Timothy C. Hain, MD Page last modified: November 19, 2011
|Micromedical Rotatory Chair||ICS rotatory chair|
The purpose of rotational testing is to determine whether or not dizziness may be due to a disorder of inner ear or brain. There are three parts to the test. The chair test measures dizziness (well jumping of the eyes really -- called nystagmus) while being turned slowly in a motorized chair (see rightward illustration above). Persons with inner ear disease become less dizzy than do normal persons. The optokinetic test measures dizziness caused by viewing of moving stripes (see leftward illustration above). Optokinetic testing is sometimes useful in diagnosis of bilateral vestibular loss and central conditions. The fixation test measures nystagmus while the person is being rotated, while they are looking at a dot of light that is rotating with them. Fixation suppression is impaired by central nervous system conditions and improved by bilateral vestibular loss.
Rotatory chair tests are usually obtained in addition to ENG (caloric) testing. Why get both when both test the same part of the ear (lateral semicircular canal) ? The reason is to add accuracy. ENG tests by themselves may be falsely positive or falsely negative. They can be falsely positive when wax blocks one ear canal. Rotatory chair testing is not affected by mechanical obstructions of the ear. They can be falsely negative particularly in situations where there is damage to each ear.
The author of this page has had extensive experience with the Micromedical rotatory chair system. My opinion is that the rotatory chair is occasionally useful in diagnostic testing. Because of it's high cost (typically about $100,000) and limited usefulness, it is best used in a diagnostic setting where multiple clinicians can use it, such as a hospital laboratory.
Rotatory chair testing is a type of "systems identification" -- engineers use this word to describe the process of attempting to figure out what a "black box" is doing, but giving it an known input, and measuring the the output. The ratio of the output to input is called the "transfer function". There are many reasonable protocols for the input. For a linear system, any protocol that includes a reasonable selection of frequency components should result in the same result -- a gain and time constant. As there are nonlinear processes in the vestibular system (such as prediction), the various methods may not always produce the same results. At present, most laboratories use either sinusoidal testing or step-testing.
The sinusoidal test protocol involves rotating the chair so that it moves sinusoidally. Because the derivative of a sine is another sinusoid, chair position, velocity and acceleration all change sinusoidally. Ordinarily one chooses a desired peak chair velocity, such as 60 deg/sec, and one also picks a series of frequencies to test covering about 0.1 to 1 hz. These frequencies cover the range of responses where gain and phase show their greatest variability when there is disease. A variant of sinusoidal testing is "sum of sines" -- SOS -- one mixes together a group of sine waves to make the input less predictable. Although the SOS appears complex, it can easily be analyzed using standard mathematical methods (i.e. Fourier analysis). A "Bode plot" -- essentially a semilogarithmic plot of vestibular gain and phase, is generally used to present results. A powerful motor is needed to attain the higher frequencies, and for this reason, sometimes testing will only include lower frequencies or the peak velocity will be reduced at the highest frequency. An example of the expected output from sinusoidal test is shown below. The upper gain plot and lower phase plot depict a normal person. The lower gain plot and upper phase plot depect expected output from someone with a unilateral vestibular loss.
The step test involves suddenly changing chair velocity (with an impulse of velocity). Step responses provide roughly equivalent gain/phase information as does sinusoidal testing. Step responses have many problems. They require a powerful chair to provide a high acceleration transient. They may be less reliable as well as somewhat more stressful to the patient, and for this reason, sinusoidal testing is generally preferred. Motion sickness is sometimes associated with prolonged vestibular responses (Hoffer et al. 2003), and for this purpose, step responses may be preferable to sinusoids. Practically though, nausea is unusual in sinusoidal testing and this is not a strong consideration.
Optokinetic testing does not actually involve a rotating chair -- instead a large pattern is rotated around the subject. OKN is much less useful than is rotatory chair testing as it is rarely affected substantially by disease. Optokinetic afternystagmus (OKAN) describes the eye movements that occur after the lights are turned out for OKN, and the subject is in complete darkness. OKAN is more sensitive to disease than OKN, but it is variable in normal subjects, which again limits its usefulness.
In VVI, a person is rotated with a visual surround or target also present. The most useful variant of this is to have a person look at (fixate) a laser that is fixed to the rotatory chair. VVI is generally a good index of ones CNS's ability to suppress nystagmus, and thus it is a measure of cerebellar and brainstem function.
OVAR is obtained by tilting the axis of chair rotation with respect to the gravitational axis. OVAR is largely a test of otolith function. While this is certainly of interest, OVAR is very nauseating and for this reason has been used little in clinical settings. In our opinion, VEMP testing is a much more practical method of assessing otolith function.
Rotatory chair tests are the "gold standard" for diagnosis of bilateral vestibular loss. One expects to see the following pattern on rotatory chair testing after a process that reduces vestibular function.
Acute Bilateral Impairment
of Vestibular Function
|Rotatory Chair Gain||Rotatory Chair Phase|
|Mild||Normal gain at all frequencies||Mild phase lead|
|Moderate||Less than 0.4 gain at 0.32 hz.||Moderate phase lead|
|Severe||0 to 0.1 gain at all frequencies||Unobtainable|
|Normal rotational chair test. Gain and phase are well within normal limits.|
|Unilateral Loss sinusoidal responses. This image is from a patient who had a right sided vestibular nerve section. Note that although the gain and phase are abnormal, there is no asymmetry.|
|Complete bilateral vestibular loss. There is no response to rotation. There is no phase because there is nothing to quantify.|
|Typical picture of suppression of rotatory chair responses. Phase is normal at all frequences. Gain is reduced at high frequencies. For more about this, see this page.|
Chronically, gain recovers at mid frequencies. A lack of recovery seen on rotatory chair testing after 2 years suggests that the test was not done properly. There are several possible reasons -- the tested individual might be taking vestibular suppressants (such as a benzodiazepine or anticholinergic), or the person might be purposefully suppressing their vestibular responses (this possibility mainly occurs in legal cases where there is a benefit to an individual in pretending to be more ill than they really are). Optokinetic afternystagmus is sensitive to bilateral vestibular loss and should be absent both acutely and chronically.
Chronic Bilateral Impairment
of Vestibular Function
|Moderate||Less than 0.4||Lead|
|Severe||Low normal at highest frequency (.32 hz), less than 0.1 at lower frequencies||High at high frequency|
In persons with unilateral vestibular loss, such as after a nerve section, there is also a typical pattern of rotatory chair testing in which the time constant is reduced and phase lead is increased. (Koizuka et al, 1995). See figure 3 above. Rotatory chair testing is thus a valuable adjunct to ENG testing by confirming an abnormality. Rotatory chair testing should not be used, by itself, for unilateral vestibular loss, as it may not be accurate in determining the side of lesion.
There are several other alternative procedures involving rotation that provide a subset of rotatory chair testing. In general, we think these are far inferior to standard rotatory chair testing. The method of identifying "abnormalities" on one varient -- the "VAT" test, noting individual points that are outside of 2-SD limits, breaks a well known statistical principle called the "Bonferonni". Without a-priori knowledge of expectations, the significance (p-value) of multiple t-tests needs to be divided by the number of measurements. In other words, just because one point is outside of the 2SD range does not mean that the person is abnormal. The range needs to be multiplied by the number of observations (making the test useless), or a more sophisticated statistic developed for normality.
Rotatory chair testing is complex and there are an immense number of errors that can occur. See this page for more detail,
Ideally subjects undergoing vestibular tests should have no sedating medications for the last 24 hours. Sometimes this is difficult, as for example, when persons are addicted to medications in the Valium family. In this situation, usually 12 hours is sufficient. More data about medication effects is found here.
We thank ICS, Interacoustic, and Micromedical Technology for use of figures of their equipment to illustrate this page.
|© Copyright April 6, 2012 , Timothy C. Hain, M.D. All rights reserved. Last saved on April 6, 2012|