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VEMP testing and SCD

Timothy C. Hain, MD Page last modified: January 21, 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.

cVEMPs 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.

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.

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 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 2016 concerning the utility in SCD. One would think that because oVEMPs are smaller and noisier than cVEMPs, they would be worse than cVEMPS. This is the most reasonable conjecture at this point.

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. 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.

 

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Copyright (c) Timothy C. Hain, MD 2002-2007
Copyright January 21, 2017 , Timothy C. Hain, M.D. All rights reserved. Last saved on January 21, 2017