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VEMP testing and SCD (Superior Canal Dehiscence)

Timothy C. Hain, MD Page last modified: June 5, 2019 button Return to testing index

INTRODUCTION

VEMP stands for vestibular evoked myogenic potentials, and SCD stands for Superior canal dehiscence. The VEMP test is very useful in diagnosing SCD.

There are several variants or "flavors" of VEMP testing. The input for the VEMP may be air-conducted sound (AC), bone-conducted sound (BC), vibration, or electrical (galvanic). The muscle being measured may be the sternocleidomastoid ("cervical"), the extraocular muscles ("ocular"), or any number of limb muscles (i.e. triceps, etc).We will use the terminology "cVEMP" to denote vestibular evoked myogenic potentials elicited from the sternocleidomastoid muscle. When we use the terms "oVEMP" or tVEMP or whatever, the small letter indicates that a muscle other than the SCM is being monitored - - such as ocular or triceps. When we use the unqualified "VEMP", we mean any vestibular evoked myogenic potential (i.e. cVEMP, oVEMP, tVEMP, etc).

Thus by combining one of the four inputs (AC, BC, vibration, galvanic), and one of the 2 main output muscles (cervical, ocular), we have the possibility of 8 different variants of VEMPs. For example, a bone-conducted ocular VEMP would be a "BC oVEMP".

An example of a positive cVEMP in SCD.

Figure left: AC cVEMP obtained in an individual with left sided superior canal dehiscence, using a Bio-Logic Navigator Pro. Right -- threshold AC cVEMP in same person. See the SCD page for his CT scan.

cVEMPS in SCD

There is no controversy that cVEMPs are useful in screening for SCD, and also that a properly done temporal bone CT scan is the "gold standard" for SCD.

In our practice in Chicago, as of mid 2019, we have done 5691 VEMPs (both cVEMP and oVEMP), and thus our experience level is very high.

cVEMPs so far have been mainly useful in documenting abnormally low thresholds in persons which largely occurs in persons with fistula or Superior Canal Dehiscence syndrome (SCD) (Brandtberg et al, 1999). Low threshold to sound, clinically, is called the "Tullio" effect. If one does not do thresholds, 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.

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 cVEMPs in a person with an air-bone gap (see hearing testing page) is also suggestive of SCD.

Fife et al (2017) reviewed several techniques of doing cVEMPs and their usefulness in diagnosing SCD. They reported that thresholds are the most useful to diagnose SCD. cVEMP raw amplitudes are not effective to detect SCD. cVEMP corrected amplitudes "may also be used to distinguish SCDS from controls". Thus according to this rather extensive review, either thresholds or corrected amplitudes can be used. We ourselves favor thresholds, and given that a VEMP is present at high intensity, we will always combine it with a low intensity stimulus to see whether or not the person's thresholds are low. We no longer use raw amplitudes as a diagnostic method for SCD. We tried and discarded using "corrected" VEMPs, because our feeling was that they added noise.

cVEMP's don't do so well in a few situations:

We have not found cVEMPs to be diagnostic of the small window fistulae that we encounter most frequently in our practice.

cVEMPs (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 cVEMP in any person with a complaint of dizziness induced by sound (Tullio's), should their regular VEMP be normal. We have stopped doing binaural VEMPs entirely.

We have also encountered a few patients with very low threshold cVEMPS (i.e. 65 dB) on both sides, who do NOT have SCD on CT scan of the temporal bone. These are generally adolescents, and we think that one should be very slow at getting CT scans of the temporal bone when one encounters this situation.

oVEMPs in SCD

An example of a positive oVEMP in SCD.

oVEMP in SCD
oVEMP obtained in a patient using sound. The right ear has clear SCD on temporal bone CT scan.

Concerning the utility of oVEMP in SCD, our current strong impression is that oVEMPs are much better than cVEMPS for detecting SCD. We prefer the absolute amplitude criterion, supplemented by the threshold criterion. In other words, we like to see both high amplitude (e.g. 20) and low threshold. We use the 500 Hz auditory input, and we gave up on doing oVEMPS using bone stimuli as it was clumsy and unnecessary. We do think that the high amplitude should probably be age adjusted. Amplitudes vary roughly 3 fold between the young and the old. (Piker et al, 2013). There are some youngsters that meet the amplitude and threshold criteria, but don't have SCD on CT scan. Below, we have indented the papers which which we don't agree, for various reasons. Sadly, this is most of them.

Janky et al (2013) reported that "OVEMPs in response to air conduction stimulation (click and 500 Hz toneburst) provide the best separation between SCDS and healthy controls and are therefore the best single-step screening test for SCDS. " We agree.

Fife et al (2017) reviewed 5 oVEMP studies in SCD, and reported a rather amazing variety in amplitudes. Criteria proposed for diagnosis of SCD based on amplitude included 1.5uV, "2.5 SD above mean", 9.3uV, and 8.25 uV. These criteria are smaller than normal oVEMPs reported by Piker et al (2013). It appears to us that technique must be wildly variable among labs. We ourselves use a much higher criterion -- 20 uV for SCD. We rather routinely get normal amplitudes of 5. Fife and associates said "oVEMP testing using either specific thresholds or amplitudes may be used in patients to aid in making an SCDS diagnosis". We think that the operational word here is "may".

Interestingly, the "ceiling" for oVEMPs seems to be much larger (compared to the mean) than cVEMPs. In other words, it is not all that unusual to see an oVEMP of 40 (as above) in SCD, in spite of the average oVEMP in our hands being only about 5. We have seen some as high as 120 ! This is the real reason why oVEMPs are superior to cVEMPs for diagnosis of SCD.

Fife et al (2017) also reported on two different threshold studies with oVEMPs, suggesting that thresholds of 102 or 99 dB were fairly sensitive. In our clinic, we look for much lower thresholds, about 65. We use the Bio-Logic NavPro, and perhaps thresholds vary according to equipment.

Manzari et al ( 2012b) suggested that "oVEMP testing with 500 Hz Fz BCV allows very simple, very fast identification of a probable unilateral SSCD." In 2013, Manzari and others suggested that oVEMP being present at 4000 hz is even better. Again, we would like to see more data. In 2015, Manzari et al reported another case.

Verrechchia et al (2016) suggested that oVEMPS were larger on the affected ear, using 125Hz single cycle vertex vibration. We are a bit dubious about the frequency content of this signal, and wonder why it is different than Manzari's report.

Govender et al (2016) reported that in 13 SCD patients, abnormal thresholds were found in 85% of air conduction cVEMPs and 62% of oVEMPs. Bone conduction brought the oVEMP sensitivity up to 83%. cVEMP's showed evidence for saturation with larger responses at smaller intensity, while oVEMPs did not. The implication of this paper is that cVEMPs are likely better than oVEMPS for diagnosis of SCD. We don't agree with this (see Janky paper above).

Verrecchia et al (2019) reported on oVEMPs in 10 patients with SCD, and compared them to 135 with no SCD (the diagnosis of SCD was partially based on their clinical impression -- we have no gold standard here). They suggested that the limits could be lower than 20 (such as 16) for a positive oVEMP. We think this is unlikely to be true, and we have not changed our clinic limits of 20.

We presume that oVEMPS that are present, in spite of a 30 dB conductive loss at 500 hZ, strongly support SCD, but we have no proof of this idea. Practically, there are not very many people with conductive hearing patterns in SCD, but quite a few people with large oVEMPs, so this observation is not very useful.

Using oVEMPs to follow SCD.

As SCD gets worse, oVEMPs can get much bigger. Because oVEMPs can grow proportionally so much more than cVEMPs, they are a far better screening tool than cVEMPs or audiograms. It would be irrational to use CT-temporal bone for screening of oVEMPS due to the high radiation load.

oVEMPs increasing with time
oVEMP done in 2016 OVEMP 2019
First oVEMP done in 2016, 48.45 uV Second oVEMP done in 2019 (top half only as wouldn't fit on page), response of 126.

This is an example of a patient being followed with oVEMPs for 3 years -- the response just keeps growing. Note that the average oVEMP is about 10, and a response of 120 is thus about 12 times the average. This is far more growth in response than might occur with a cVEMP.

oVEMPs are not perfect at diagnosing SCD -- a false positive.

oVEMPs are not always correct
Positive oVEMP negative CT
oVEMP suggesting SCD (amplitudes of 21 and 23, low threshold on L).
oVEMPs are not always correct
Negative CT images
CT scan of temporal bone showing normal SCC on both sides.

Although oVEMPS are very good for diagnosing SCD, i.e. they are sensitive, they are not 100% specific. The case illustrated above had a very large oVEMP on both sides as well as a low threshold on the left (at 60). Nevertheless, the CT-temporal bone did NOT show SCD as is clearly shown above, on either side. This patient meets criteria for Meniere's disease, and perhaps has a dilated utricle.

It is our impression that these false positives are more common in younger patients (the one above was roughly 40).

Autoplugging -- and other false negatives

An interesting interaction between technique and sensitivity is that oVEMPs are done upright, and cVEMPs (at least in our setting) are done supine. This means that in patients with a large dehiscence, there might be plugging of the open canal by dura while upright, and opening up while supine. This hypothesis has yet to be tested.

Another possible reason for a false negative oVEMP is poor technique. If the electrodes are positioned wrongly, there may be no response at all. oVEMPs are trickier to do than cVEMPs, as the muscles are much smaller.

Finally, lack of cooperation on the part of the patient might prevent a good recording. If the patient pretends to look upward, but actually looked straight ahead, this would reduce the response.

References:

Copyright (c) Timothy C. Hain, MD 2002-2007

Copyright June 5, 2019 , Timothy C. Hain, M.D. All rights reserved. Last saved on June 5, 2019