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VEMP testing and SCD

Timothy C. Hain, MD Page last modified: December 30, 2017 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 the end of 2017, we have done 4122 VEMPs (both cVEMP and oVEMP), and thus our experience level is 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

These papers are somewhat divided as of 2017 concerning the utility of oVEMP in SCD. The numbers are not large in reports. Our current impression is that oVEMPs are 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. about 17) 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.

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. It appears to us that technique must be wildly variable among labs. We ourselves use a much higher criterion -- 17 uV for SCD. We rather routinely get normal amplitudes of 5. We have done many oVEMPs. 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".

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.

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 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.

 

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Copyright (c) Timothy C. Hain, MD 2002-2007
Copyright December 30, 2017 , Timothy C. Hain, M.D. All rights reserved. Last saved on December 30, 2017