Fixation-off sensitivity (FOS) is a phenomenon induced by elimination of central vision/fixation, and may either manifest clinically with seizures or only represent an EEG abnormality. FOS is characterized by posterior or generalized epileptiform discharges that consistently occur after closing of the eyes and last as long as the eyes are closed. It is most commonly encountered in patients with idiopathic childhood occipital epilepsies, but may also be observed in cases of symptomatic or cryptogenic focal and generalized epilepsies, as well as in asymptomatic non-epileptic individuals. FOS should be differentiated from pure forms of scotosensitivity, in which EEG discharges or epileptic seizures are elicited by darkness, and from epileptiform discharges triggered by eye closure, which refer to eye closure sensitivity. Although FOS is probably associated with occipital hyperexcitability its intrinsic epileptogenic potential is presumed to be low.
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