The VHIT is an instrumented bedside technique used to diagnose reduction in vestibular function in one ear vs. the other.
An examiner abruptly accelerates and then decellerates the head, moving the head in rapidly at high speed and then stopping it.
The graphic above shows a positive VHIT in a patient with unilateral vestibular loss. For head thrusts to the right, head and eye are similar. For head thrusts to the left, there are many large "covert" saccades in the middle of the eye traces. This rather clearly shows a compensated patient who has a complete unilateral vestibular loss. (This device is the Interacoustic VHIT).
The graphic above shows a positive VHIT in a patient with bilateral vestibular loss. For head thrusts to either side the eye is quiet for the first 100 msec, and then there are many large "covert" saccades . This rather clearly shows a compensated patient who has a complete bilateral vestibular loss. (This device is the Interacoustic VHIT).
It should be said that in neither of these patients was the VHIT used to establish the diagnosis. The diagnosis was already well known from other tests (VENG and rotatory chair).
The VHIT test is a commercial version of the HIT test (head impulse test) using sophisticated eye tracking and head velocity transducers. This is presently the most useful version of the HIT test, mainly because it can detect covert saccades.
The VHIT test users are growing very rapidly worldwide, except in the USA where adoption has been greatly limited by the lack of a health insurance mechanism to bill for the test. In our opinion, the VHIT is obviously a useful diagnostic device. We think that the time to do a VHIT and the utility of the test is roughly the same as a caloric test. We are presently dubious that anything but the "horizontal" test has much diagnostic utility. If/when a billing code is developed, we hope that the description of the procedure is narrow enough to prevent fraudulent billing for "fake" VHIT implementations, such as the field has seen in many other vestibular testing procedures.
Different versions of the VHIT
That being said, there are several commercial versions of the VHIT -- including (at least) one marketed by Interacoustic, one by GN-otometrics (GNO or ICS), one by Micromedical technology, and one by Synapsis. The first three devices all share similar technological features -- an eye camera worn by the patient, a head velocity sensor, and a head-mounted calibration arrangement.
A full "test" can be done in about 12 minutes. We have both the Interacoustic and GNO versions in our clinic, and agree that the test can be done quickly, but due to problems with eye tracking, and the overhead in analyzing, realistically this seems to us to be a 30 minute procedure. This compares poorly to the bedside "HIT" which can be done in 15 seconds.
These devices differ fairly markedly, and so far, little comparison data is available between the devices. Bansal and Sinha (2016), using the ICS device, reported that in normal subjects the mean VOR gain was greater for every test involving the right side compared to the left (2016). We have so far not noticed this in our use of the Interacoustic VHIT, or previous studies using scleral eye coils.
In theory, the VHIT device can be used to assess vertical canal pairs as well as horizontal canals. However, as there is no method of validating the device for vertical canal testing, other than known surgical lesions, it would seem difficult to be sure that the device really works. Additionally, as none of the currently available VHIT deviecs measures torsion, they all use vertical instead, which means that the response could be coming from canals other than what one supposes -- i.e. if you are supposedly moving the head in the plane of RALP, but in reality, the head is moving more in the sagittal plane, you are not really measuring RALP but rather a mix of RALP and LARP. This problem could be solved with a device that monitored torsion -- but so far, this is technically not feasible.
The ability to detect "covert" saccades is what makes the VHIT device better than just doing the HIT test by hand. On the other hand, acutely in unilateral loss, there are no covert saccades and the bedside "HIT" test works very well and also doesn't require one to master a new device. We have observed patients with "covert" saccades as early as 1 week post vestibular neuritis, while on large amounts of meclizine. We have also observed "overt" saccades in patients more than 1 year "out" from unilateral loss. Thus, it would seem that there is a lot to learn about the supposed ability of covert/overt saccades to detect compensation.
Some enthusiasts about the VHIT test, suggest that there is no longer any need to do rotatory chair or VENG tests. This is a naive viewpoint as these older and time-tested vestibular tests provide information that the VHIT lacks. The rotatory chair provides low-frequency information. We have encountered patients with very clear abnormal rotatory chair tests, but very normal VHIT tests. This makes perfect sense from a physiological perspective. This means that the two tests are not identical.
The ENG provides excellent side of lesion information. The VHIT test provides very high frequency information -- part of the whole picture, but it doesn't replace the other vestibular tools.
At this writing (12/2015), we think that the main use of the VHIT device is to determine how well compensated patients are to known unilateral or bilateral vestibular loss. We also see it as a good "tie breaker" when there is disagreement between ENG and rotatory chair. Something to add on, not a replacement.
We think that the initial diagnosis of unilateral vestibular loss will likely be made by the time-tested ENG device in the lab, and by simple clinical signs (vibration, HSN) at the bedside. Bilateral loss will continue to be diagnosed with the rotatory chair, as VHIT can miss well compensated bilateral weakness.
The VHIT could potentially allow clinicians to determine whether persistent symptoms are related to poor compensation.
Abnormal VHIT test with "backup" saccades. Patient with migraine and fluctuating bilateral hearing loss.
Back-up saccades are almost always a central sign, suggesting brain disease rather than ear disease. The reason is that the VOR gain is tightly controlled by the brain. Should one's VOR gain be too high, the brain would rapidly suppress it. Thus back-up saccades should mean that there is a cerebellar disturbance.
Detecting backup saccades pattern requires that one check the velocity profile (as shown above), for spikes of eye velocity that occur during the head thrust, and go in the opposite direction as the VOR. This pattern is seen above. The velocity regression plot (not shown) is normal.
This observation is potentially very important. Small "backup" saccades are still saccades - -they are very fast. The VHIT device can "see" tiny backup saccades that one cannot observe with one's naked eyes. Thus finding "backup" covert saccades should be a unique capability of the VHIT, that cannot be duplicated by either the VENG or Rotatory chair test.
We have observed this pattern in people who don't look where they are instructed. If a normal person is told to look away from the target, many backup saccades are generated. This suggests to us that backup saccades may be a sign of uncooperative patients.
We think that this observation needs more research. We are somewhat doubtful that finding this in one of 25 trials has much meaning, as suggested by Hueberger et al (2015), but it is very much worth checking as the VHIT device can provide data about this potential central sign unavailable through any other method.
- Uncooperative patients not looking at the target. This is an obvious problem as the current system software does not check for eye position prior to head thrusts. This is a flaw.
- Choi et al (2014) observed a similar pattern in two patients with "acute cerebellar dysfunction".
- One would also think that patients with myasthenia gravis, after taking their medication, might also show backup saccades.
- One might also conjecture that in Meniere's disease with hydrops, vestibular function might improve due to larger "pipes" in the inner ear, and result in a transient backup saccade pattern. As Meniere's is usually unilateral, one might expect some asymmetry.
|Normal VHIT test in extremely ataxic patient with downbeating nystagmus -- VHIT doesn't work for this group.|
Central vestibular disorders:
VHIT is not useful for strokes, brain tumors (of anything but the 8th nerve), cerebellar degenerations, nystagmus disorders (such as downbeating nystagmus), Chiari malformations -- basically anything other than unilateral or bilateral vestibular loss. This lack of sensitivity to anything other than unilateral vestibular loss has been used -- basically when people fail the VHIT, they are unlikely to have any of the above. The main potential exception to this general rule is longstanding cerebellar disturbances (see above).
We do think that interpreters of VHIT should be watching out carefully for backup saccades (see above), as this is a central sign similar to overshoot saccadic dysmetria. Stay tuned.
Partial vestibular disorders, especially bilateral.
r-chair VHIT conflict Case 1: Highly abnormal rotatory chair with decreased gain, increased phase and asymmetry -- same person as right Normal VHIT in same person as on left side
r-chair VHIT conflict2 Case 2: Highly abnormal rotatory chair with decreased gain, increased phase and asymmetry -- same person as right Extremely abnormal VHIT in same person as on left side
The two cases above illustrate that results of the VHIT and rotatory chair can be wildly different. This suggests that there is a lot of "play" in the high-frequency VHIT. Much more than in the better established Rotatory chair test. Of course, a gold standard is not available here. We think that when there is doubt, the rotatory chair test should win.
What might be going on here ?
Perhaps explaining case 1, VHIT only detects rather severe unilateral or bilateral vestibular loss. Normal persons can compensate and raise their high frequency gain, sacrificing their low frequencies. As the VHIT does not monitor low frequency responses, someone with as much as a 50% loss of vestibular function, equally distributed on both ears, could go entirely undetected by VHIT. Thus the VHIT would be a bad test to monitor early aminoglycoside ototoxicity. This is illustrated by the first case above.
In case 2, the VHIT appears to be worse than the rotatory chair. It is clear that there were very good head thrusts, and also a large number of covert saccades. Thus this seems to be a situation where the high-frequencies are "out", and the low-frequencies are still present. This particular lady had migraine as well. Perhaps her pathway to compensation involved covert saccades rather than increasing high-frequency VOR gain.
In other words, one might conjecture that people can take different pathways to compensation after a bilateral loss. Some increase the high-frequency gain at the expense of the low-frequency gain. Others use covert saccades instead, and do not adjust their high-frequency gain upward. If this is true, VHIT is shown to be at best, just a partial measure of vestibular function. Rotatory chair covers more ground.
VHIT is also reported to be insensitive, not performing as well as caloric testing (Bell et al, 2015). Of course, the greater specificity of theVHIT test compared to calorics, adds value. Additionally, VHIT and calorics may be sensitive to different conditions.
Mysterious vestibular disorders
VHIT is not a good way to evaluate an unknown source of dizziness, or for that matter as a screening test for dizziness, because it is sensitive to just a few uncommon conditions. A broader test such as VENG or rotatory chair is much more rational.
All three of these vestibular tests supposedly measure vestibular function -- so what do you do when they disagree ?
VHIT normal, Rotatory chair abnormal.
The rotatory chair and VHIT both measure high-frequency VOR, and the VHIT does not measure low-frequency VOR. Thus the rotatory chair can be abnormal at low frequencies, and the VHIT can wrongly suggest that vestibular function is normal.
The rotatory chair is somewhat dependent on having working saccades, while the VHIT is not. While the explanation is complicated, the rotatory chair can be abnormal in persons with no saccadic eye movements, but the VHIT can be normal. Here, the VHIT is (probably) correct.
Rotatory chair normal, VHIT abnormal.
This is usually an VHIT artifact -- i.e. Rotatory chair is correct. The rotatory chair has a lot more data to work with than the VHIT, and the VHIT is vulnerable to technical artifact from time to time.
VENG normal, VHIT very abnormal, Rotatory chair also abnormal.
We have encountered this pattern, and have no good explanation. One would think that it might reflect rapidly fluctuating vestibular function, but we are not aware of a disorder that can go from normal to unilateral loss in a day.
This device looks as if it should be easy, but it can be very difficult to get it right. The frames of the interacoustic device need to be very tight. The head has to be moved in the right direction, right speed, and without any wobble at the end. One cannot hold onto the goggles by thei frame or their straps. Any of these errors make the procedure useless. We think that one should plan to be "trained" for several sessions over a month -- ideally, about 4 1-hour sessions, for most to learn how to do the VHIT.
The Interacoustic and GN-otometrics VHIT devices are developed by two different groups, and differ substantially. The GNotometric VHIT device (ICS) is a goggle that measures the right eye alone. If you have a false right eye, or a droopy right eye, it doesn't work. The Interacoustic device is more adjustable, as the camera can be positioned on either eye. Both devices are somewhat fragile as they both carry a mirror attached to the head, which can be broken.
The Micromedical VHIT uses two cameras placed within the same goggles as their VENG system.
The Synapsis device is very different in that there is nothing attached to the head -- it is perhaps best to test small children as otherwise the performance degradation would be unacceptable. We have not had any direct experience with the Micromedical or Synapsis implementations of the VHIT.
VHIT testing is currently not wide-spread. In Chicago, we offer it at Chicago Dizziness and Hearing. It is both available as part of a new-patient PT evaluation as well as "a la carte". In other cities, it should be possible to find practices that offer it by asking the vendors of these devices.
The VHIT appears to be most useful as part of a rapid battery of bedside tests. (Mandala et al. 2008). It provides unique information about compensation (covert saccades). It ultimately might end up being primarily useful to vestibular physical therapists.