How to perform the Velocity Step Test on the Rotary Chair

This video by Jack Bennett - International Clinical Trainer at the Interacoustics Academy - describes the clinical process, interpretation and clinical utility of the Velocity Step Test on the Rotary Chair.

You can read the full transcript below.

What is the Velocity Step Test?

The velocity step test is a test of vestibular ocular reflex on the rotary chair. It is performed in darkness. So with the goggles on, and the patient is given a mental alerting task. In this case listing boys names for each letter.

How to perform the Velocity Step Test

The patient is accelerated up to a fixed velocity, where they will be spun for 60 seconds. And then at the same rate, they will decelerate back to still, and again will be recorded for 60 seconds. This will happen in one direction and then the other direction. 

Initially, up to a slow velocity. In this case, 50 degrees per second. And then later, up to a faster velocity, which will be 180 degrees per second. The first 60 seconds are known as "per rotation" or during the rotation and then the patient will stop and we begin the "post rotation" portion of the test.

You can see the nystagmus is significant as soon as the patient stops.

What does the Velocity Step Test measure?

While the acceleration occurs, nystagmus is measured, the peak slow phase velocity is recorded, and the time constant is measured. The time constant is the amount of time it takes for the nystagmus to reduce by 63% or to 37% of that peak slow phase velocity. The time constant is also measured for the post rotation period. Again, this is the amount of time it takes for the nystagmus to reduce from peak slow phase velocity to 37% of peak or reduced by 63%.

How to perform the Velocity Step Test continued

The patient is now rotated in the other direction with the same acceleration and deceleration and at the same velocity. This is so that the time constants from both sides can be compared, giving a time constant asymmetry measurement. Having completed rotations in both directions at the lower velocity, we now accelerate the patient to a higher velocity.

By now accelerating the patient to a higher velocity, we are saturating the response from the cupula. This means it is generating maximal neural stimulation. By doing this, the contribution from the inhibitory side is negligible. And therefore, we can have a good understanding of each ear individually and how they are generating nystagmus responses to rotational acceleration.

This way, the velocity step test can significantly improve our confidence in stating unilateral vestibular deficit.

How to interpret Velocity Step Test results

Having accelerated the patient to a much higher speed, when the patient now comes to rest, you can see the slow phase velocity that is generated peaks at a much higher velocity than in the slower velocity test. Although nystagmus is still generated for a longer amount of time than in the low velocity test, we are still using the time constant criteria of reduction to 37% of peak slow phase velocity or a reduction of 63%.

When measurements are completed at a slow and high velocity in both directions, we can remove the goggles from the patient and begin to review the results. Although the step velocity test is the oldest rotary chair test, it is worth noting that ordinarily, it is only measured if indicated by the sinusoidal harmonic acceleration test.

When reviewing the results, we have three main parameters:

• Gain as a percentage
• Time constant
• Time constant symmetry

1. Gain

The gain assesses whether the amount of nystagmus produced was proportionate to the amount of acceleration. This gives a gross understanding of the vestibular function. You can see that the normative data is quite broad. There is a low sensitivity for any deficits in gain percentage measurements.

2. Time constant

The most diagnostically significant parameter in the velocity step test is the time constant which, again, is the amount of time it takes for the slow phase velocity of the nystagmus to decrease to 37% of its peak velocity. Reduced time constants may be associated with unilateral or bilateral vestibular pathology, or even central vestibular involvement. It has also been suggested that abnormally long time constants may be associated with motion intolerance or central vestibular pathology.

3. Time constant symmetry

Abnormalities in the time constant symmetry can be useful to determine the weaker side in unilateral vestibular pathology. This is particularly true with the higher velocity step test. Edits can be made to the nystagmus recorded using the edits tools tab. And the time constant for each condition can also be adjusted by moving the slider. This is to suit the needs of different clinics and different normative data.

Summary

By measuring the vestibular system time constant, you can evaluate both the peripheral vestibular response to the rotational stimulus as well as the central velocity storage mechanism. This makes the velocity step test a useful test for aiding in the diagnosis of a variety of vestibular disorders.