The role of occipital cortex hyperexcitability in visual snow syndrome

Visual snow syndrome (VSS) is an emerging clinical entity, mainly characterized by persistent, bilateral, whole-visual field, disturbing, small flickering dots or pixelation, floaters, palinopsia, nyctalopia, photopsia, and photophobia. Patients with VSS also describe associated symptoms such as tinnitus, concentration difficulty, lethargy, depression, anxiety, and irritability, all of which affect the patients' quality of life. The consistency of these visual and nonvisual symptoms has recently led to proposed criteria for VSS. The diagnosis relies on the exclusion of other ophthalmic or neuropsychiatric disorders. Interestingly, many patients may have a comorbid migraine, and the symptoms were previously attributed as a persistent visual phenomenon in migraine. VSS is listed in the International Classification of Headache Disorders-Third Edition Appendix as a complication of migraine; however, VSS is a new disease entity distinct from persistent migraine aura. Some patients with VSS recall exposure to illicit hallucinogenic drugs, amphetamines, cannabis, or Lysergic acid diethylamide (LSD). The stereotypic clinical characteristics of VSS are currently well defined, and the pathophysiology is under investigation. Due to the subjective perceptual descriptions of patients with VSS, objective electrophysiologic parameters and functional brain imaging studies using magnetic resonance imaging and positron emission tomography are warranted for defining the quantifiable and reliable outcome measures. Patients with migraine, idiopathic occipital epilepsies, patients with Alice in Wonderland syndrome, patients with Charles Bonnet syndrome, visual hallucinations in recovery from cortical blindness, and recreational 3,4-methylenedioxymethamphetamine/ecstasy users have been suggested to have occipital cortex hyperexcitability. VSS is attributed to involving a dysfunctional magnocellular pathway, thalamocortical dysrhythmia, dysfunctional central visual processing, and occipital cortex hyperexcitability as possible underlying mechanisms. This review will focus on the role of occipital cortex hyperexcitability in VSS and hopefully provide insight into its pathophysiology and therapeutic strategies.