Timothy C. Hain, MD
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We will discuss the main subcategories of vestibular tests next.
The purpose of the ENG is to determine whether or not dizziness may be due to inner ear disease. There are four main parts to the ENG. The calibration test evaluates rapid eye movements. The tracking test evaluates movement of the eyes as they follow a visual target. The positional test measures dizziness associated with positions of the head. The caloric test measures responses to warm and cold water circulated through a small, soft tube in the ear canal.
The ENG test is the gold-standard for diagnosis of ear disorders affecting one ear at a time. For example, the ENG is excellent for diagnosis of vestibular neuritis. The ENG is also useful in diagnosis of BPPV and bilateral vestibular loss, although the rotatory chair test is better at the diagnosis of bilateral vestibular disorders than is the ENG. The calibration and tracking tests are intended to diagnose central nervous system disorders, such as cerebellar degenerations. These tests are generally insensitive compared to an examination by a neurologist or an MRI scan. ENG, however, is much less expensive than an MRI in most institutions.
In the author's clinical practice, ENG equipment supplied by Micromedical Technologies, Inc. is used, supplemented by an ICS caloric irrigator. We find that the Micromedical equipment produces excellent eye movement recordings, although the analysis procedure is not optimal. This is especially true for oculomotor testing. In our opinion, Micromedical's technical support is exemplary.
There are several other vendors, and in fact the ENG industry is presently undergoing somewhat of a consolidation. For example, systems by ICS (AKA GN otometrics; Schaumberg Illinois) and Interacoustics are illustrated above also. We have not used personally ENG equipment sold by ICS but we have a generally good opinion of it. We have used equipment sold by Interacoustics, but we prefer the Micromedical system. We have also used equipment from Synapsis (a French company), but ultimately we were dissatisfied with the software. We strongly suggest avoiding ENG systems sold by small companies and especially by companies that refuse to support their software. One should have a 1-month return option built into any ENG purchasing agreement.
The present best method to measure eye movements is an infrared/video system, as illustrated above. Other methods include electrooculography (EOG), and infrared reflectance. Video systems are usually more accurate than the older EOG (electrooculography) method because they are less sensitive to lid artifact and are not affected by electrical noise generated by muscle. Infrared reflectance is little used in recent times because of nonlinearity (basically inaccuracy). EOG testing may still have a place in persons with very small eyes that cannot be tracked with video. For persons who are buying a new system, we strongly recommend getting a video system. A single camera is sufficient for most purposes.
Some IR video systems offer digital recording of eye movement video's. While "neat", we have not found this to be useful from a clinical standpoint.
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|
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. Two tests use active head movement (autorotation)-- brand names for these devices are "VAT", and the "VORTEQ". Both of these tests provide a part of the the rotatory chair test information (the high-frequencies), and measure something a little different -- the contribution of the inner ear, cognitive input, and neck inputs to nystagmus rather than the contribution of the inner ear alone (Dell Santina et al, 2002).
If you have a rotatory chair test, there is no need to get a "VAT" or "VORTEQ" test as the information supplied is largely redundant. It is possible to have a moderate loss of vestibular function on a rotatory chair test, but have the VAT/VORTEQ test miss the diagnosis entirely. This is particularly possible in a person who has had a few years to compensate for a bilateral vestibular loss. However, if a rotatory chair test is not available, these test may have some value.
VAT tests have been used in unilateral syndromes(Perez et al. 2003), but we think that caloric testing would be superior
We thank ICS, Interacoustic, and Micromedical Technology for use of figures of their equipment to illustrate this page.
Rotatory Chair testing (emedicine site).References: