In Case You Missed It! “Localised Increase in Cerebral Perfusion in Patients with Visual Snow”

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Francesca Puledda, Christoph J Schankin, Owen O’Daly, Dominic Ffytche, Ozan Eren, Nazia Karsan, Steve C R Williams, Fernando Zelaya, Peter J Goadsby

Highlights & Conclusion

“Patients with VSS had higher rCBF than controls over an extensive brain network, including the bilateral cuneus, precuneus, supplementary motor cortex, premotor cortex and posterior cingulate cortex, as well as the left primary auditory cortex, fusiform gyrus and cerebellum. These areas were largely analogous comparing patients either at rest, or when looking at a ‘snow-like’ visual stimulus. This widespread, similar pattern of perfusion differences in either condition suggests a neurophysiological signature of visual snow. Furthermore, right insula rCBF was increased in VSS subjects compared with controls during visual stimulation, reflecting a greater task-related change and suggesting a difference in interoceptive processing with constant perception of altered visual input.

The data suggest VSS patients have marked differences in brain processing of visual stimuli, validating its neurobiological basis.


“We aimed to investigate changes in regional cerebral blood flow (rCBF) using arterial spin labelling (ASL) in patients with visual snow syndrome (VSS), in order to understand more about the underlying neurobiology of the condition, which remains mostly unknown.”


“Visual snow is a neurological condition defined by the presence of a continuous and unremitting visual disturbance in the form of uncountable tiny dots covering the whole visual field.1 Patients affected by visual snow syndrome (VSS) experience a complex array of neurological and visual symptoms in addition to the static itself, such as palinopsia, entoptic phenomena, nyctalopia and photophobia.2 Visual snow represents a spectrum type disorder that at its worse manifests with most of these additional symptoms, as well as with distressing comorbidities such as migraine and tinnitus3; in such cases, the condition is perceived as very disabling.4

Although the pathophysiology of VSS remains largely unknown5 a growing body of literature has started offering some insight on the possible biological mechanisms underlying the condition. Behavioural6 and neurophysiological studies7 8 have shown patterns of changes pointing to increased cortical excitability and visual cortex dysfunction. Through neuroimaging, it has been possible to determine that VSS is characterised by altered metabolism of the extrastriate visual cortex9 10 as well as structural changes involving the visual system, and further expanding beyond it.9 11 12

Arterial spin labelling (ASL) is a quantitative, non-invasive functional MRI technique that has evolved considerably in the last decade13; this method exploits the phenomenon of neuro-vascular coupling to use resting perfusion as an indirect but sensitive marker of neuronal activity.

In this study we investigated intrinsic differences in brain activity in patients with VSS compared with controls, determining differences in regional cerebral blood flow (rCBF) using ASL. Given that VSS is characterised by an abnormal perception, in order to differentiate changes due to altered visual processing from those of the ongoing visual snow effect itself, we studied subjects both at rest and during a visual task that simulated the visual snow experience. We hypothesised those areas in the visual network would be characterised by changes in cerebral blood flow in VSS subjects, indirectly reflecting changes in neuronal activity. A further hypothesis was that these areas would not show particular changes in different states of brain activity, since visual snow is a continuous phenomenon that does not appear to be influenced by external conditions.”

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