VHIT conflict between other vestibular tests
This material is intended for clinicians and vestibular scientists. See also: HIT test
Timothy C. Hain, MD Page last modified: December 17, 2016
The VHIT is an instrumented technique used to diagnose reduction in vestibular function in one ear vs. the other, or bilateral loss. An examiner abruptly accelerates and then decellerates the head, moving the head in rapidly at high speed and then stopping it. The VHIT is currently popular, somewhat less expensive than alternatives, and the more enthusiastic users claim that it can replace the older repertoire of vestibular testing, such as for example, rotatory chair testing and caloric testing. If this conjecture were true, then the VHIT should supply the same data as the other two.
Some very enthusiastic articles about VHIT have been published. MacDougall, Weber, McGarvie and Halmagyi (2009), from the Australian group that commercialized a variant of the VHIT, studied 8 patients with vestibular lesions, and reported that it was 100% sensitive and 100% specific. The elusive perfect vestibular test !
This page provides some examples and discussion considering why this is not always the case.
Weber et al (2009) reported that VHIT "detects gentamicin ototoxicity". The implication is that VHIT detects bilateral vestibular paresis. Here we mean to point out that this is sometimes untrue.
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.
r-chair VHIT conflict 3 Case 3. Near complete loss on rotatory chair testing, and complete loss on caloric. Much better performance on VHIT.
Case 3 had bilateral sequential vestibular neuritis. Both the caloric test and Rotatory Chair test document severe weakness. The VHIT test however, produces a gain of about 0.66, which falsely suggests that the patient has nearly intact vestibular function. This is almost certainly due to relative preservation of high frequency gain. The VHIT is wrong.
r-chair VHIT conflict -- VHIT worse than R-chair and HSN/Vibration.
Case 4. Near complete loss on rotatory chair testing, and complete loss on caloric. HSN is weak, and vibration nystagmus is absent. VHIT shows very severe L loss, more so than would seem possible with unilateral loss.
Case 4 had an apparent left sided vestibular neuritis, but with some symptoms as well on the right. In particular, a weak BPPV was seen on the right. Vibration and HSN nystagmus are not in keeping with 100% loss as suggested by VHIT and ENG. This may be a situation where there is a partial bilateral, and the patient has chosen to "optimize" for good VOR gain to the right, and just given up on left. It is not entirely clear what is going on here, but perhaps different patients can choose different compensation strategies.
All three of these vestibular tests -- vhit, rotatory chair, caloric, supposedly measure vestibular function -- so what do you do when they disagree ?
VHIT normal, Rotatory chair abnormal.
This is a fairly common pattern because the VHIT measures high frequencies ALONE, the rotatory chair measures high and low frequencies, and the caloric low frequencies alone. Generally speaking, the low frequencies are more sensitive than the highs, and thus the VHIT can fail more easily than the rotatory chair.
In partial bilateral loss, the rotatory chair will be abnormal at low frequencies, but the VHIT can wrongly suggest that vestibular function is normal. Thus the VHIT creates false NEGATIVES.
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. So in persons with saccadic problems (usually rare cerebellar patients or PSP), the Rchair can be false POSITIVE. This is a very extremely rare situation.
Rotatory chair normal, VHIT abnormal.
This is usually an VHIT artifact -- i.e. Rotatory chair is correct. The second example above illustrates a situation where the VHIT suggests complete loss but the rotatory chair, partial loss. The rotatory chair has a lot more data to work with than the VHIT, and the VHIT is vulnerable to technical artifact, especially goggle slippage, poor head movement technique, failure to calibrate, looking at the wrong place.
VENG normal, VHIT 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.