Timothy C. Hain, MD
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|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 this 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 reminent from the saccule's use as an organ of hearing in lower animals.
|Figure2. VEMP circuitry. Sound stimulates the saccule, which activates the inferior vestibular nerve, lateral vestibular nucleus, medial vestibulospinal tract ipsilaterally, and then the sternocleidomastoid muscle in the neck (Colebach et al, 1994; Uchino et al, 1997; Kushiro et al, 2000; Murofushi et al, 1996; Wilson et al, 1995).|
The pathway for the VEMP test 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) and the leg muscles via the lateral vestibulospinal tract (LVST). Sound evoked VEMPS are almost completely unilateral. (Colebach et al, 1994; Uchino et al, 1997; Kushiro et al, 2000; Murofushi et al, 1996; Wilson et al, 1995)
|Figure 3: 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 3 illustrates a VEMP test. 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 disurbances. 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).
The VEMP response consists of an initial positivity (p13) followed by a negativity (n23), see figure 3 above. It is an evoked potential. Later components have a lower stimulus threshold and are non-vestibular.
Tinnitus is a relative contraindication to click and tone-burst VEMP testing and an alternative stimulus should be considered in these subjects.
EMG electrodes are applied to the middle third of anterior neck muscles (sternocleidomastoids) and the supine patient holds their head up unsupported, using the anterior neck muscles. Another method of obtaining activation is to have patients sit upright with their chin turned over the contralateral shoulder to tense the SCM muscle. Subjects are instructed to tense the muscle during acoustic stimulation, and relax between runs. If the neck muscles are not activated, no VEMP is produced. The reflex scales to tonic EMG.
Loud clicks or tone bursts (typically 95-100 DB nHL or louder) are repetitively presented to each ear in turn at 200 msec intervals. The optimum frequency lies between 500 and 1000 Hz. The sternum is used as a reference and the forehead as a ground. Myogenic potentials are amplified, bandpass filtered (5-1K Hz), and averaged for at least 100 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. Because of the high intensity of the sound used to evoke these responses, calibrated headphones should be used. In our opinion, a minimum of three repetitions should be obtained on both sides, to be sure that the VEMP is reproducible or absent, as the case may be. An exception to this can be made if the first two repetitions are of large amplitude and nearly identical (e.g. see figure 2). In general, 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.
There are some difficulties intrinsic to this methodology. Assuring neck muscle activation is the biggest one. While one can run a VEMP very successfully with the patient's head being held up vs. gravity, this is tiring. VEMP's 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. Without EMG normalization, the limits of normal are very very large (i.e. 20 to 400).
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 this stimulus compared to a click is that it requires lower absolute stimulus intensities. 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.
Bone conduction VEMPs
Skull taps and bone conduction tones can also be used to elicit VEMP's. 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 VEMP's, using frequencies of about 200 Hz. Clinical bone vibrators need 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)
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. However, 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), because presumably most of the. 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. While it should detect a complete vestibular nerve loss, it is generally not challenging to detect a complete vestibular nerve lesion using ENG and other modalities, and thus it would simply offer a technically more challenging method of establishing a complete vestibular nerve lesion. Galvanic VEMP's may nevertheless prove useful using threshold or latency information. Galvanic VEMP's are not suppressed by anodal current, which suggests that VEMP's do not require the irregular afferents (Bacsi and Colebach, 2003). More study is needed.
A closely related test to the VEMP was described very recently 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 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 VEMP's are presently attributed to the saccule, the data presented so far suggests that hearing is not necessary for VEMP's. 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 VEMP's, which would be against this idea (Watson and Colebatch, 1998).
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), or absent (no reproducible wave). We consider these very conservative criteria. . Decreased amplitudes and increased threshold is seen on persons 70 and older. We have not found VEMP's to be of great utility in the diagnosis of SCD, perhaps because these patients are so rare, but we have found them useful in the diagnosis of vestibular neuritis.
What data is there in the literature ?
The VEMP literature is rapidly increasing and it seems that there are considerable valuable diagnostic information to be obtained. VEMP's 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). 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 ? 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 VEMP's in a person with a conductive hearing loss is also suggestive of SCD.
Whether or not the VEMP test can be used to detect saccular and saccule pathway damage remains to be seen. Suggesting that it can, 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, VEMP's 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. 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.
VEMP's would be expected to be reduced or absent in persons with bilateral vestibular loss, such as due to aminoglycoside ototoxicity. Only a few patients have been studied so far (Murofushi et al, 1998). In our practice, we have 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 questionsed 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 VEMP's, one cannot clearly relate an absent VEMP to absent saccule function in this situation.
It would be of great interest to know whether VEMP's are absent in persons with intact hearing, but no vestibular responses. We have tested several patients with intact hearing, but greatly reduced vestibular responses (one due to gentamicin, the other idiopathic), and found preserved VEMP's. This may be because the saccule is spared, or because VEMP's in humans can be generated by the cochlea too.
In Otosclerosis, VEMP's should be absent. A person with a present VEMP and conductive hearing loss may have SCD.
In Meniere's disease, 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, perhaps due to saccular dilitation. Absent VEMPs in advanced disease may represent collapse of the saccule. 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 (2004), we would like to see confirmation of this by others.
Prolonged latency of VEMP's has recently been suggested to be a sign of a retrocochlear (vestibular nerve) lesion, such as is found in vestibular neuritis. Abnormal VEMP's (asymmetrical or long latency) are reported in about 25% of persons diagnosed with vestibular neuritis (Murofuschi et al, 1996).
VEMPS are generally absent or reduced in persons with acoustic neuroma.
VEMPs are often asymmetrical in spasmodic torticollis (Colebatch, Di Lazzaro et al. 1995).
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). VEMP's test mainly 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.
Although VEMPs can be obtained in people with complete hearing loss, the hearing loss has to be of the sensorineural type. Persons with conductive hearing loss often do not have VEMP's, presumably because the sound stimulus, conventionally delivered by earphones, does not get to the saccule. This means that VEMP's 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.
Failed vestibular nerve section: 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. VEMP's might, in theory, be useful for detecting residual function in the inferior vestibular nerve. However, as VEMP's require a very strong stimulus, it seems unlikely that they would be very sensitive. Study of this question is needed.
VEMP tests are commonly performed by an audiologist or an electrophysiology technician. Audiologists are often associated with otolaryngology practices (ENT doctors), while electrophysiology technicians are often associated with Neurology practices. It is not a difficult test and does not require much training for one to perform. Thus technically, either setting is quite reasonable. Because hearing testing is absolutely required to interpret the test to be sure there is no conductive hearing loss (see above), often the most convenient process is to have an audiologist do the VEMP test and audiogram in a single sitting. We think that a team combining an experienced audiologist and otoneurologist is optimal.
VEMP testing is a very useful but still rapidly evolving and immature technology. If you are thinking about doing VEMP testing, unless you feel comfortable with electrophysiology, it might be best to wait for a year or two while the market settles down. If you are in the market for a new device, we think it is best to get a multipurpose machine - can it do ECOG testing ? ABR testing ? Other types of evoked responses such as SSEPs and VEPs ? Does it do OAE's ? Does it interface with NOAH ( a clinical database) ? If it is an external device, how does it connect to your host computer (USB is best, serial is worst). Is there a possibility of the device supporting an EMG feedback display and normalization of responses to the EMG ? We do not know of any commercial device that can do all of these things, but in a few years, it is likely that this will be the state of the art. At this writing (7/2004), a stand-alone "box" should cost about $10,000. Examples of vendors include Bio-Logic, GN-otometrics, and Nicolet.
Be extremely sure that the VEMP machine has a calibrated sound output. Because sound levels are loud, and thresholds are critically dependent on only 10 db steps of loudness, it is critical to be sure that you are getting the right sound volume. A 10 db difference between ears might obliterate a VEMP.
Exercise due diligence. Ask the company for references -- who is using their equipment already ? Will they talk with you ? What has been their experience ?
Be especially sure to consider the company's technical support. As VEMP testing in general is evolving rapidly right now, it is very likely that you will need technical support. Be very cautious if you cannot reliably reach technical support when you call them, or if technical support is an option that costs more money. Look also to see what the device does "out of the box", and whether or not additional software is needed to do what you want. If you purchase a unit, we also suggest insisting on a 1 month return, should the unit not work out in your environment. Because the technology is evolving rapidly, you may wish to rent or lease equipment rather than buying it outright.