This material is intended for clinicians and vestibular scientists. See also: bedside version -- HIT test
Timothy C. Hain, MD Page last modified: December 17, 2016
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", defined as horizontal only, can be done in about 12 minutes. We would expect a test that included the other 2 plans would take perhaps another 10 minutes, for a total of about 22 min. 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.
MacDougall et al (2016) recently reported a variant VHIT test where patients were asked to view a laser dot that moved with their head. This protocol then asks patients to turn OFF their VOR, rather than to use it. The authors state that "anticompensatory" saccades are elicited in normal controls, but are less commonly found in persons who have less VOR to suppress. This makes sense. Our thought however is that this paradigm adds another variable to the mix -- how well people can suppress their VOR, and that it is inferior to the ordinary HIT protocol. We think that this procedure adds noise to the HIT test.
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.
Covert saccades in patient with complete vestibular loss on R.
More about VHIT and compensation is here. The ability to detect "covert" saccades, as illustrated above, 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. Or to say this a different way, you don't need the VHIT test for acute dizziness -- the HIT test does perfectly well. 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.
Bottom line regarding what is VHIT good for:
At this writing (9/2016), 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.
Backup saccades ?
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.
Meniere's disease ?
One would think that fluctuating gain in Meniere's disease might be easily detected with the VHIT. Theoretically, gain should go up in hydrops, and drop after hydrops is resolved (i.e. during an attack). In this regard, Lee et al (2016) reported widely variable results in 14 patients during attacks of MD. They tested all 6 canals in many patients, providing a profusion of results. The gain was either normal (67%), decreased (19%)or increased (14%). Results were not very different towards the good ear either. On the other hand, caloric responses were decreased in 64% towards the "bad" ear.
While at first look, this report suggests VHIT is not an effective method in a Meniere's attack, we don't think the story is over here. Looking for a change in vestibular function, rather than an absolute value might be more effective. Additionally, using the vertical canal VHIT may have diluted the data with random error, as vertical canal VHIT techology is primative (lacking torsional eye movement processing).
|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 - -e.g. loss of half of vestibular function.
The VHIT test just tests the high-frequency VOR. It doesn't test the entire spectrum. Would you test someone's color vision using just "blue", and ignore red and green ? Unsurprisingly then, the VHIT can fail in partial lesions.
Normal VHIT in man with acoustic for about 20 years Large acoustic neurinoma in man with normal VHIT.
We have encountered patients with large acoustic neurinoma's, with absolutely normal VHIT tests. This man has only a small amount of clinical findings -- and in spite of this substantial size mass (clearly a tumor). The images above show that VHIT is not a "screening" test for acoustic neurinomas (sadly).
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.
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. While the ICS device is easier to secure to the head, certain types of faces seem to result in left-right asymmetry in VOR gain.
For all devices, the head has to be moved in the right direction, right speed, and with little wobble at the end. One cannot hold onto the goggles by their 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.
When one does not move the head with high acceleration, the VHIT test becomes less sensitive. Weber et al (2008) reported that as accelerations increased from 750 to 6000 deg/sec**2, ipsilesional gains decreased greatly, increasing sensivitiy. This is very logical, as if one doesn't do the test fast enough, it shouldn't work. Of course, to do the test with the highest sensitivity, one must turn the head with the highest acceleration, which could lead to safety issues.
We are simply unconvinced that the oblique VHIT testing is useful. Without measuring the full vector of eye movement, i.e. torsion, there are simply too many things that can go wrong.
The Interacoustic and GN-otometrics VHIT devices differ substantially. The GN-otometric 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 heavy goggle limits the acceleration that one can apply.
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. It does not replace standard testing (i.e. calorics and R-chair) as it has no low-frequency data and also does not isolate the stimulus to one ear (such as does calorics).