Training in VNG
Performing optokinetic testing (VNG)
So the next test in the VNG test battery is the optokinetic test. The optokinetic test allows for the eyes to follow a moving target whilst the head remains still.
How to change the stimulus
Now what I want to do before starting this test is just show you that, depending on which patient you have, you can change the stimulus for the optokinetic test. The most common stimulus that we use with adults is the checkerboard. However, to make things a little bit more interesting, we can also change these to images. This is particularly useful when testing infants and children.
For the purposes of testing Tess, today, we're going to be using the checkerboard.
How to instruct your patient
Tess, you're going to see a stimulus on the screen and it's going to move from one side to another. So the checkerboard is going to be moving over from the left to the right or from the right to the left. What I want you to do is just to look straight at the center of the screen, and just watch the pattern pass by.
Any questions before we begin? Fantastic.
Test procedure
In the optokinetic test, what we're going to do is test two conditions, a slow condition and a fast condition.
Slow condition
If you can just look at the center of the screen for me, and we can go from there. The test will last 15 seconds Tess. Okay, here we go. Counting down from 15. What we can see on the screen is that the target is moving leftwards, 20 degrees per second. And Tess' eyes are moving rightwards in relation to that stimulus.
Once the stimulus stops, we're now ready to now move it in the opposite direction. So what we're going to do is now move the stimulus rightwards, 20 degrees per second. Okay, are you ready Tess? Yes. Here we go. Excellent.
With the optokinetic stimulus it can be quite demanding to look at. So you don't always have to do the test for the entire duration. If you have a sufficient amount of nystagmus, then it's always okay that you can stop a little bit early.
Fast condition
Now that we've done the slow stimulus, what we're going to do now is increase the speed from 20 degrees per second to 40 degrees per second. So let's start that one now. Are you ready Tess? Excellent. Here we go. So, quite a little bit different this time. That stimulus is moving at a much faster speed.
So again, what I'm going to do is I have sufficient data here so I can stop the test. And then give Tess just a couple of seconds to rest before we do the final stimulus. Okay, so let's move into the final screen. And then test. Are you ready for the last stimulus? Yes, excellent. You're doing a great job so far. Here we go. Excellent. And if I stop the test, here, we have sufficient data.
Results screen
What we can see on the screen is both the raw tracings on the bottom and then the gain values at the top. And what we can see is the movements at both 40 degrees and 20 degrees for the left. And for the right. The gray area is the normal threshold data and we can see that Tess' responses are way above that line indicating that Tess has good optokinetic function.
Example of an abnormal optokinetic response
What we can see is that in the low frequencies, the gains are within the normal range. However, we see reduced gains in the higher frequency testing condition. This can be seen sometimes in central patients, patients who have motion sensitivity, or people who are generally older in age.
That concludes the optokinetic test. And now we're ready to move on to the next test within the VNG test battery.
Description
The optokinetic test uses a similar system to the smooth pursuit system and in fact the smooth pursuit system is implicated in this test. The difference is that the stimulus in optokinetic testing should be "full field" meaning that it occupies all of the patient's field of vision.
It can be used a part of a test battery approach to determining the presence of central lesions.
This video demonstrates a patient undergoing optokinetic testing.
Presenter
