In the video, you will learn about the core fundamentals of rotational chair testing. We will cover the two most commonly performed tests in the rotational chair: the sinusoidal harmonic acceleration (SHA) test and the velocity step test. It will cover the test theory and interpretation as well as frequently asked questions. To guide you through this video, we have the pleasure of learning from the true expert of rotational chairs, Dr. Chris Zalewski.
Find some of the burning questions following the webinar below.
For me – and it's echoed within the literature – rotational testing makes for a very nice first line defense. It provides a very low frequency response and a high frequency response. This provides a nice, informative, broad frequency response to the vestibular system.
I generally do rotational chair testing first for all my patients in so much that if I have a normal 0.01 Hz, low frequency SHA response, chances of finding a caloric abnormality are slim. This has been documented in the literature.
So, if I have a pretty normal response in the low frequencies, I don't feel compelled to do calorics, which we know most of our patients don't enjoy anyway. If I record a high frequency normal response, chances are the vHIT is going to be somewhat normal as well.
So, I consider it a very nice first investigative clinical assessment of our vestibular system. I do that for most of my patients, particularly those who are pediatric, as they get a little shied away from the caloric sometimes.
I don't think you necessarily have to test them all. I do because I'm in a research facility and want that that broad spectrum response. But I do think it's important to at least have one within the velocity storage mechanism range, which is 0.01 or 0.02 Hz.
Ideally, I think 0.01 Hz is an ideal frequency, because neighboring frequencies tend to have highly correlated responses. The very low frequency response 0.01 Hz can also be highly correlated to the caloric.
If you wanted to use the chair as kind of a first clinical investigation, I would strongly recommend 0.01 Hz, because it can give you a lot of information as to whether or not you maybe even need to do caloric irrigations. I would include a mid-frequency response probably around 0.8 Hz or 0.16 Hz and then certainly, I think, a good valid high frequency at 0.64 Hz. If I had to limit it to three, I'd pick the two ends of the spectrum and then one in the middle.
If we compare vestibular loss to hearing loss then unlike presbycusis which traditionally impacts the higher frequencies for hearing, vestibulopathy mainly affects low frequencies first. Reduced gain at these frequencies is likely due to the weak stimuli that the vestibular system receives with these types of head movement.
The stimulus must incorporate and provide enough afferent drive to drive the VOR. Here we need to consider the cupula displacement model. The slower you move the head, the weaker the displacement of the cupula. This means weaker afferent drive and reduced velocity storage time and energy to work with to generate an appropriate VOR. That's commensurate with your head movement. So, the lower stimulus of head movements is generally impacted first for providing an effective VOR for that system.
So, your vestibular rehabilitation exercises are going to increase your gain through the compensation process that we should all generally go through. But you must remember that your abnormal phase is always a consequence of the repaired gain.
You basically must sacrifice phase if you want to have effective compensation of gain. You can go the other way around. You could probably fix phase, but you're never really going to compensate gain. And for me – at least I think for most of our patients – they would probably rather have compensated gain.
I don't think phase is going to be a very intuitive measure to measure productivity of vestibular rehab. You certainly can measure that through a VOR gain response as it returns to normal. But I think you're always going to see that permanent phase lead in your traditional model.
Then as far as using it to compensate the vestibular system, a lot of VOR exercises depend on your vision. Since vestibular testing and the rotational chair is done in the dark, it's probably not going to be as effective as you would find any other vestibular compensation process that you would do with a visual fixation target. I'm not sure use of the chair is going to be as effective in that process of returning gain to within-normal limits.
It does help us. Think of the scenario, you are hanging out in a circle with your friends, and you’re just on your feet, moving extremely slowly. It can be difficult for the vestibular system to process those extremely slow head movements as the cupulas aren't generating a very strong, robust afferent drive.
So, we need help in producing and processing those really slow head movements and that's where velocity storage comes in. Granted, the visual system is also helping out as well, but the velocity storage mechanism was solely designed to help augment really low frequency head movements in order to generate and process an appropriate VOR response.
So, we don't really need it for high frequency head movement information or when we process daily life activities. But when we have velocity storage of phase abnormalities for higher frequencies, where we don't necessarily expect them, it could suggest the central pathology where you're recruiting velocity storage for head frequency movements, where you really don't need it.
In 20 years, I've never had someone produce any emesis on my chair, or caloric. You have to realize that the healthier the stimulus system, the more they're going to respond. I also run a 300-degree step test for research investigative purposes looking at a variety of different step increments to see how the vestibular system responds across an array of stimuli.
But I do find the 240-degree step test to be quite tolerable for patients. In fact, I would even go as far to say a lot of kids love it. I find that the most noxious response that I generally hear from patients is with SHA low frequency 0.01, 0.02 Hz. The vestibular system does not really appreciate low frequency stimuli. It was never really designed to sense those really low frequency movements efficiently and effectively, so it often produces some degree of nausea. But I do find that most patients tolerate it well.
It's all related to math. There's no magic here. A linear regression is rise over run. But the decay of nystagmus is not linear, it's nonlinear. So, when you want to find the rise over run for a nonlinear regression, it's the inverse of the log e and "e” is Euler’s numbers. It's the inverse of a logarithmic function, which is 37%.
If the time constant is not at least 10 seconds, we would consider that an abnormal time constant for a 60-degree step test. So, my limits are generally: the lower limit is about 10 seconds, and the upper limit is anywhere from about 32 to 33 seconds.
I do not look at time constants currently for the 240-degree step test. One reason for that is because the robust stimulus is so fast that the cupula displacement is so great that the VOR output is so high. Therefore, the system has a hard time kind of holding on to that VOR output level and the system just wants to dump that.
So, you end up having these time constants on 240-degree step tests that are kind of where cupula mechanics are below at about eight seconds or so, where traditionally on a 60-degree step test we'd call that abnormal. So, time constants on a 240-degree step test are generally awfully fast and not very clinically informative, simply because that's a lot of velocity afferent drive stored energy to hold on to. The system generally just wants to dump it as fast as it can.
We've seen that in my clinic, yes. The short time constants are fairly solid with a peripheral or a central dysfunction as we noted before. But for those patients who have a really long time constant – that is, the propagation of the VOR is really hyper extended – this would generally signify a central problem with the over storage or the lack of dispelling of the VOR over a standard normal period of time.
Migraine is actually a fairly decent example of that. Migraine inhibits or disrupts the way our central system generally processes stimuli and can give that prolonged effect. Migraine patients often report prolonged dizziness, following things like amusement park rides, or car rides or things of that nature.
Not every migraine patient will have an extended VOR time constant, and I usually use about 30 to 33 seconds for the cut off for that. But if you do have a patient that has time constants at around 35 seconds or longer, it's generally something that I would associate with a central pathology and certainly migraine would fall within that.
I personally don't like to do testing when BPPV is active. BPPV is an intermittent disorder, it becomes active and is a hyperactive response on the side that is affected. Rotational chair is a dynamic test, and I know that BPPV is most commonly occurring in the posterior canal and during rotational testing, we're impacting the horizontal canal.
But I have seen – even in caloric irrigations – where you have abnormal results on someone with the active BPPV. I would much rather clear that before assessing what the horizontal canal is doing just in case there's central compensation mechanisms occurring in response to that chronic, active BPPV.
It's a long answer, but there's no real straightforward answer with the exception that a preference of mine is to clear the BPPV first.
There's two ways to answer this: on a 60-degree step test or a 240-degree step test.
If you're doing a high frequency, 240-degree step test, it's probably a more tolerable stimulus than the caloric, and it certainly is faster, and it can give you laterality information. Particularly when you have a patient that's probably not going to be very amenable to caloric testing. So, I like the 240-degree step test in that regard.
If I wanted to argue for a 60-degree step test, I think it offers a better stimulus to investigate velocity storage mechanisms than SHA and phase. Your phase in SHA – although it's a fairly stable parameter – is impacted by gain. Gain can be impacted by a number of patient factors such as fatigue or mental suppression.
During the step test protocol, it is a really good stimulus to produce that propagated VOR response that can give really good insight to velocity storage mechanisms, which becomes really important. Velocity storage is not only important because it offers our ability to augment low frequency processing of low frequency information, but it's also integral to compensation mechanisms as well. So, it can give insight into that as well so that becomes important as well.
But I'd be a big proponent of any clinic that wanted to institute 60- and 240-degree step testing within their clinic for those reasons.
I don't really think any one vestibular test should be used to assess a patient. It should be a variety. So, although I've been stressing rotational testing as a good first line clinical assessment measure, it should probably never be used alone.
We all tend to do things a little bit different in the vestibular world, particularly when it comes to VEMP, vHIT, and things of that nature. Our norms aren't necessarily really stratified very well between age groups either. So, acquiring your own norms and in relative to how you're performing each test in the clinic is probably extremely important.