Nystagmus (nye-STAG-muss) refers to rapid rhythmic back-and-forth involuntary eye movements, usually side to side, rarely in the vertical plane. A very mild degree of nystagmus called micronystagmus is present normally in everyone. The eyes normally flutter back and forth very rapidly; the amplitude of the swings is very small, and cannot ordinarily be seen. This occurs so that the visual receptors in the retina can complete their refresh cycle after discharge. If this micronystagmus ever stops for an instant, you will see a strange tree-like pattern before your eyes which represents the blood vessels on the surface of the retina.

Visible, abnormal nystagmus may be caused by abnormality of any one of the three basic mechanisms that regulate position and movement of the eyes: fixation (focusing on and tracking objects), conjugate gaze (keeping the eyes parallel so that the images coincide), or vestibular mechanisms (the balance organs). This last is easily demonstrated: spin around and around until you feel dizzy, then observe the temporary nystagmus of your own eyes caused by the vestibular stimulation of spinning.

Nystagmus can sometimes be more pronounced and thus visible, or even disconcerting. Nystagmus can occur benignly with no evidence of any significant medical condition. The blind have a particular type of nystagmus, described as "roving." It is disconcerting for some to see, and is one of the reasons the blind may wear dark glasses. A newborn baby with visible roving about of the eyes or jerking nystagmus should definitely get immediate ophthalmologic attention, because this could indicate blindness or congenital cataract. Other causes of nystagmus include drug toxicity, notably some anti-seizure medications such as Dilantin® (phenytoin). It may also herald serious neurological disease, to include brain tumor. Obviously, any new onset of nystagmus should be promptly evaluated.

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