ENG and VNG testing in persons who can't see
Timothy C. Hain, MD
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ENG/VNG testing requires well functioning eyes and is greatly affected by vision.
In persons who can't see at all, there is almost nothing to be gained by doing an ENG. Instead, it makes much more sense to do a VEMP test, as it does not require vision.
In persons who have limited vision, one must interpret results based on their type of visual disorder. It is important to measure visual acuity and document it in your report !
If the person uses contacts - -there is no difficulty. If they use glasses for distance vision, then after the glasses are removed for testing, the person may be (nearly) blind ! Blind people can't track.
In persons who can't see out of one eye, they usually can't keep the other eye "calibrated". Measurements in the blind eye are useless.
Here, subjects can't see eccentric targets and may need to search for them, causing the appearance of ocular dysmetria as well as long latency.
An interesting example of this is shown in the saccadic trace below, from a patient with Retinitis Pigmentosa. Persons with RP eventually develop tunnel vision. Thus, when the dot is moved for the saccade paradigm, a person with RP has to find it, as in essence, it has simply disappeared.
|Extremely dysmetric saccades in person with retinitis pigmentosa (RP), which is an ocular disorder which impairs peripheral vision. Latency is long, saccades both under and overshoot.|
Because central vision is spared in RP, smooth pursuit may be normal. This is illustrated on the page that discusses tracking disorders.
Subjects can't fixate well, especially on small targets.