Timothy C. Hain, MD
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Optokinetic nystagmus, or OKN for short, is the eye movement elicited by the tracking of a moving field. It differs from smooth pursuit which is the eye movement elicited by tracking of a single distinct target. As moving fields contain within them distinct targets, OKN generally contains within it smooth pursuit. As a consequence, usually OKN performance (gain -- ratio of eye tracking velocity to target velocity), exceeds that of smooth pursuit.
|Example of OKN being elicited within a rotatory chair enclosure.|
There are many ways to elicit OKN. The stimulus that most closely matches the definition of OKN is to use a full field surround such as is shown above and below. Even these are compromises as individual bars can be tracked, and there are usually distinct objects in the visual field that can be fixated. Accordingly, the "best" OKN stimulus is real, physical surround that rotates, containing a pattern which has no distinguishing features such as a random dot pattern.
It is not easy to build a good OKN full field stimulator and clinicians as well as commercial vendors have attempted to use other devices.
ENG vendors often sell a "light bar", consisting of an array of LED lights that can be controlled in such a way to produce moving bars of light. This stimulus is clearly not a full field, contains easily tracked lights, and mainly is an alternative method of eliciting smooth pursuit. In the author's opinion, this type of OKN has no diagnostic value over simple pursuit.
Another commercial variant is to use a video-projector to display a moving field on a portion of a screen. This method is a little better, as an appropriate pattern may have no individually trackable portions, but still it does not produce a full field and because of this is vulnerable to persons fixating on the edges of the projected image. In the author's opinion, this method of eliciting OKN is of unproven value but worth considering.
Clinicians sometimes may try to elicit OKN using simpler devices such as the drum below. This device, not even pretending to be a full field, is even more of a "pursuit" stimulus than the larger drum methods shown above. Still, it can occasionally be useful in evoking convergence retraction nystagmus. In the author's opinion, this type of drum very little utility.
"OKN" can also be elicited by strips that contain squares of alternating colors, tape measures, and even busy ties. The usefulness of these devices, in the author's opinion, is questionable.
OKN is much less useful than is rotatory chair testing as it is rarely affected substantially by disease.
Optokinetic afternystagmus (OKAN) describes the eye movements that occur after the lights are turned out for OKN, and the subject is in complete darkness. The darkness must be 100% -- small light leaks will obliterate OKAN.
OKAN is only elicited by large field stimuli. It can be easily elicited by the complete surround methods shown above.
|Time constant of OKAN in normal subjects||Peak slow-phase velocity of OKAN in normal subjects|
OKAN is more sensitive to disease than OKN, but it is variable in normal subjects, which limits its usefulness. The figures above show the distribution of normal values for OKAN time-constant and slow-phase velocity from Tijssen et al (1989).
|Directional Preponderance (DP) of OKAN in normal subjects|
Optokinetic testing does not actually involve a rotating chair -- instead a large pattern is rotated around the subject. OKN is much less useful than is rotatory chair testing as it is rarely affected substantially by disease.
Optokinetic afternystagmus (OKAN) describes the eye movements that occur after the lights are turned out for OKN, and the subject is in complete darkness. OKAN is more sensitive to disease than OKN, but it is variable in normal subjects, which again limits its usefulness.
In VVI, a person is rotated with a visual surround or target also present. The most useful variant of this is to have a person look at (fixate) a laser that is fixed to the rotatory chair. VVI is generally a good index of ones CNS's ability to suppress nystagmus, and thus it is a measure of cerebellar and brainstem function.