Timothy C. Hain, M.D.• Last edited on January 30, 2019
The inner ear vestibular system is comprised of linear and angular motion sensors. The linear sensors are called the otoliths, and the angular sensors are called the semicircular canals. There are two otolith organs (utricle and saccule), and there are three canals (horizontal, anterior, and posterior). These motion sensors provide a 6-degree of freedom signal (3 dimensions of angular and linear each), to the brain, from each ear.
Bilateral vestibular damage, in the past, meant that both horizontal semicircular canals were damaged. The other parts of the ear - -2 vertical canals and 2 otoliths, were only quantifiable at autopsy, and not much of that kind of data was available. In recent years however, newer vestibular tests have been developed, some of which claim to measure function of the other 4 sensory organs (i.e. 2 otolith organs, 2 vertical canals).
|Name of test||Claims to measure||Reliability||Limited to|
|Caloric||Horizontal semicircular canals||Moderate, range of normal between 20-100, false positives are common.||Low frequencies, canals|
|Rotatory Chair||Horizontal semicircular canals||Moderate, but both sides together. Device is very expensive.||canals|
|VHIT-horizontal axis||Horizontal semicircular canal/canals||Moderate, high frequencies only||High frequencies, canals|
|VHIT-RALP||Right-anterior/Left Posterior pair||Low, high freq only (because of technical limits on tracking torsion)||High frequencies, canals|
|VHIT-LARP||Left Anterior/Right Posterior pair||Low, high freq only (because of technical limits on tracking torsion)||High frequencies, canals|
|cVEMP||Saccule||Moderate, amplitudes vary widely||Unclear correlation between response and real world function|
|oVEMP||Utricle||Moderate||Unclear correlation between response and real world function|
This larger array of procedures allows us to dream of the holy grail of vestibular diagnosis - -measure all 5 sensory organs ! Sadly, as of 2018, these procedures are not entirely reliable, but they are much better than 25 years ago !
At this writing, only the caloric and rotatory chair tests are well established for diagnosis of bilateral vestibular loss. For example, Sargent et al (1997) used these tests to establish bilateral vestibular loss. So we have a low-frequency test (calorics), and a mixed frequency test (rotatory chair).
The horizontal VHIT is very valuable for diagnosis, but it is less sensitive than the rotatory chair. It misses the low-frequency cases.
The RALP/LARP VHIT tests are not clinically relevant because of their unreliability -- a fundemental technical issue is their lack of ability to measure torsion. They are suitable perhaps for research papers, but not for clinical work.
The cVEMP and oVEMP tests are age dependent -- they do not work at the ages where most bilateral loss occurs. In other words, workers that attempt to establish bilateral vestibular loss get no added information from these studies in persons over the age of 60.
A particularly useful method of diagnosing bilateral loss is with the rotatory chair. I wrote an article about using the "Gain-TC" product to diagnose bilateral loss (Hain et al, 2018). Papers that rely on unreliable measures as listed above are proliferating, but recall that these methods are not reliable.
Some authors suggest that "Cluster analysis" can be used to find patterns in dizzy patients with two sets of unreliable tests - -VHIT and VEMP. (Tarnutzer et al, 2018). In our view, this pathway may lead to some confusion as the reliable tests for bilateral loss (i.e. rotatory chair, caloric) were not included in their analysis.
Caloric testing for bilateral loss
ENG test. Contemporary ENG methodology uses video-ENG, caloric irrigation, and computer control . See here for more details. Absent caloric Responses. A total response of less than 20 is abnormally low.
Caloric testing is much easier to access than rotatory chair testing, being available almost universally, but it is also prone to false positives. While the rotatory chair assesses all frequencies, both high and low, the caloric test only assesses low frequencies (roughly 0.003 Hz). The caloric test is more sensitive than the VHIT test, but it takes more time. It has quite a bit of variabilty as well, which limits its usefulness. The average caloric response is about 100, and thus the criterion of 20 means that someone must lose about 80% of their low frequency response.
Rotatory Chair testing for bilateral vestibular loss.
Note that the rotatory chair test is generally needed to categorize bilateral patients. Caloric testing by itself is not sufficient, as because of the high variability of caloric tests, even absent caloric responses are sometimes encountered in otherwise normal individuals (Furman and Kamerer, 1989). Although recent authors suggest that a limit of 27 deg/sec. is sufficient (Zapala et al, 2008), in our opinion, this limit is not generally applicable because of the wide variation in how testing is performed, and because of the intrinsic variability of ENG responses.
Rotatory Chair ENG caloric responses Mild Increased phase, steeper than normal slope to gain vs. frequency plot. Normal and symmetric. Total response >= 27 when done with water. Moderate Increased phase, steep slope of gain vs frequency plot, gain greater than 0.2 at highest frequencies Total response between 0 and 27 Severe No response at all frequencies except (possibly) highest (0.64 Hz), gain less than 0.4 at 0.64 hz No response to ice water irrigations
(These categories are based on testing done at the author's clinic, and might not be applicable to other protocols at other institutions). Pathologic correlation is minimal for these categories -- but recent data suggests that "severe" losses are associated with roughly an 80% or more loss of hair cells. The "mild" bilateral loss pattern resembles that which occurs when one ear is not working (i.e. 50% loss).
Rotatory chair test The rotatory chair test is the gold standard for diagnosing bilateral vestibular loss. Abnormal rotatory chair test in person with bilateral vestibular loss. There is decreased gain at low frequencies and increased phase at lower frequencies.
The rotatory chair test is useful to document the characteristic reduced responses to motion of both ears and also in assessing compensation and the partial recovery that nearly always occurs over time. However, the VHIT test (more recent) is better at assessing recovery and compensation.
The rotatory chair test assesses both high and low frequencies. It is the only "full spectrum" vestibular test -- the alternatives just cover a portion of the bandwidth of the vestibular system (see below). This is why the rotatory chair test is the "gold standard".
Rotatory chair testing generally improves with time -- high frequency gain eventually becomes normal or nearly so, several years after exposure. Recovery at high frequencies is felt to be related to non-vestibular sensory input, and does not necessarily correlate with severity of vestibular injury. Low frequency responses (e.g. < .04 Hz) usually remains depressed. Optokinetic afternystagmus is abolished in significant bilateral vestibular loss (Hain and Zee, 1991). Rotatory chair testing is the "gold standard".
A recent advance in rotatory chair testing is to use the "Gain-TC product (see Hain, Cherchi and Perez, 2018). This allows one to both quantify the amount of vestibular loss as well as provides a good framework to follow it over time.
Visual suppression is the process of attempting to keep the eyes on a target moving with the person, while their head is being rotated. Visual suppression testing can be useful in detecting patients who are pretending to have gentamicin ototoxicity in a hope of being compensated. The idea here is that bilateral patients have a much easier time than normal patients doing suppression, as they have nothing to suppress. Patients who are pretending to have bilateral loss, sometimes are unable to stop their eyes from jumping while suppressing and rotating. We think the best way to do this is with the rotatory chair.
The "VHIT" test, can be used to detect bilateral loss. The image above shows a severe case. As of 2018, VHIT is quick and fairly effective, and it has supplanted the VAT test (see below). While some think it has also supplanted the rotatory chair and caloric test, this is actually not true. The reason is because the VHIT tests high frequencies alone, roughly 3 hz, and can miss a low-frequency pattern vestibulopathy. VHIT, for example, might thus conflict with caloric testing which is mainly a low-frequency test. VHIT's main advantage over the rotatory chair is that it is a less expensive device for clinicians, and also because it is very quick. The VHIT can be used to detect worsening in vestibular function, but it does not provide as clear a paramater regarding total vestibular function as the rotatory chair, gain-TC product (Hain, Cherchi and Perez, 2018).
Recent authors have been reporting values for variant VHIT tests in bilateral loss (e.g. Tarnutzer et al, 2018). The RALP and LARP variants of the VHIT test are not reliable right now because of technical issues with measuring ocular torsion.
VEMP testing is nearly always reduced in bilateral vestibular loss due to aminoglycoside ototoxicity, and the combination of absent VEMP's and absent calorics is probably nearly as good as rotatory chair testing for diagnosis of bilateral vestibular loss, given that technique is good. There are two types of VEMPS: cVEMPs and oVEMPS. cVEMPs are often preserved in patients with bilateral vestibular neuritis. oVEMPS are usually gone. The reason for this is presumably that vestibular neuritis generally affects the superior vestibular nerve.
VEMPs are also reduced in older persons, and their utility diminishes after the age of 60. This is a huge problem -- as most patients with bilateral vestibular loss are on the older side of 60.
Moving platform posturography is always abnormal, but it is not specific (i.e. many other disorders also impair posturography). Allum and others recently concluded that diagnosis of bilateral vestibular loss using posturography is best achieved using measure of trunk control following pure toe-up rotational perturbations under eyes-closed conditions (Allum et al, 2001). This is not a paradigm that is routinely available.
Older tests for bilateral vestibulopathy that are no longer used
The Vestibular Autorotation Test (VAT) is a variant rotational test where the subject moves their head themselves instead of being rotated by a motorized chair. Another brand name is the Vorteq test. VAT testing is probably less sensitive to bilateral loss than is ordinary rotatory chair testing because of a tendency for high-frequency VOR gain to recover via non-vestibular mechanisms. There has also been substantial Medicare fraud using the VAT test in certain settings. This is a high-frequency test, like the VHIT.
As of 2018, the rotatory chair is the "Gold standard" test for bilateral loss. After this, an extremely low horizontal VHIT test is specific but insensitive, as it misses low-frequency issues. All of the rest of our currently available clinical measures of vestibular function have serious flaws and should not be relied upon for clinical diagnosis.