Timothy C. Hain, MD Page last modified: October 4, 2013
|Figure 1: Schematic of the utricle and saccule. These sensory organs in the inner ear primarily respond to linear acceleration such as due to orientation to gravity, but the saccule is also somewhat sensitive to sound. This is the basis of the VEMP test.|
The purpose of the VEMP test is to determine if the saccule, one portion of the otoliths, as well the inferior vestibular nerve and central connections, are intact and working normally. The saccule, which is the lower of the two otolithic organs, has a slight sound sensitivity and this can be measured. This sensitivity is thought to be a remnant from the saccule's use as an organ of hearing in lower animals.
|Figure 2. VEMP circuitry. Sound stimulates the saccule, which activates the inferior vestibular nerve, lateral vestibular nucleus, 11th nerve nucleus, and then the sternocleidomastoid muscle (mostly ipsilaterally).|
The pathway supposed to account for the VEMP response is shown in figure 2 above. Sound stimulates the saccule, traverses the vestibular nerve and ganglion to reach the vestibular nucleus in the brainstem. From there, impulses are sent to the neck muscles via the medial vestibulospinal tract (MVST). For most muscles, the net effect of saccule stimulation is inhibition, but excitation from electrical stimulation of the saccule has also been reported in some muscles in animals (Uchino et al., 1997;Kushiro et al., 1999).
While the clinical literature concerning VEMPs suggest that they are almost entirely saccular in origin, the animal literature does not support this claim. Many studies of monkeys show that all 5 vestibular endorgans phase lock to loud clicks (Xu et al, 2009). Nevertheless, at the present writing (2011), it seems reasonable to assume that while things are not entirely clean, the main source of the VEMP is the saccule.
Sound evoked VEMP recorded from the neck are claimed to be almost completely unilateral. (Colebach et al, 1994; Uchino et al, 1997; Kushiro et al, 2000; Murofushi et al, 1996; Wilson et al, 1995), but in clinical practice this can't be counted upon (see example below).
|Figure 3 : Equipment used to record a VEMP, a Bio-Logic Navigator Pro.|
VEMPs are recorded using an evoked response computer, a sound generator, and surface electrodes to pick up neck muscle activation or other muscles if this is of interest. Figure 3 above illustrates the basic rather minimal equipment needed. In the author's laboratory, a Bio-Logic Navigator Pro does nearly all of the work, and sends the results to a desktop computer.
VEMP testing is not hard, but there are a lot of technical pitfalls. The basics can be learned by a technician in about 30 minutes. It is a very big response, and as long as the person doing the test is attentive to details (getting the sound in both ears with proper placement of inserts or headphones, having the person lift their head through the entire trial, electrodes in the right place with proper impedance), it is very straightforward. The details (see below) take much longer to learn.
The figure below illustrates a VEMP test printout. The VEMP response consists of an initial positivity (p1 or p13) followed by a negativity (n1 or n23), see figure 4 below. It is an evoked potential. Although P1 is positive, it is shown negative on many VEMPs, because of electrode placement (basically putting them on backwards). The most reliable measure of the VEMP response is the amplitude (Isaradisaikul et al, 2008).
Later VEMP components have a lower stimulus threshold and are thought to be nonvestibular. This is not well understood and we are frankly dubious about this idea - -they might simply be part of the same spike train. As VEMP's are easily elicited without the need for EMG rectification, and EMG spikes occur at roughly the same latency as the waves in a VEMP, coherent spike trains are a reasonable alternative explanation. In other words, later waves may all be part of the same response. In our opinion, there is no reason at all to buy a VEMP system that does rectification. This is a worthless "feature". To be fair though, there are some that differ from this opinion (Lee et al, 2008).
Higher than normal thresholds or low amplitudes may be found in persons with saccule disorders as well as conductive hearing loss. Reduced amplitudes are commonly found in vestibular nerve disturbances. Lower than normal thresholds as well as asymmetrical amplitudes are found in persons with Tullio's phenomenon, which is dizziness induced by sound. Prolonged latencies of P13 may be found in central disturbances (Murofushi et al, 2001), but practically these are very rarely encountered, and technical error is the source of most prolonged latencies.
|Normal VEMP using a Bio-Logic Navigator Pro.. The main potential, P1, is located at about 13 msec. Each side is about 250 microvolt in size (which is far above the lower limit of normal, about 70). There is an electrical artifact seen at 0 msec for the left side. This can be ignored.|
The general rule of thumb with hearing and VEMP's is that conductive hearing loss obliterates VEMP's, and sensorineural hearing loss does little or nothing to VEMP's. The two plots below illustrate this.
|Figure: VEMP obtained in an individual with a modest left sided conductive hearing loss, using a Bio-Logic Navigator Pro. The VEMP on the right was normal, and the VEMP on the left, entirely absent. P1 designates the potential that occurs at 13 msec (often called P13)|
|Figure: VEMP obtained in an individual with a profound right sensorineural hearing loss, using a Bio-Logic Navigator Pro. This shows that (sensorineural) hearing is not necessary to obtain a VEMP. This recording was obtained using binaural stimulation (see comments regarding this method).||Audiogram in person with the VEMP shown to the right. There is a profound hearing loss, presumably sensorineural.|
A potential pitfall in a person with a complete sensorineural hearing loss, is that one has no way of determining whether they also have also have a conductive component to their hearing loss. For example, someone might have far advanced otosclerosis. Thus, one could falsely conclude that there was no vestibular function on one side based on an absent VEMP. Bone VEMP's are one way to get around this problem.
Basic head positioning and electrode layout
Best practice, as shown below, is to apply EMG electrodes to the middle third of anterior neck muscles (sternocleidomastoids) and the supine patient holds their head up unsupported, using the anterior neck muscles. Subjects are instructed to tense the muscle during runs of acoustic stimulation, and relax between runs. If the neck muscles are not activated, no VEMP is produced. The reflex scales to tonic EMG -- once again -- if you don't activate the neck muscle, you don't get a response. A corollary, is that if you get a response without neck muscle activation, it isn't a VEMP (maybe a PAM ? see below).
The EMG electrodes should not be the kind that use alligator clips to attach to foil, because the head moves around during the VEMP. These types of electrodes generate massive amounts of noise with movement. Instead, electrodes should be used that are "hard wired" to the cable.
People with fat necks have lower responses, as the signal from the muscle is underneath the fat and has to go a longer distance. Similarly, people with long necks have later responses, as the signal again has to go a longer distance (Chang et al, 2007)
Some patients are unable to hold their heads up at the angle shown. In this case, some experts recommend simply tilting the entire body up by about 30 degrees, so that there is less torque needed by the patient to hold their head up (Colebatch, personal communication). We think this is an excellent idea, but at present there are no amplitude norms for this procedure.
|Figure: Recommended patient positioning for VEMP testing. During the test, the patient lies flat on their back, lifting the head off the table.|
Another (but bad) method of obtaining activation is to have patients sit upright with their chin turned over the contralateral shoulder to tense the SCM muscle. We think this is a bad idea because you have no way of knowing how much tension the patient is producing, it is tiring, and patients can do something different on the right than on the left, without you knowing. It is also well known that head position on body can change VEMP responses. This adds another variable.
Because the response is generally ipsilateral (carefully note the qualification), one theoretically can use bilateral stimuli and bilateral recording to reduce the number of trials (Huang et al, 2006; Young, 2006). We tried this method in our clinical practice, but discarded it for reasons explained below. It has the advantage of using the same stimulus when recording each side, which reduces some of the considerable variability. The limits of normal for the amplitude with the head-raising technique are roughly 70 to 700. Regarding the upper limit, there is no disease that we know of that can be diagnosed by larger than normal VEMPs on both sides, but in our clinical experience, we have seen this primarily in persons with hyperacusis.
|Monaural VEMP that shows the basic pitfall of binaural/bilateral recording. Because there is a substantial response from the contralateral side on the left, one could assume wrongly that there was a present VEMP even in a person with a vestibular nerve section. We do not recommend binaural stimulation.|
We think that binaural stimulation is generally a bad idea. The figure above illustrates why we have discarded this technique. While VEMP's are "generally ipsilateral", this doesn't mean that they are always, 100% ipsilateral. Artifacts can also go across the midline, which reduces much of the value of using a binaural stimulus. Because the sternum is rather close to the sternocleidomastoid muscles, there can also be artifact due to "volume conduction" -- meaning electrical activity from one side getting confused with the other (Li et al, 1999). The bottom line, is that if you are trying to "rule out" any vestige of remaining vestibular function -- don't use a bilateral VEMP. A corollary is that if you really care that what you are measuring reflects the side you think you are measuring, don't use a bilateral VEMP. In our opinion, this pretty much eliminates the technique.
Loud clicks or tone bursts (typically 95-100 DB nHL or louder) are repetitively presented to each ear in turn at 200 msec intervals (5/second). The optimum frequency lies between 500 and 1000 Hz. The sternum is generally used as a reference and the forehead as a ground. There is some evidence that linked wrists might be a better choice for a reference though (Li et al, 1999).
We generally use a Bio-logic Nav-Pro set up with the parameters documented here.
Note that binaural presentations of sound is not recommended (by us anyway). It is faster but it reduces ones ability to localize the side of lesion because of crossover. Our recommendation, based on some unfortunate clinical experiences where binaural stimulation lead us astray, differs from others in the literature (e.g. Young, 2006).
Myogenic potentials are amplified, bandpass filtered (30-3K Hz), and averaged for 200 presentations. The response evoked in the neck EMG is averaged and presented as a VEMP (see figure 2). The latency, amplitude, and threshold for the p13-n23 wave is measured. The amplitude is the most reliable measure (Isaradisaikul et al, 2008). Latencies are less reliable but are useful in deciding whether a particular waveform is a "VEMP" or just noise.
Because of the high intensity of the sound used to evoke these responses, carefully checked inserts should be used. Headphones are less reliable than inserts, because small errors in headphone placement can result in substantial changes in sound intensity, and loss of a VEMP. When the head is being held upright, headphones can shift easily.
The VEMP is generally quick and easy to obtain because it is a strong potential and only requires about a minute of stimulation to get 100 presentations. We usually use 200 presentations ourselves. This means that you can easily repeat the VEMP test. In our opinion, a minimum of two repetitions should be obtained on each side, to be sure that the VEMP is reproducible or absent, as the case may be. Three is generally aimed for. An exception to this can be made if the first two repetitions are of large amplitude and nearly identical (e.g. see figure 2). We generally use monaural recordings.
We recommend tone bursts rather than clicks -- here is the reasoning. A similar response is produced using tone bursts instead of clicks (Murofushi, Matsuzaki et al. 1999; Welgampola and Colebach, 2001; Cheng, Huang et al. 2003). Either 500 or 1000 hz tones are presented at a 5/second rate. They suggested using an intensity of 120 db SPL. A stimulus duration of 7 msec was found optimal. The advantage of the tone burst stimulus compared to a click is that it requires lower absolute stimulus intensities. This is important if you are using equipment that doesn't produce optimally loud stimuli (see below). We suspect that it produces longer and more variable latencies. At the present writing however, the clinical value of measuring latency (other than being sure you have a VEMP), remains somewhat elusive. Amplitudes are much more reliable.
Rauch et al (2004) also advocate using tone bursts, and suggest a 500 hz frequency is optimal. They suggest monitoring ongoing EMG activity to ensure that the SCM muscles are activated as without muscle activation, a VEMP does not occur. In their study, a VEMP was judged absent when no replicable response was observed and enough responses were averaged for residual noise to be less than 3 uv. They suggest that thresholds are more useful than amplitudes. We do not agree -- we feel that amplitudes are more useful than thresholds, given a well standardized protocol. The literature suggests that amplitudes are more reliable than thresholds too (Isaradisaikul et al, 2008).
Practically, one cannot do both -- 3 repetitions as well as thresholds, because of fatigue. We have also found many patients with low thresholds, but no dehiscence. See more discussion of the problems inherent in doing thresholds below under technical pitfalls.
Nearly all VEMP problems are caused by operator error. The VEMP is a relatively new test, and so far, manufacturers have not built into the protocols methods of quality assurance. When VEMP's make no sense in the overall clinical context, we think it is a good idea to just repeat them on a later visit, and inservice the technician if there is a big discrepancy
Assuring neck muscle activation is the biggest problem. While one can run a VEMP very successfully with the patient's head being held up vs. gravity, this is tiring. A common quality control problem in the VEMP is an overly nice technician who allows the patient to put their head down during the test. VEMPs can be run with the head being actively turned to one side, thus fatiguing only one side rather than both, but this procedure also has it's pitfalls, as it is less reliable and produces smaller potentials (Wang and Young, 2006). A suggestion for any manufacturer who might be reading this is to add a method of determining if the head is off the table (a simple pressure switch would work). A few more comments about VEMP biomechanics are here.
In persons who can't cooperate, assuring neck muscle activation is a gigantic problem. Consider, for example, attempting to do a VEMP in an infant. How do you ensure that the neck muscle is activated ? This is an intrinsic problem with doing VEMP's in very young children, and perhaps it should just not be attempted at all. At a minimum, VEMP's done in persons who are (perhaps) not cooperative should be done with equipment that can monitor the EMG.
Another technical "gotcha" in the VEMP is a sound not getting to the ears. This is very very important ! Common things that go wrong here are asymmetrical placement of inserts, wax occluding one side, or a defective sound generator (the Bio-logic ones seem to go bad very often, when used for VEMPs -- it has happened to us 3 times in just 2 years !). Because VEMP's are not far above the threshold provided by most evoked potential equipment, little things like not putting the insert in as far in one ear as the other can make a big difference. Just 10 dB can be important. Regarding checking of the inserts, we suggest a sound-check with every single VEMP. The easiest way to do this is to run an initial VEMP without lifting the head -- this both assess for PAM artifact (see below), as well as can provide a good time to do a sound check. It would make sense for the VEMP protocol to include a threshold check at 500 hz, using the inserts and transducer of the evoked potential machine, but so far, this is not available.
Thresholds can also be problematic. There are several major problems. The first is that there is a subjective element to picking a threshold. One person's response might be another person's noise. The second is that if you do thresholds, you can't do a lot of repetition (because people get tired). The third is that they don't always work -- low thresholds are NOT always accompanied by radiographic evidence of superior canal dehiscence. We have encountered several patients with thresholds of 60, but normal temporal bone CT's. Our present position is that thresholds should not be done routinely, as they prevent one from doing a reliable amplitude.
|VEMP artifact (squiggles at start) due to technical error in placement of sound generator too close to electrodes.|
Electrical artifact. VEMP's are huge (compared to ABR or ECochG) and there should not be much random electrical artifact. If you see stimulus artifact, or sinusoidal undulations to the trace, it is very likely that the electrodes are bad, someone put the sound generator box too close to the electrodes, the impedance was too high, or the evoked potential machine needs service. In our experience with the latter, the thing that routinely goes bad with the evoked potential machine are the connectors and the insert driver.
In the VEMP trace above, there is stimulus artifact. After it went away when the sound generator was moved, we realized that the sound generator cannot be very close to the EMG leads. The sound generator produces a magnetic field which induces a strong artifact, if one places it close to one of the electrical leads.
Other artifact. Occasional patients have "VEMP like potentials" (? VLP), that are not VEMP's.
In some instances, this is caused by the "posterior auricular muscle (PAM)", a sound evoked twitch of the ear. The PAM is a small muscle behind the ear that can "wiggle" the ear. Actually, there are three of these muscles -- posterior, superior and anterior auricular muscles - -all vestigial. PAM is innervated by the facial nerve. The latency of the response is about 11 msec, making it overlap with the VEMP. (Funahashi et al, 1992). The literature about PAM is confused -- some authors may have mistaken the much larger VEMP's for PAM responses, and vice versa. See Gibson (1978) for a review of the older literature.
To rule out PAM potentials, you can run your VEMP initially or perhaps in between other runs, without contracting the SCM.
In other instances, this is due to "volume conduction" -- electrical activity from one side showing up on the other side. Volume conduction problems can be best solved by monaural stimulation. For reasons developed above, we think that a run or two of monaural stimulation is a very good idea.
Logical inference. While VEMP's are "generally ipsilateral", this doesn't mean that they are always, 100% unilateral. If you are trying to "rule out" any vestige of remaining vestibular function -- don't use a binaural stimulation VEMP. As nearly always one is trying to localize the side of lesion, we think that binaural VEMP's are best avoided. An exception is when one is trying to diagnose bilateral vestibular loss, when it is OK.
The term "rectification" is used in a a confusing way by some to indicate normalization -- the unrectified VEMP signal is divided by an averaged pre-stimulus rectified surface EMG (Lee et al, 2008). The reasoning is that the size of the surface EMG signal may vary according to electrode placement or differences in muscle activation or body build. Chang et al (2007) showed that people with larger amounts of neck subcutaneous tissue (i.e. fat), have smaller VEMPs. This is reasonable and just common sense, and a good reason for normalization.
Variability due to these technical problems might be reduced by adjusting for these factors. This technique would seem quite reasonable for adjusting for differences in electrode placement as well as neck anatomy. It would seem to us to be not quite as reasonable for adjusting for differences in muscle activation, because this might easily change during the 2 minute VEMP itself.
We have experimented with this technique ourselves and decided that it was worse than using a very careful electrode placement and having patient's lift their head up straight (ensuring equal activation of both SCM). In other words, we discarded it.
On the other hand, Lee et al reported that rectification was better (2008). They did not use our favored technique -- instead of lifting the head up straight, they had subjects lift and turn their heads. Because this method introduces the variability due to head turning, in our opinion, their experimental design was simply designed to fail. Overall, it seems to us that their method may be better than the (poor) method of head-turning as it controls for intermittent effort which is a problem with the head-turning methodology. We presently continue to hold to the opinion that a careful, unrectified head-straight lifting VEMP is superior. However, as technology improves, we may change our mind.
In this regard, we would like to see a VEMP output that provided the clinician more data -- the "raw" VEMP amplitude, the "raw" integrated and rectified EMG, and the "normalized" values. We would also like to see a VEMP machine that printed out the entire raster of EMG activation during the trial -- to assure that the patient was activating their neck and that the electrodes remained firmly attached.
There is room for other improvement -- for example, a VEMP stimulus that had "gaps" in the sound stimulus might allow a clever algorithm that used the gaps to compute a more useful normalization signal. Also, the term "rectified VEMP", should be replaced with the more reasonable term "normalized VEMP". We don't think the term "corrected" VEMP is a reasonable one, as "correction" covers too much ground.
Recent workers have claimed that bone-conducted tone bursts activate the vestibular system, producing potentials in the extraocular muscles (e.g. Jombic and Bahyl, 2005). There is some evidence that these are utricular in origin. Much more work is needed to characterize these responses. While it would be wonderful to have a practical utricular test, we are a bit dubious that things are so simple.
We have been able to record OVEMPs using a "mini-shaker" oscillator. However, we are not presently sure what part of the ear is being measured and as well, we think that calibration is an issue. Some authors suggest that it is a utricular response (Manzari et al, 2012a). Evidence for this assertion remains very circumstantial.
Manzari et al ( 2012b) suggested that "oVEMP testing with 500 Hz Fz BCV allows very simple, very fast identification of a probable unilateral SSCD." We are dubious that this low-gain response could be preferable to cervical VEMP testing, and would like to see more data from other groups too. In 2013, Manzari and others suggested that oVEMP being present at 4000 hz is even better. Again, we would like to see more data.
Taylor et al (2012) reported that increased oVEMPs, compared to normal VEMPS, were found in a single patient with bilaterally enlarged vestibular aqueducts. Interesting I suppose, but this obviously needs to be confirmed with, lets say, 10 cases.
We have studied acoustically generated VEMP's in the gastrocnemius and triceps (Ruddissil et al, 2008; Cherchi et al, 2009). These responses occur at much longer latency than SCM VEMP's. Their clinical utility remains to be established.
Skull taps and bone conduction tones can also be used to elicit VEMPs. Taps can be delivered to the forehead or lateral skull, with some differences in polarity and sidedness of the resulting potential. Bone conduction tone bursts also can evoke VEMPs, using frequencies of about 200 Hz. Clinical bone vibrators generally require additional amplification to produce strong enough stimuli for VEMP testing. Bone conducted VEMPs are not as well lateralized as click evoked VEMPs. (Sheykholeslami, Murofushi et al. 2000)
McNerney and Burkhard (2012) compared air to bone conducted VEMPs, using 500 hz for both. A TECA unit was used for the testing. BC stimuli were set to 120 db pFL, using a B071 bone vibrator. We are not entirely sure what a "pFL" is -- presumably something related to bone rather than air conduction. We are not at all sure how pFL levels of 120 db relates to the ordinary problem with clinical bone vibrators that only produce 60 db.
Well, anyway they found that amplitudes were less variable for bone conduction. This is unsurprising as bone conduction stimulates both ears and one would think that averaging across two ears would be more reliable than one. They also found that bone masking reduces amplitudes of air conducted VEMP's. To us this suggests that a component of air-conducted VEMP response is either due to startle or multisensory convergence. To summarize, although BC VEMP's are undoubtedly more reliable, their obvious problem lies in their inability to confine the stimulus to one ear. We would like to see the a similar study done in persons with conductive hearing loss.
Galvanic stimuli can also produce a VEMP (Watson and Colebatch 1998). This technique bypasses the saccule and thus might be used to separate end-organ from nerve and more proximal lesions. Technically, averaging of electrical consequences of galvanic input is difficult because there is a large electrical artifact associated with the stimulus itself. This has been handled by subtraction methodology. A typical protocol uses 4 ma cathodal current pulses, of 20 ms duration, administered at 2 Hz, with 128 stimuli total (Bacsi et al, 2002). Logically, to prevent artifact, it would seem better to record from muscles that are far away from the current input (ear), such as in the legs or arms.
Galvanic and acoustic VEMP's activate different populations of vestibular afferents. Galvanic stimulation excites distal vestibular nerve afferents, particularly those with irregular discharges, but any selectivity on this action on afferents arriving from the different vestibular end organs is uncertain. (Bacsi, Watson and Colebatch, 2002)
As the entire vestibular nerve is stimulated by galvanic input, one would expect that galvanic VEMPs would be insensitive to partial nerve lesions (i.e. failed vestibular nerve section), but also quite sensitive to complete vestibular nerve loss. Thus an absent galvanic VEMP might be used as a rationale to avoid doing more surgery.
For the same reason, galvanic VEMPs should also not be able to differentiate between endorgan (saccule) damage and inferior vestibular nerve damage because one would expect that the galvanic VEMP would be present even if the inferior vestibular nerve were damaged.
Galvanic VEMPs may nevertheless prove useful using threshold or latency information. Galvanic VEMPs are not suppressed by anodal current, which suggests that VEMPs do not require the irregular afferents (Bacsi and Colebach, 2003). Galvanic VEMP's may be more reliable than acoustic VEMP's and thus be a better method of monitoring vestibulospinal connections through the spinal cord than acoustic VEMP's. More study is needed.
A closely related test to the VEMP was described by Halmagyi and others (2003). Event triggered averaging is used to detect electro-oculographic responses to loud clicks -- intensities ranging from 80 to 110 Db. 128 clicks were delivered at a rate of 5/s from 60 to 110 db, in 10 db steps. Normal subjects have no or a very low amplitude response of < 0.25 deg at 110 db. The latency was 8 msec. This test is not generally available, but appears promising. The technology is very similar to VEMP, and perhaps might even be obtained with similar instrumentation. It may be a good candidate to replace the Tullio test.
What does it test ?
Figure 2 illustrates the pathway for the acoustic VEMP response, which includes the saccule, the inferior vestibular nerve, the vestibular nucleus, the medial vestibulospinal tract, the accessory nucleus, the 11th nerve, and finally the sternocleidomastoid muscle. Abnormal VEMPs might be caused by abnormalities in any of these structures. The sound induced VEMP also requires conduction of sound to the inner ear, which means that an intact middle ear is needed.
While VEMPs are presently attributed to the saccule, the data presented so far suggests that hearing is not necessary for VEMPs. This does not exclude the possibility that hearing is sufficient for a low-level VEMP, as it is unusual to encounter a human subject with a well documented vestibular lesion that is confined to the saccule. There are some data however suggesting that vestibular nerve section abolishes VEMPs, which would be against this idea (Watson and Colebatch, 1998). It is also possible that hearing is synergistic with vestibular input -- i.e. you get more of a response with multisensory convergence. We are presently of this opinion, but these are issues that need to be worked out.
In our clinic setting in Chicago, we consider VEMPS to be abnormal when they are very asymmetrical (one is 2 times or more as large as the other -- an RVR of 33% or greater), low in amplitude (less than 70 for a young population), or absent (no reproducible wave, or P1 latency outside of our norms). We use tone-burst VEMPs, which produce modestly larger responses than clicks. We do this because our equipment (Bio-Logic Nav-Pro), peculiarly enough, does not produce as loud a stimulus as we would prefer (only 95 dB). As mentioned above, we tried bilateral binaural recording and stimulation, but we switched back to monaural stimuli because of some bad diagnostic experiences where it appeared that there was a VEMP with binaural stimulation, but there was clearly no VEMP with monaural stimulation.
|Figure: VEMP amplitude to clicks, as a function of age, from Su et al (2004).|
Concerning effects of age, 3 studies have been done -- using 3 different techniques. For clicks, compared to young persons, decreased amplitudes (roughly a factor of two) are seen on persons 70 and older (Su et al, 2004). Similarly, also using clicks but not using commercial equipment, Ochi reported amplitudes of approximately 250 for 20 year olds vs. 90 for 80 year olds.
Basta et al reported similar results for 500 hz tone bursts (2007) but with lower overall amplitudes using a Viking system. These systems seem to produce lower amplitude results, perhaps due to differences in calibration. It has been our general experience that the device that we use (Bio-Logic NavPro) produces larger potentials than most competing devices.
|Latency (p13)||Su et al||11.33+-.82||Rarification click|
|Latency n23||Su et al||18.24 +- 1.33||Rarification click|
|Amplitude p13-n23||Su et al||126+-49.6||Click|
|IAD ratio||Su et al.||0.16 +- .12||Click|
At the present writing (2011), it appears to us that in normal persons, and using the Bio-logic Nav-Pro, average VEMP amplitude decline from approximately 150 at the age of 20 to approximately 75 at the age of 70.
The VEMP literature is rapidly increasing and it seems that there are considerable valuable diagnostic information to be obtained.
See the following sections for brief discussions of VEMPS, illustrated with traces from our practice, in
VEMPs in Superior Canal Dehiscence Syndrome -- works
|Figure left: VEMP obtained in an individual with left sided superior canal dehiscence, using a Bio-Logic Navigator Pro. Right -- threshold VEMP in same person. See the SCD page for his CT scan.|
VEMPs so far have been mainly useful in documenting abnormally low thresholds in persons with the "Tullio" effect, which largely occurs in persons with fistula or Superior Canal Dehiscence syndrome (SCD) (Brandtberg et al, 1999). If one does not do thresholds (we don't do them routinely), there nearly always is an amplitude asymmetry in this syndrome, as well as a very large VEMP in an ear with a air-bone gap. VEMP's normalize after surgery to plug the superior canal (Welgampola et al, 2008). This can be interpreted in several ways -- the saccule may be less stimluated after canal plugging, or the canal may be less stimulated after canal plugging.
However, they seem likely to be of much wider utility than this because they offer an objective method of assessing the vestibular nerve, including a portion of it for which there is no other available clinical test. The essential bits of information that might be useful are: 1). is the VEMP present at abnormally low threshold on either or both sides ? and 2). Is the VEMP absent on one side at a high threshold ? These two bits of information tell one whether there is Tullio's, and also whether there may be damage to the saccule, inferior vestibular nerve or it's projections.The presence of VEMPs in a person with an air-bone gap (see hearing testing page) is also suggestive of SCD.
We have not found VEMPs to be diagnostic of the small window fistulae that we encounter most frequently in our practice.
VEMPs (using bilateral, binaural method), using amplitude criteria, are not always successful in detecting bilateral SCD. For this, one needs either a temporal bone CT or threshold VEMPs. We recommend doing a threshold VEMP in any person with a complaint of dizziness induced by sound (Tullio's), should their regular VEMP be normal.
Vemps as a test of vestibular nerve disease -- sometimes works
|VEMP obtained in an individual with left sided Ramsay-Hunt syndrome, using a Bio-Logic Navigator Pro. Ramsay-Hunt is a facial weakness, sometimes combined with 8th nerve neuritis, due to a recurrence of Herpes Zoster (the chicken-pox virus).|
Suggesting that VEMPs are sensitive to saccule pathway disease, Ochi and associates (2003) reported use of VEMPs to diagnose vestibular neuritis involving the inferior division of the vestibular nerve. Because the saccule is supplied by the inferior division, VEMPs should be absent in this situation. VEMP does not distinguish between the saccule and inferior vestibular nerve, and available techniques seem unlikely to be able to resolve between these two.
As most types of vestibular neuritis spare the inferior vestibular nerve, VEMPs would be expected to generally be normal in vestibular neuritis. In fact, in our clinical practice in Chicago, we sometimes use the pattern of very abnormal VENG, accompanied by very normal VEMP to strongly suggest Vestibular neuritis -- i.e. we use it to exclude total vestibular nerve disease rather than to detect vestibular nerve disease. We also use similar logic to diagnose bilateral vestibular neuritis. When VEMP's are abnormal in vestibular neuritis, they recover more rapidly than canal related tests (Kim et al, 2008)
Galvanic VEMP stimulation stimulates the entire vestibular nerve and accordingly would be expected to be normal even if the inferior vestibular nerve were damaged. Thus an absent sound-VEMP and present galvanic-VEMP would not differentiate between a saccule lesion and an inferior vestibular nerve lesion.
Prolonged latency of VEMPs has recently been suggested to be a sign of a retrocochlear (vestibular nerve) lesion, such as is found in vestibular neuritis. We are somewhat dubious about this and think that in this case the first wave of the VEMP may simply be missing, but later waves related to cochlear function are preserved, causing the appearance of a prolonged latency. Abnormal VEMPs (asymmetrical or long latency) are reported in about 25% of persons diagnosed with vestibular neuritis (Murofuschi et al, 1996). Prolonged latency also occurs in persons with long necks (of course).
|VEMP asymmetry in patient with small intracanalicular acoustic neuroma, without any hearing deficit.|
In Vestibular neuritis, VEMP's reportedly normalize more rapidly than canal related tests (Kim et al, 2008). However, this conclusion is suspect as the VEMP technique used in this study (head-turning) is intrinsically flawed (see discussion above).
VEMPS are generally absent or reduced in persons with acoustic neuroma (see figure above).
Failed vestibular nerve section: Not very likely to work
VEMPs should be and nearly always are absent in persons with vestibular nerve section.
|Expected absent VEMP in person with Right sided VNS. See this page for more about this patient.|
Vestibular nerve sections fail to control intractable vertigo due to Meniere's disease in about 5% of patients. When they fail, the question can arise whether the nerve section was incomplete. VEMPs might, in theory, be useful for detecting residual function in the inferior vestibular nerve, as this branch of the vestibular nerve is sometimes intermingled with cochlear fibers (Lehnen et al, 2004). However, as VEMPs require a very strong stimulus, it seems unlikely that they would be very sensitive. Head impulse tests for the posterior canal reveal residual function more more frequently than do VEMPs (Lehnen et al, 2004). More study of this question is needed.
A potential pitfall in doing VEMP's in persons with VNS are contralateral responses and extraneous responses (mainly posterior auricular muscle responses). You should NOT do bilateral stim VEMP's in persons with VNS. You should also check for the PAM (by doing a test with the head resting on the table) if there is something that appears to be a response on the sectioned side.
VEMPs as a test for bilateral vestibular loss -- works !
|VEMP in person with complete bilateral vestibular loss due to gentamicin, but with some hearing. The potentials shown are replicable but they lack the characteristic below-baseline displacement for P1 (see below).|
VEMPs would be expected to be reduced or absent in persons with bilateral vestibular loss, such as due to aminoglycoside ototoxicity.
Gentamicin obliterates VEMPs in guinea pigs. (Cheng et al, 2010)
Only a few patients have been studied so far with gentamicin (Murofushi et al, 1998). In our practice, we have tested many patients with bilateral vestibular loss and normal hearing, and find that it is a good test (Hain et al, 2006). We have also tested two deaf patients with bilateral loss, one due to Cogan's syndrome and another due to a Mondini malformation, and found absent or nearly absent responses in both. This is as would be expected if one believes that the saccule is affected in these conditions. Nevertheless, this conclusion can be questioned as a problem intrinsic to testing persons with bilateral hearing loss is that one does not know if they might also have a conductive hearing disturbance superimposed on the sensorineural loss. Because conductive hearing loss obliterates sound-induced VEMPs, one cannot clearly relate an absent VEMP to absent saccule function in this situation.
VEMP's also appear to be absent after unilateral gentamicin treatment used for Meniere's disease (Helling et al, 2007). They may be useful in deciding whether or not more drug is indicated. A possible confounding problem is that there may be middle ear disease after the injection due to the perforation required for transtympanic gentamicin.
One might think that as VEMP's assess the saccule (an otolith organ), and because BPPV is thought to be due to another otolithic disease (loose otoconia from the utricle), that there might be abnormal VEMP's in persons with BPPV. Yang et al (2008) asked this question, and reported that latency was slightly increased. As we are dubious that latency is a useful measure at all, we are also dubious about the use of VEMP's in BPPV.
VEMP's in Otosclerosis - -should be absent.
In Otosclerosis, VEMPs should be absent. A person with a present VEMP and conductive hearing loss may have SCD.
VEMPS in Meniere's disease -- doesn't work.
In our own (large) clinical experience, we have not found VEMP's to be of any use in Meniere's disease. However, others have reported different experience.
It has been reported that low amplitude of VEMPs may be found in the affected ear (Waele, 1999) and a substantial proportion of subjects show no VEMP, or a higher threshold (Rauch et al, 2004). VEMP amplitudes can be increased in early Meniere's disease, as well as fluctuate oppositely to hearing, perhaps due to saccular dilatation (Young et al, 2002). Absent VEMPs in advanced disease may represent collapse of the saccule. One might also logically suppose that there should be a point in which the saccule is normal in size, in the middle. It seems to us that these observations just suggest that VEMP's can be any size in Meniere's disease -- big, normal or small, and that they are unlikely to be useful for diagnosis.
Rauch and associates have reported that VEMP's show different "tuning" in Meniere's disease. The idea seems to be that as patients with Meniere's have a low-tone hearing loss, they might also show alteration at low-tones in VEMP's. In our opinion, this is not a very logical idea as the cochlea and saccule have very different mechanical organizations. Nevertheless, in several papers, Rauch and colleagues showed that the 500 hz tone-burst VEMP has the lowest threshold, and that this "favoring" of 500 hz over 250 and 1K is largely lost in persons with Meniere's disease (Rauch et al, 2004). It is our feeling that this is simply impractical. It is just too difficult to obtain VEMP thresholds at 3 frequencies.
It has recently been proposed that VEMPs that increase on glycerol loading or furosemide injection are suggestive of Meniere's disease (Shojaku et al, 2002; Seo et al, 2003). At the present writing (2007), we would like to see confirmation of this by others. If enlarged VEMPs are due to saccular dilatation, one would expect the opposite effect.
Rauch et al reported that threshold differences in VEMPs, particularly at 250 hz, were about 80% accurate in detecting the side of lesion in persons already known to have Meniere's disease from audiometric and clinical data (Rauch et al, 2004). Lacking a pathological "gold standard" diagnosis, however, it is difficult to know how to interpret this observation.
Osei-Lah and others (2008) recently reported that the mean interaural amplitude difference ratio is significantly higher in stables Meniere's disease as compared to acute Meniere's disease. Thresholds and latencies were not useful.
VEMP's may be useful in monitoring low-dose gentamicin treatment for Meniere's disease though, because VEMP's are reduced in this situation (Helling et al, 2007; Picciotti et al, 2005). So far, we have seen some confirmation of this idea. Look here for a case illustration.
We tentatively think that it is a good idea to get a VEMP prior to ITG, and another one if there appears to be a treatment failure.
VEMPs in central disorders -- rarely works.
We have not been impressed with sensitivity of VEMPs to central disorders such as brainstem strokes. The difficulty seems to be that the latencies in VEMPs (at least with tone bursts), are so variable that there is inadequate sensitivity to disease. This may be an artifact of our methodology (tone burst), which is temporally a longer stimulus than a click. We don't have any examples to show here, so we will just mention what is in the literature.
VEMPs are often asymmetrical in spasmodic torticollis (Colebatch, Di Lazzaro et al. 1995).
VEMP's are reduced in progressive supranuclear palsy (PSP), according to Liao et al (2008). PSP is a rare degenerative disorder of the CNS that generally results in death within 5 years of diagnosis. The average VEMP amplitude was reported to be 54, with a range of 16.8 to 214. This range overlapped substantially with age-matched controls, but may be another helpful method of identifying PSP. No data is presently available concerning progression of VEMP's in PSP patients over time. One would expect a decline. These patients are unusual however and a longitudinal study would not be easy to carry out.
VEMPS, like other evoked potential tests, can also be abnormal in central diseases such as multiple sclerosis (MS). (Shimizu, Murofushi et al. 2000; Versino, Colnaghi et al. 2002; Murofuschi et al, 2001) and brainstem stroke (Chen et al. 2003). VEMPs test mainly measure lower brainstem function (medulla), while the ABR also tests upper brainstem function (medulla pons and midbrain). Here, latency measures would seem more logical than amplitude measures.
|Normal VEMP in patient with Machado-Joseph disease (Sca-3). The amplitude is low (30-50) but the latency is normal in this middle aged individual with genetically proven sca3 for 5 years, with gait ataxia, a small brainstem, and bilateral upgoing toes.|
VEMP's are reported abnormal in certain cerebellar degenerations (i.e. Machado Joseph disease), but normal in others (e.g. OPCA, cortical cerebellar degeneration). (Tagegoshi and Murofushi, 2000). As VEMP circuitry is not thought to involve the cerebellum, it would be surprising to find abnormal VEMP's in any purely cerebellar condition. In our own practice (see above), but we have only studied two patients and found abnormally reduced amplitude for age in one and an absent VEMP in the other. As Machado-Joseph affects the brainstem, the mechanism for the reduced VEMP may lie in damage to other locations than the cerebellum.
VEMPs in hearing disorders -- needs more data.
Although VEMPs can be obtained in people with complete hearing loss, the hearing loss has to be of the sensorineural type. Just because VEMP's can be "obtained" it doesn't mean that they are uncorrelated with hearing loss, and in our experience, both hearing and vestibular function correlate with VEMP amplitude. However, this conclusion is not (so far) well documented in the literature.
Persons with conductive hearing loss, even just a small amount such as 10-15 dB, often do not have VEMPs, presumably because the sound stimulus, conventionally delivered by earphones, does not get to the saccule. The saccule has a high threshold, and if you are stimulating the ear close to that threshold (i.e. 95 dB), it is easy to drop below it. This means that VEMPs are less useful in older persons, who often have a component of conductive hearing loss due to otosclerosis and related disorders, and also should be interpreted with a recent audiogram, including bone and air conduction testing, in hand. A way around this may be to do bone-conduction VEMPs when the air-conduction VEMP is absent. While this substitutes for the bone conduction audiogram, it requires more testing and potentially may fatigue the patient. Also, bone-conduction stimuli are generally not very strong.
Several groups (Wang and Young, 2007; Akin et al, 2012) reported that VEMP's are reduced in persons with noise induced hearing loss. They attributed this effect to saccule damage. While theoretically possible, we are dubious that a structure that requires 70db to elicit a minimal response would be very sensitive to noise. The saccule would more reasonably be damaged by excessive changes in linear acceleration as it is loaded by the otoconia.
Instead our opinion is that there simply is reduction of VEMP's from hearing reduction. Similarly, Zhou, Kenna, Stevens and Licameli (2008) reported VEMP's are reduced in children with sensorineural hearing loss, and attributed this effect to saccule damage. We think that these authors made the same logical error as Wang and Young. VEMP tests are generally normal in sudden hearing loss (Wu and Young, 2002).
VEMP tests are commonly performed by an audiologist or an electrophysiology technician. Oddly enough, VEMP's are sometimes even done by physical therapy practices. Audiologists are often associated with otolaryngology practices (ENT doctors), while electrophysiology technicians are often associated with neurology practices. VEMP's are easy to obtain, but there are many pitfalls involving hearing.
We strongly suggest having an audiologist or audiology technician do this test.
We do not think that this test should be done by people who are not familiar with hearing -- i.e. physical therapy practices or general neurology practices, because a good working understanding of how hearing interacts with VEMPs is essential. We shudder to think of what might go on in a practice where testing is being done universally without any knowledge of whether the patient has a conductive hearing deficit, or sound-checks being done on the equipment.
Regarding interpretation, it is simply not reasonable for general audiologists, who have no neurological background, to make inferences about brain function. Similarly, in our opinion, most physical therapists do not have appropriate audiology training to interpret VEMP's. Because the interpretation process involves both peripheral and CNS pathways, we think that a team combining an experienced audiologist and otoneurologist is optimal.
See this page for a discussion of what we have found to work.
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