Training in VNG
Performing saccade testing (VNG)
The next test in the VNG test battery is the saccade test. In the saccade test, we're going to do ask the patient to follow a target, which is moving randomly. That random movement can be horizontal, vertical, or a combination of horizontal and vertical movements. Today, we're going to do horizontal eye movements and horizontal saccades.
How to instruct your patient
Tess, we're going to measure your saccade system. You're going to see the target on the screen, and that target is going to jump around in a horizontal fashion. I want you to look at the target. And as soon as it moves, move to the next target or move to where the target is on the screen.
I want you to keep your head still and only move your eyes when looking at the target. Also, please don't anticipate where the target is going to come. So don't try and guess. Just wait for the target to move. But when it does, move as quickly as you possibly can.
Any questions before we begin? Fantastic. Okay, so let's start the test.
Test procedure
If you can look at the center dot for me, please Tess. And here we go. So this saccade test lasts around 45 seconds. We need to look at several different eye movements of different sizes to assess the saccade system. On the screen, we can see a yellow line representing the target. And the blue and red lines represent the left and right eyes, respectively.
So you're doing really well Tess, only 20 seconds to go. As the test continues, we can see that data is being extracted and displayed on the right-hand side of the screen. We can see latency, velocity, and accuracy. So we can start to make these interpretations whilst the test is running.
Now that the test is completed, we can now see that we come into the test analysis screen. We have the latency, velocity, and accuracy of the eye movements all displayed. We see a red diamond in the velocity trace. This tells us that the velocity trace on this occasion was outside of the threshold limits set within the system. Therefore, we would want to repeat the test to see if that abnormality is truly an abnormality, or if there was some error during the recording.
Abnormal saccade test
What we can see in the results is that when the patient is looking leftwards, we can see a reduction in the velocity and an increase in the latency. However, when they're looking in both directions, we can see that they're demonstrating hypermetric saccades on the left eye. This is another indication of disconjugate eye movements.
So that concludes the saccade test and we're now ready to move on to the next test within our VNG test battery.
Description
The saccade test (a test of the central vestibular system) assesses the patient's ability to move their eyes rapidly, from one focal point to another. As this is a random saccade test, the eye movements are reflexive rather than voluntary.
You can test in both horizontal and vertical directions. Abnormalities can include:
1) velocity of eye movements, either too slow or too fast
2) accuracy of eye movements, either overshooting or undershooting
3) latency of eye movements (reaction time), usually this is only a long latency as abnormally short latencies are most likely due to the patient anticipating the movement
The results can provide very specific site of lesion information. This video shows a horizontal saccade test being performed on a patient.
Presenter
