory analysis on functional network were employed to detect the neuroplasticity of brain networks. Results: After neuro-navigated rTMS intervention, the episodic memory of aMCI patients and Montreal Cognitive Assessment score of two groups were significantly improved. Increased FA values of right anterior thalamic radiation among aMCI patients, while decreased functional network properties of thalamus subregions were observed, whereas similar changes not found in AD patients. It is worth noting that the improvement of cognition was associated with the neuroplasticity of thalamic system. Conclusion: We speculated that the rTMS intervention targeting left angular gyrus may be served as a strategy to improve cognitive impairment at the early stage of AD patients, supporting by the neuroplasticity of thalamic system.

Refined Analysis of Chronic White Matter Changes after Traumatic Brain Injury and Repeated Sports-Related Concussions: Of Use in Targeted Rehabilitative Approaches?

Traumatic brain injury (TBI) or repeated sport-related concussions (rSRC) may lead to long-term memory impairment. Diffusion tensor imaging (DTI) is helpful to reveal global white matter damage but may underestimate focal abnormalities. We investigated the distribution of post-injury regional white matter changes after TBI and rSRC. Six patients with moderate/severe TBI, and 12 athletes with rSRC were included ≥6 months post-injury, and 10 (age-matched) healthy controls (HC) were analyzed. The Repeatable Battery for the Assessment of Neuropsychological Status was performed at the time of DTI. Major white matter pathways were tracked using q-space diffeomorphic reconstruction and analyzed for global and regional changes with a controlled false discovery rate. TBI patients displayed multiple classic white matter injuries compared with HC (p < 0.01). At the regional white matter analysis, the left frontal aslant tract, anterior thalamic radiation, and the genu of the corpus callosum displayed focal changes in both groups compared with HC but with different trends. Both TBI and rSRC displayed worse memory performance compared with HC (p < 0.05). While global analysis of DTI-based parameters did not reveal common abnormalities in TBI and rSRC, abnormalities to the fronto-thalamic network were observed in both groups using regional analysis of the white matter pathways. These results may be valuable to tailor individualized rehabilitative approaches for post-injury cognitive impairment in both TBI and rSRC patients.

Can diffusion tensor imaging (DTI) outperform standard magnetic resonance imaging (MRI) investigations in post-COVID-19 autoimmune encephalitis?

BACKGROUND

Neurological and psychiatric manifestations related to severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection are widely recognised. Standard magnetic resonance imaging (MRI) investigations are normal in 40-80% of symptomatic patients, eventually delaying appropriate treatment when MRI is unrevealing any structural changes. The aim of this study is to investigate white matter abnormalities during an early stage of post-COVID-19 (coronavirus disease 2019) encephalitis while conventional MRI was normal.

METHODS

A patient with post-COVID-19 autoimmune encephalitis was investigated by serial MRIs and diffusion tensor imaging (DTI). Ten healthy control individuals (HC) were utilised as a control group for the DTI analysis. Major projection, commissural and association white matter pathways were reconstructed, and multiple diffusion parameters were analysed and then compared to the HC average using a z-test for serial examinations.

RESULTS

Eleven days after the onset of neurological symptoms, DTI revealed early white matter changes, compared with HC, when standard MRI was normal. On day 68, DTI showed multiple white matter lesions compared with HC, visible at this time also by the MRI images, indicating inflammatory changes in different association and projection white matter pathways.

CONCLUSION

We confirm a limitation in the sensitivity of conventional MRI at the acute setting of post-COVID-19 autoimmune encephalitis. A complementary DTI investigation could be a valuable diagnostic tool in early therapeutic decisions concerning COVID-19-related neurological symptoms.

Delayed Onset of Inhibition of Return in Visual Snow Syndrome

Visual snow syndrome (VSS) is a complex, sensory processing disorder. We have previously shown that visual processing changes manifest in significantly faster eye movements toward a suddenly appearing visual stimulus and difficulty inhibiting an eye movement toward a non-target visual stimulus. We propose that these changes reflect poor attentional control and occur whether attention is directed exogenously by a suddenly appearing event, or endogenously as a function of manipulating expectation surrounding an upcoming event. Irrespective of how attention is captured, competing facilitatory and inhibitory processes prioritise sensory information that is important to us, filtering out that which is irrelevant. A well-known feature of this conflict is the alteration to behaviour that accompanies variation in the temporal relationship between competing sensory events that manipulate facilitatory and inhibitory processes. A classic example of this is the “Inhibition of Return” (IOR) phenomenon that describes the relative slowing of a response to a validly cued location compared to invalidly cued location with longer cue/target intervals. This study explored temporal changes in the allocation of attention using an ocular motor version of Posner's IOR paradigm, manipulating attention exogenously by varying the temporal relationship between a non-predictive visual cue and target stimulus. Forty participants with VSS (20 with migraine) and 20 controls participated. Saccades were generated to both validly cued and invalidly cued targets with 67, 150, 300, and 500 ms cue/target intervals. VSS participants demonstrated delayed onset of IOR. Unlike controls, who exhibited IOR with 300 and 500 ms cue/target intervals, VSS participants only exhibited IOR with 500 ms cue/target intervals. These findings provide further evidence that attention is impacted in VSS, manifesting in a distinct saccadic behavioural profile, and delayed onset of IOR. Whether IOR is perceived as the build-up of an inhibitory bias against returning attention to an already inspected location or a consequence of a stronger attentional orienting response elicited by the cue, our results are consistent with the proposal that in VSS, a shift of attention elicits a stronger increase in saccade-related activity than healthy controls. This work provides a more refined saccadic behavioural profile of VSS that can be interrogated further using sophisticated neuroimaging techniques and may, in combination with other saccadic markers, be used to monitor the efficacy of any future treatments.

Visual Snow Syndrome as a Network Disorder: A Systematic Review

Aim: By reviewing the existing clinical studies about visual snow (VS) as a symptom or as part of visual snow syndrome (VSS), we aim at improving our understanding of VSS being a network disorder. Background: Patients with VSS suffer from a continuous visual disturbance resembling the view of a badly tuned analog television (i.e., VS) and other visual, as well as non-visual symptoms. These symptoms can persist over years and often strongly impact the quality of life. The exact prevalence is still unknown, but up to 2.2% of the population could be affected. Presently, there is no established treatment, and the underlying pathophysiology is unknown. In recent years, there have been several approaches to identify the brain areas involved and their interplay to explain the complex presentation. Methods: We collected the clinical and paraclinical evidence from the currently published original studies on VS and its syndrome by searching PubMed and Google Scholar for the term visual snow. We included original studies in English or German and excluded all reviews, case reports that did not add new information to the topic of this review, and articles that were not retrievable in PubMed or Google Scholar. We grouped the studies according to the methods that were used. Results: Fifty-three studies were found for this review. In VSS, the clinical spectrum includes additional visual disturbances such as excessive floaters, palinopsia, nyctalopia, photophobia, and entoptic phenomena. There is also an association with other perceptual and affective disorders as well as cognitive symptoms. The studies that have been included in this review demonstrate structural, functional, and metabolic alterations in the primary and/or secondary visual areas of the brain. Beyond that, results indicate a disruption in the pre-cortical visual pathways and large-scale networks including the default mode network and the salience network. Discussion: The combination of the clinical picture and widespread functional and structural alterations in visual and extra-visual areas indicates that the VSS is a network disorder. The involvement of pre-cortical visual structures and attentional networks might result in an impairment of "filtering" and prioritizing stimuli as top-down process with subsequent excessive activation of the visual cortices when exposed to irrelevant external and internal stimuli. Limitations of the existing literature are that not all authors used the ICHD-3 definition of the VSS. Some were referring to the symptom VS, and in many cases, the control groups were not matched for migraine or migraine aura.