BACKGROUND: Visual snow (VS) is a constant visual disturbance described as 
flickering dots occupying the entire visual field. Recently, it was 
characterized as the defining feature of a VS syndrome (VSS), which includes 
palinopsia, photophobia, photopsias, entoptic phenomena, nyctalopia, and 
tinnitus. Sixty percent of patients with VSS also experience migraine, with or 
without aura. This entity often is considered psychogenic in nature, to the 
detriment of the patient's best interests, but the high frequency of similar 
visual symptoms argues for an organic deficit. The purpose of this review is to 
clarify VSS as a true entity and elaborate the nature of individual symptoms and 
their relationship to each other.
EVIDENCE ACQUISITION: The literature was reviewed with specific regard to the 
clinical presentation and psychophysical, neurophysiological, and functional 
imaging studies in patients with defined visual disturbances that comprise VSS.
RESULTS: Consideration of the individual symptoms suggests that multiple factors 
are potentially involved in the development of VSS, including subcortical 
network malfunction and cortical hyperexcitation. Although there is substantial 
overlap between VSS and migraine syndromes in terms of co-occurring symptoms, 
both neurophysiological and neuroimaging studies provide substantial evidence of 
separate abnormalities of processing, supporting these as separate syndromes.
CONCLUSIONS: VSS is likely associated with either hyperactive visual cortices 
or, alternatively, impaired processing of simultaneous afferent information 
projecting to cortex. VSS likely results from widespread disturbance of sensory 
processing resulting in sensory misperception. There may be a number of 
syndromes associated with impaired sensory processing resulting in sensory 
misperception, including migraine, persistent perceptual postural dizziness, and 
tinnitus, which overlap with VSS. Elucidation of abnormality in one defined 
syndrome may provide a path forward for investigating all.