The Sensitivity of Vestibular Evoked Myogenic Potentials in the diagnosis of Semicircular Canal Dehiscence

Abstract

Background: Semicircular Canal Dehiscence (SCD) syndrome is a rare disorder involving the inner ear.  A dehiscence is an absence of an area of bone, and in this case of SCD it occurs in the superior semicircular canals (SCC) creating a mobile third window in the system.

Stimulus evoked electromyography changes can be recorded from various muscles.  Two types of vestibular evoked myogenic potentials (VEMP) are recordable.  The first is the cervical VEMP, which is recorded from the sternocleidomastoid muscle on the neck.  The other is the ocular VEMP, which is usually recorded below the eye.  Both cVEMP and oVEMP have been previously been used clinically in the diagnosis of SCD.

Objective: To establish if there is sensitivity between cVEMP and oVEMP in the diagnosis of SCD.

Method: A literature search using the keywords “semicircular canal dehiscence” and “vestibular evoked myogenic potentials” was performed using SCOPUS and ScienceDirect databases.  These results were narrowed to include studies of adults with SCD, testing oVEMP and cVEMP for the period of January 2012 to December 2016.

Results: Of the 61 results, 4 were considered relevant, two of the studies researched both oVEMP and cVEMP, while the others researched oVEMP and cVEMP independently.  The analysis of the research focused on the tone used, the electrode montage and the amplitude measured.

Conclusion: In the diagnosis of SCD, oVEMP tested using a modified electrode placement using a bone vibration at the Fz position develops a large amplitude that is significant in patients with a SCD.

Table of Contents

Click to expand Table of Contents

Abbreviations

1. Introduction

1.1 Symptoms of SCD

1.2 Types of VEMP

2. Objectives

3. Method

3.1 Inclusion/Exclusion Criteria

3.2 Search Strategy

3.3 Assessment of Study

3.4 Data Analysis

4. Results

4.1 oVEMP Results

4.1.0 oVEMP Protocol

4.1.1 oVEMP Stimulus and Recording Parameters

4.1.2 oVemp Response Parameters

4.2 cVEMP Results

4.2.0 cVEMP Protocol

4.2.1 cVEMP Stimulus and Recording Parameters

4.2.2 cVemp Response Parameters

5. Discussion

5.1 Electrode Montage

5.2 Tone

5.3 Amplitude and Latency

6. Conclusion

7. References

8. Appendices

List of Tables and Figures

Figure 1 – CT Image of SSCD

Figure 2 – VCR Pathway

Figure 3 – cVEMP Response

Figure 4 – oVEMP Response

Table 1 – Study Summary

Figure 5 – oVEMP amplitude

Figure 6 – cVEMP threshold

Abbreviations

SCD– Semicircular Canal Dehiscence

SCC – Semicircular Canals

VEMP – Vestibular Evoked Myogenic Potentials

oVEMP – ocular Vestibular Evoked Myogenic Potentials

cVEMP – cervical Vestibular Evoked Myogenic Potentials

CT – Computed Tomography

EMG – Electromyography

VCR – Vestibulo Colic Reflex

N – Negative Peak

P – Positive Peak

ms – Milliseconds

TB – Tone Burst

BC – Bone Conduction

BCV – Bone Conduction Vibrations

Fz – Frontal zero

Cz – Central zero

ACS – Air Conducted Sound

dB – Decibel

SPL – Sound Pressure Level

µV – microVolts

pSP – peak-equivalent sound pressure level

nHL – normal Hearing Level

1. Introduction

Semicircular Canal Dehiscence (SCD) syndrome is a rare disorder involving the inner ear that was first described by Lloyd Minor in 1998 (Minor et al., 1998).

A dehiscence is an absence of an area of bone, and in this case of SCD it occurs in the superior semicircular canals (SCC) creating a mobile third window in the system. This third mobile window is likely to create a path of lower impedance for the transmission of pressure and acoustic energy to the vestibule, making the vestibular system more sensitive to sound and pressure changes (Chilvers & Davies 2015).

In a normal functioning ear sound travels along the external ear, into the middle ear moving the stapes, and this sound pressure enters via the oval window to dissipate the fluid, not through the normal pathway to the cochlea but through the labyrinith (Ward et al., 2017).

The aetiology of SCD is highly contentious, there are many theories as to why these dehiscence’s occur, it is unclear whether it is congenital (Takahashi et al., 2012), acquired (Parlea et al., 2012) or a two hit phenomenon (Minor et al., 2000).

One theory is that there is a “first event” where some patients who are born with a thin or absent bone overlying the superior semi-circular canal and that a “second event” such as a head trauma or a valsalva manoeuvre (when a person forcefully expires against a closed glottis) that causes the onset of the signs and symptoms of SCD.

Another theory to explain SCD is that dural pulsations (caused by pressure changes in the vascular system of the brain) over the arcuate eminence (a prominence on the anterior surface of the petrous portion of the temporal bone indicating the position of the superior semi-circular canal) result in progressive loss of bone.

Theories that support progressive bone loss of the SCC are supported by observations that the prevalence of SCD increases among older adults (Nadgir et al., 2011).

Figure 1. CT Image of SSCD.Computed tomography scan through the right mastoid cavity and middle cranial fossa. Superior semicircular canal indicated with asterisk. Dehiscence between the superior semicircular canal and the middle cranial fossa indicated with arrow. (Jacqueline et al., 2009)

Overall, the lack of agreement regarding the aetiology of SCD suggests that the syndrome is multifactorial.

1.1 Symptoms of SCD

Patients with SCD syndrome can present with a range of auditory and/or vestibular signs and symptoms that are associated with a bony defect of one or both superior semi-circular canals.  The audiological symptoms are due to the presence of the mobile third window, which results in air conducted sounds losing energy and so an increased threshold is need to hear these sounds.  However in bone conducted sounds, the opposite is true, the low impedance caused by the dehiscence allows sound to travel through the perilymph in the inner ear via the labyrinth resulting in bone conduction sound being heard better than normal.  These results can be distinguished in an audiological appointment where PTA is performed (Ward, et al., 2017).  Vestibular signs and symptoms include vertigo and oscillopsia (movement of objects in the visual field) in response to sound (Tullio phenomenon) and/or pressure (Hennebert sign) and chronic disequilibrium, while auditory complaints include autophony (perception of one’s own body sounds at unusually high levels e.g. eyes moving), bone-conductive hyperacusis (sensitivity to low frequency sounds e.g. heartbeat) and pulsatile tinnitus(Minor et al., 1998).

The diagnosis of SCD syndrome can sometimes be difficult due to the variety of signs and symptoms that are not necessarily unique to SCD, thus “mimicking” other diseases (Merchant et al., 2007).  The audio-vestibular examination for patients with SCD requires a detailed medical history, physical examination, audiometric testing, vestibular evoked myogenic potential (VEMP) testing and imaging by using high resolution computed tomography (CT).

1.2 Types of VEMP

VEMP recording comprise of short latency surface electromyography potentials (EMG – which is a measurement of the electrical activity in muscles as a by-product of contraction) (Colebatch & Halmagyi., 1992).  They may be recorded from over the sternocleidomastoid (SCM) muscles of the neck – termed cervical VEMP or cVEMP (Colebatch et al., 1994),or through the inferior oblique muscles below the eyes during upward gaze – termed ocular VEMP or oVEMP (Rosengren et al., 2005).

cVEMP assess vestibular function through the vestibulocollic reflex (VCR), the VCR arc is often less often studies in literature compared to the well studied VOR (Vestibular Ocular Reflex).  Research presumes the pathway of the VCR is evoked by both of the otholitic organs, their corresponding vestibular nerve fibers, the vestibular nuceli, the vestibule – spinal tract, accessory nucleus, the accessory cranial nerve and then the SCM muscle (Rosengren, Welgampola & Colebatch., 2010).  The VCR is thought to allow for the stability of the head in space as it acts directly on the neck muscles (Mudduwa, Kara, Whelan and Banderjee 2010).

Figure 2. VCR Pathway.  Diagram of the presumed VCR pathway.

The origin of the oVEMP is also controversial with no consensus among researchers.  Some research consider the utricle to be responsible for the response (Manzari et al., 2010), this is the most widely accepted hypothesis, however other research has named the saccule as the organ responsible,(Murofushi & Curthoys, 1997), others suggest both may be responsible (Curthoys & Vulovic.,2011).

Research does agree that the response is mediated by the superior vestibular nerve.  In studies in which patients have a superior vestibular neuritis, the response for oVEMP is absent, and the response for cVEMP is present.  In patients with inferior vestibular neuritis, results show oVEMP present and cVEMP absent (Shin et al., 2012).  These results however do not help in the argument of which otolithic organ is affected.  All of the afferent fibres of the utricle pass through the superior vestibular nerve, whereas the saccule fibres have a pathway that runs through both the superior and inferior vestibular nerve.  Therefore it is summarised that the superior vestibular nerve innervates the utricle, part of the saccule and the lateral SCC.  The inferior vestibular nerve innervates the posterior semicircular canal and most of the saccule.

The cVEMP is composed of two sets of waveforms.   The labelling system used when measuring myogenic potentials was established by Yoshie and Okudaira (1969).  The first potential has a positive peak (p) with an average latency of 13 milliseconds (ms), followed by a negative peak (n) with an average latency of 23 ms, making it known as p13–n23 complex.

Figure 3 illustrates the p13 – n23 complex response waveform in an adult with a normal functioning vestibular system

The oVEMP is also composed of two sets of positive and negative waveforms. The first potential has a negative peak (n) with an average latency of 10 ms, followed by a positive peak (p) with an average latency of 15 ms, making it known as n10–p15 complex.

Figure 4 illustrates the n10 – p15 oVEMP complex response waveform in an adult with a normal functioning vestibular system

If is not absolutely certain whether the threshold or the amplitude of the oVEMP or cVEMP to air conducted  or bone conducted sound, or low or high frequency stimulation along these pathways has the greatest sensitivity and specificity for the diagnosis of semicircular canal dehiscence (Manzari et al., 2012, Zuniga et al., 2013).

2. Objectives

The primary research question of this study is to establish if there is sensitivity between cVEMP and oVEMP in the diagnosis of SCD. Do the differences between oVEMP and cVEMP, change the diagnosis of SCD?

3. Methods

3.1 Inclusion/Exclusion criteria for studies

This review considered the following research methods for inclusion; systematic reviews and non-randomised case control trials.  To be included for appraisal, the publications must meet the following inclusion criteria:

a) articles published from January 2012 to December 2016;

b) articles available in English;

c) articles available in full;

d) articles where participants were male or female,

e) only articles were the SCM was used to measure cVEMP,

f) articles that used either oVEMP or cVEMP individually or together to investigate SCD.

Papers were excluded from the review if the patient group, intervention or outcome measures were not relevant to the clinical question.  The following exclusion criteria were also adopted: duplicates, notes, dissertations, letters, conference papers and editorials. Paediatric articles were excluded and animal studies were excluded.

3.2 Search strategy

The scientific database SCOPUS and Science Direct, were used to enable a multi-pronged strategy, as they cover multiple health related databases and they have full Med Line and PubMed coverage. The strategies searched can be seen in Appendix 1.

The 61 papers identified from the search were examined for their relevance to the clinical question posed and the inclusion/exclusion criteria.  If ambiguous measures were reported these papers were also excluded.

3.3 Assessment of study

Selected studies were also measured for their evidences and appropriateness to the clinical question.  The following criteria appraisal areas were assessed:

1. The population studied (A sufficient risk factor group to control group population),
2. Presence of a healthy control group,
3. Methodology (The procedure of physically measuring the VEMP),
4. Measurement methods similar in all studies,
5. Confounding factors accounted for,
6. Suitable data analysis,
7. Precise values of results,
8. Plausibility of results,
9. Results in comparisons to other available evidences.

3.4 Data analysis

The analysis of the material found was performed at this stage.  First the duplicate references in the databases consulted were eliminated.  Second, by virtue of reading the abstracts, articles that did not meet the established objectives were excluded. (Appendix 2. Process of selecting evidence).

Table 1 -Study Summary

Study	Method	Population/   Sample	Outcome   Measure	Results
Study 1:   Zuiga et al (2013)   Appendix        3	Prospective case – control Study	Case Group: 29 patients with confirmed SCDS   Control Group: 25 age matched volunteers	Ocular and Cervical VEMP’s responses to air conduction sound – cVEMP thresholds, oVEMP n-10 and peak to peak amplitude	cVEMP threshold showed sensitivity and specificity ranging from 80-100% for diagnosis of SCD. oVEMP amplitudes demonstrate sensitivity and specificity >90%
Study 2:   Milojcic et al. (2013)     Appendix        4	Retrospective review   (2004–2010)	Case Group 1: 58 affected SSCD ears in patients (15-26 years)   Case Group 2: 26 unaffected ears of unilateral SSCD (15 to 65 years) Case Control: 21 ears of healthy subjects (21-52 years)	Results of pure tone audiometry and cervical evoked myogenic potentials (cVEMP)	SSCD ears differed from normal ears or unaffected ears of unilateral SSCD patients their cVEMP thresholds at 500 Hz were significantly lower. Logistic regression indicated that the probability of predicting SSCD from the cVEMP thresholds was significant.
Study 3:   Verrcchia et al (2016)     Appendix        5	Prospective case – control study	Case Group 1:   15 patients with unilateral SSCD (23-63 years) Case Group 2: 20 patients with unilateral vestibular loss (18-63 years) Case Group 3: 15 healthy controls* (23-63 years) * unaffected ears from unilateral SSCD group	Results of oVEMP to low frequency vertex vibration (125 Hz)	Amplitude analysis of oVEMP evoked by low-frequency vertex bone vibration stimulation is an additional indicator of SCD syndrome and might serve for diagnosing SCD patients with coexistent conductive middle ear problems
Study 4:   Manzari et al. 2012     Appendix        6	Prospective case -control study	Case Group:   26 patients with a unilateral SSCD (23-85 years) Case Control: 27 healthy subjects (19-71)	Results of oVEMP and cVEMP  to low frequency vertex vibration (500 Hz)	When testing 500 Hz Fz BCV an asymmetrical oVEMP n10 with significantly increased amplitude of contralateral oVEMP n10 (compared with population values of healthy subjects) is a simple useful indicator of SSCD, confirmed by the Cz response. oVEMP testing with 500 Hz Fz BCV allows very simple, very fast identification of a probable unilateral SSCD

4. Results

4.1 Ocular Vestibular Evoked Myogenic Potentials

4.1.0 oVEMP Protocol

Study 1  The participants lay semi recumbent with their upper bodies elevated at a 30 degree angle from the horizontal plane.  The electrode montage consisted of a non-inverting electrode placed on the cheek approximately 5 mm below the eye lid and centred beneath the pupil, the inverting electrode was placed 2 cm below the non-inverting electrode, and the ground electrode was placed on the manubrium sterni.

Study 3  The participants remained seated with their head held so that the Reid’s plane was horizontal (Reid’s plane is described as the head upright, it is typically tilted about 7 degrees nose up with respect to the horizontal plane).  The electrode montage consisted of two pairs of electrodes placed under each eye directly below the pupil, the inverting electrode was placed in the area of the infra orbital ridge, while the non-inverting electrode was placed 2 cm below the inverting electrode. The ground electrode was placed on the uppermost part of the sternum.

Study 4  The participants lay supine on a bed with their head supported in a pillow but positioned so that their head was pitched slightly nose down, with the chin close to the chest. The electrode montage consisted the active electrode being places on the infraorbital ridge (lower margin of the eye socket) directly beneath the centre of the pupil, the reference electrode was placed in a 1.5 cm line away from the active electrode.  The ground electrode was on the chin.

4.1.1 oVEMP Stimulus and recording parameters

Study 1 used a commercial EMG system for VEMP testing.  ACS was delivered monaurally via TDH – 49 calibrated headphones.  Two types of ACS were delivered:

1. 0.1-ms, 105dB nHL (140dB SPL), clicks of a positive polarity at a repetition rate of 5 per second,

and;

2. 500 Hz, 125 dB SPL TB of positive polarity, with a linear envelope, at a repetition rate of 5 per second.

One hundred sweeps were averaged for each VEMP stimulus.

Study 3 used a handheld mini shaker generated bone – conducting stimuli producing 125 Hz TB bone conducting (BC) stimuli a total number of 64 stimuli were delivered during each sequence at 5 stimuli per second.  This cycle was repeated 3 times with short intervals between cycles.  A total of 192 stimuli were delivered to each patient.

Study 4 used 500 Hz Bone conduction vibrations (BCV) delivered by a 4810 mini shaker.  The mini shaker was applied at the Fz position (midline of the forehead at the hairline) as well as the Cz position (top of skull).  The BCV mini shaker was driven by computer generated waveform, usually consisting of 50 repetitions of a 500 Hz tone burst last a total of 7 msec. Stimulus level after amplification was about 130dB force level (0dB force level = 1µN)

4.1.2 oVEMP Response parameters

Study 1 when using the TB oVEMP for 500 Hz the results of the N10 wave was significantly higher in those with SCD than compared controls (median: SCD 23.7 µV; controls 2.3 µV). N10 amplitudes of ≥9.3 µV were reported as having 100% sensitivity and specificity.

The peak to peak amplitudes measure agreed with the previous results (median; SCD 48.9 µV; controls 3.8 µV). Peak to peak amplitudes of ≥17.1 µV were also reported as showing 100% sensitivity and specificity.

When Study 1 reported on the other 16 patients with SCD that underwent click evoked oVEMP, it was found that amplitudes were significantly higher for those with SCD (N10 amplitudes medians: SCD 18.4 µV; controls 1.65 µV. Peak to peak medians: SCD 35.2 µV; controls 3.55 µV).  Peak to peak amplitude ≥9.9 µV has a sensitivity and specificity of 100% for SCD.

When Study 3 reported the oVEMP amplitude results in response to 125 Hz bone conducted stimuli, all 15 SCD ear were found to have a larger amplitude than those recorded for the SCD population’s healthy ear side (SCD 53.0 µV; controls 17.2 µV).  A complete separation of SCD ears from the healthy control ones occurred at 44 µV.  This produced a sensitivity of 73%.

When Study 4 reported on the oVEMP amplitudes of 26 patients with SCD, the n10 amplitude for the oVEMP stimulated with 500 Hz BCV at the Fz position had mean amplitudes of 22.4 µV, with the healthy controls reported as having mean amplitudes of 5.6 µV.

When Study 4 reported on oVEMP amplitudes of the n10 also stimulated by 500 Hz BCV at the Cz position, the average in the SCD population was 10.5 µV and in the healthy population was 0.1 µV.

Figure 5 – Displaying results of N10 oVEMP amplitude using different tone types and positions

4.2 Results - Cervical Vestibular Evoked Myogenic Potentials

4.2.0 cVEMP Protocol

Study 1 participants lay semi recumbent with their upper bodies elevated at a 30-degree angle from the horizontal plane. They were instructed to lift their heads from the head rest by flexing their necks to provide tonic background muscle activity. To ensure adequate SCM activation, the tester monitored that EMG activation was kept at or above 50 µV.

In Study 2 participants activated the SCM by turning their head towards the contralateral shoulder during recording while seated upright in a chair.  Subjects were given feedback by the tester to maintain consistent contraction of the muscle.

In Study 4 participants lay supine on a bed.  To activate the SCM they were required to lift their head from the pillow to tense both the SCM muscles while recording.

The electrode montage for Study 1 consisted of a non-inverting electrode at the midpoint of the SCM, an inverting electrode placed at the sternoclavicular junction and a ground electrode placed on the manubrioum sternui.  The electrode placement for Study 2 in the recording of the ipsilateral SCM, used a non-inverting electrode on the belly of the SCM and the inverting electrode on the tendon attaching to the clavicle, the ground electrode was placed on the forehead.

The electrode placement for Study 4 was not disclosed only that the skin was prepped over the SCM muscle and the surface EMG electrodes were used to record the responses.

4.2.1 cVEMP Stimulus recording parameters

Study 1 cVEMP thresholds were recorded by presenting clicks in decrements of 5dB nHL.  This study used a commercial electromyogtraphic (EMG) system for VEMP testing.

Study 2 used a custom –programmed evoked potential system for both the signal generation and cVEMP recording.  Stimuli were gated tone burst– 2 cycle rise without a plateau.  TDH – 49 headphones were used to deliver frequencies of 250, 500, 750, and 1,000 Hz.  A stimulus rate of 13 per second was used.  cVEMP thresholds were recorded by first obtaining a recording to a high level usually 123 dB pSP, and then decreasing the stimulus by 10dB steps until no cVEMP was recorded. (TB level was then raised by 5dB for the last cVEMP recording.

Study 4 Bone conduction vibrations (BCV) were delivered by a 4810 mini shaker.  The mini shaker was applied at the Fz position (midline of the forehead at the hairline).

4.2.2 cVEMP Response parameters

Study 1 reported that thresholds of SCD participants (median – 75dB nHL) were significantly lower relative to controls (median – 95dB nHL).  A cut-off threshold value of ≤ 85dB nHL provided the combination of best sensitivity (86%) and specificity (90%).

Study 2 reported that cVEMP threshold in the SCD population had a significant lower result at 500 Hz. When ACS was measured at 250 Hz, 500 Hz and 1000 Hz, the following results were achieved.

Figure 6 – Displays the cVEMP thresholds at 250,500 and 1000 Hz of Study 2.

Study 4 reported that cVEMP average amplitude of the ipsiSSCD at position Fz was 186 µV, which was significantly greater than the average cVEMP amplitude for healthy controls 128.6 . The Cz response to the ipsiSSCD was recorded as 98.6 µV, compared to the 15.4 µV in the healthy population.  This result was noted, as the, ipsisSSCD cVEMP to Cz stimulation being significantly smaller than ipsiSSCD cVEMP to Fz stimulation.

5. Discussion

A systematic review was performed to determine the effect of stimulus type and level, electrode montage/muscle activation method and amplitude of responses for participants in the four studies mentioned.  Due to slight differences in the methodology of the studies, it was sometimes difficult to make direct comparisons between the results.

5.1 Electrode montage

The electrode montage in the studies review differed in both the oVEMP and the cVEMP.  The oVEMP electrode placement in studies reviewed have a very similar configuration, with the montage consisted of two pairs of electrodes placed under each eye directly below the pupil, the active (non-inverting) electrode was places in the area of the infra orbital ridge, while the reference (inverting) electrode was placed below the active electrode.  The ground electrode was placed at different areas in each study; namely the mandibular sterni, sternum, and the chin.  The distance between the active and reference electrodes varied.

The electrode montage of the oVEMP is very important, in a study by Piker et al. 2011, ten healthy volunteers were investigated to establish test – retest reliability using a specific electrode configuration, as well as effect of the reference electrode on measuring oVEMP.  (The classic electrode configuration tested was; active electrode placed infraorbital beneath each eye at 1cm, reference electrodes directly underneath at 3cm and ground electrode was placed at Fpz {centre of forehead just below hair line}).  The results of this investigation showed identical results between the test and the retest (10 weeks later).

However when the classic configuration was tested it was found that both the active and reference electrodes recorded the evoked potential.  It was found that with this configuration the reference is not a neutral electrode.

In all the studies analysed the electrode montage may have had a configuration that allowed the reference electrode to be stimulated by the evoked potential, as the reference electrode was positioned at either 1.5 cm or 2 cm under the active electrode.  To allow for this stimulation, a different electrode configuration should be sought.  Leyssens et al., 2017 it is suggested that a “nose reference” electrode configuration should be used, using a more lateral placement of the active electrode (just below the lateral canthus and more on the IO muscle) with the reference electrode to the medial canthus on the nose.  This montage allowing a distance ranging from 3 and 4 cm, depending on facial features.

This highlights the reliability of the electrode montage of the oVEMP and the new montage detailed above would allow for a more accurate result to be procured.

The electrode placement of the cVEMP in study each of the cVEMP although slightly diverse descriptions were given, the studies mostly corresponded with the popular electrode montage reported in literature.  This popular electrode montage consists of the active electrode being placed on the upper third or half of the SCM muscle, with the reference electrode over the lateral end of the upper sternum and the ground electrode at Fz position (Zhou & Cox., 2004).

In Sheykholeslami Murofushi and Kaga study in 2001, in which the effect of electrode location on ithe SCM muscle to test VEMP was investigated.  It was concluded that in the 15 patients tested the middle part of the SCM muscle was the optimal location for recording consistent responses of vestibular evoked myogenic potentials.

It is however how the SCM muscle is activated that causes much dispute when testing cVEMP.  The three studies included in this review all take a different approach to activation of the SCM.  There are two basic techniques used to activate the SCM during cVEMP testing, head rotation or neck flexion.  Both of these activation methods can be achieved in various positions i.e. seated, supine or semi recumbent.

Neck flexion can be attained by lifting head against gravity in the supine position (as in study 1 and 4), or by pushing head forward against a padded bar (Welgampola & Colebatch., 2005).  Head rotation may be accomplished in the seated position (as in study 2), when the participant turns their head toward their shoulder and so the SCM becomes activated.  Isaacson et al., 2006 compared three methods of SCM muscle activation for cVEMP testing and found that when the participant lay supine with their neck rotated to the side lead to the most amplitudes being measured.

Concurrently Wang and Young in 2006, compared the head rotation in the sitting position and the head elevation method with the head in the midline position. This research found that the head rotation method may function as an alternative movement when eliciting cVEMPs in those who cannot endure SCM muscle contraction by head elevation. However, this lower response rate with smaller amplitude precludes the repeated use of the head rotation method.  It was thus concluded that when VEMP responses cannot be produced by the head elevation method, the head rotation method should be used to reduce false-negative results.

In the studies analysed for this research the head elevation method was using in all except study 2.  The use of the head rotation method in this research may have had an influence on the VEMP threshold frequencies of this study.  However with the inclusion of the audiometric test, that noted when an air bone gap was present, may mitigate the VEMP results, as if was found that participants with SSCD had significantly larger air bone gaps at 250 Hz and 500 Hz, and cVEMP thresholds at 500 Hz were significantly lower.  This indicates that recent audiometric threshold is necessary when testing cVEMP and that 500 Hz tone burst ACS should allow for a SSCD cVEMP thresholds to be diagnostic when compared with a healthy ear.

It must be also noted that the use of head elevation in Study 1 and Study 4 did not produce a results that wouldindicate a difference between head elevation and head rotation.

5.2 Tone

As previously mentioned the oVEMP involves the excitatory pathway from the utricle to the contra-lateral inferior oblique muscle.  In the studies review the transducers stimulated both the air conduction pathway as well as the bone conduction pathway.  This comparison between oVEMP with stimulation by air conduction and by bone conduction should be considered with caution, due to the mechanisms stimulus of these types of transduction are different.  As these stimuli activate different otology pathways, differences in the responses and frequency of the oVEMP may be expected.

However it should be noted that in a study by Curthoy (2012), the hypothesis that measuring oculomotor responses to 500-Hz ACS and 500 – Hz Fz BCV records predominantly utricular function, whereas measuring neck muscle responses to these stimuli probes predominately saccular function was tested.  So it is possible to probe the utricular and saccular function separately because of the different neural projections, not because of the stimuli.

The ACS was analysed in Study 1 in which 16 patients underwent click evoked oVEMP’s.  The result of this test using a tone burst produced significant SCD oVEMP results when compared to the healthy controls.  The use of clicks did not produce as large of a response between the groups.

The oVEMP was more sensitive for detecting SCD when using BC stimulation in comparison to the cVEMP with BC, both for the measurement of threshold and amplitudes.  BC oVEMP thresholds levels achieved 83 percent sensitivity and specificity and BC evoked amplitudes in oVEMP had over 80 percent sensitivity and specificity.  125 Hz used in Verrecchias’ study cause almost a threefold difference in the ears with SCD compared to the healthy ones. BC stimulation also overcomes the problems of a conductive hearing loss in SCD, where as AC testing can be compromised.

The use of high frequency stimulation of 4000 Hz has also been researched (Manzari et al., 2013 & 2015).  In this research oVEMP at 4000 Hz was not detectable in healthy controls, but in patients with CT confirmed SCD oVEMP was detected using both ACS and BCV stimuli.  However this measurement has only ever been researched by Manzari et al., and more data is needed to corroborate these results.

5.3 Amplitude and Latency

Amplitude and Latency are important parameters to consider when testing both cVEMP and oVEMP.

Zuniga et al, 2013 concluded that in oVEMP a n10 (N1) amplitude of greater than 9.3 µV and a peak to peak amplitude (N1-P1) of greater than 17.1 µV exhibited 100 percent sensitivity and specificity for SCD.  Kanter and Gurkov (2014) argued that this result was because the oVEMP are likely to reflect utricular function.

In terms of cVEMP in the three studies consider for this research, amplitude and latency are not well reported in the studies.  The measurement of cVEMP varied from threshold results measured in nHL, threshold results measured in pSP and amplitude measured in µV, so not allowing for comparison between the results.  It is however possible to compare the oVEMP and cVEMP amplitudes, especially in reference to Study 4, as both were measured using the same technique.  This study also allowed for the comparison of the position to apply the BCV mini shaker, which was somewhat ambiguous in Study 3 as to what position the mini shaker was placed.  The comparison of the oVEMP and cVEMP at position Fz, leads us to believe that the oVEMP at this position is a clear indication of a SCD.  The oVEMP and cVEMP at the Cz position although does change the results between SCD and healthy studject, it is not as obvious and so therefore is only useful in clarifying the result at the Fz position.  The reason for this change is due to the position of the stimulus changing the direction to which the otolithic organs are activated.  It would seems that the Fz position allows for greater movement of the hair cells that have an effect on the IO and the inferior rectus muscles, that movement from the Cz position (Lin et al., 2010).

In Verrecchias’ study it was concluded that oVEMP stimulated by low frequency bone vibrationsallowed for the amplitude of the oVEMP to identify SCD with a sensitivity of 87% and a specificity of 93%.

Limitations of the studies used are the age profile of the participants, as many of the studies used age matched comparisons.  The general agreement is that age has a definite consequence on the vestibular system.  Welgampola and Colebatch (2001) have shown a reduction in amplitude and increase in thresholds for click evoked cVEMPs after the age of 60 years.  Age seems to cause a similar appearance on oVEMP results (Colebatch et al, 2013).  It is likely that the age matching of healthy subjects lead to more skewed results and the use of health controls less than 60 years of age could give a better separation of results.

It is important to note that even with a simplified recording process and low performance cost, it is necessary to have uniformity in measuring the parameters in clinical application.  A standardised methodology is fundamental to achieve reliable and sensitive test results.

6. Conclusion

In conclusion, the findings of this report emphasise that even with the simplest recording method, it is essential for clinical application that this test has uniform parameters. Standardisation of the methodology is essential for the recording of reliable and sensitive data.  All the studies examined suggest that VEMP is a reliable diagnostic tool; however oVEMP was identified as the most sensitive test in the diagnosis of SCD.  With the change of electrode montage to laterally of the IO and medially too the canthus of the eye, use of low frequency bone conduction stimuli at the Fz position, for this test the amplitude of the wave should be sufficiently coherent at a 90 percent sensitivity to diagnose SCD.

Although VEMP in the case of SCD has been well diagnosed, there is diminutive information in the literature regarding reproducibility, given the results reported in this literature; features of VEMPs are very sensitive to variables that may be influenced by the examiner. The field should therefore work on a better standard for VEMP recordings.

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8. Appendices

Appendix 1

Search Strategy

Strategy            Combinations

Strategy 1         ((“semicircular canals”[MeSH Terms] OR (“semicircular”[All Fields] AND “canals”[All Fields]) OR “semicircular canals”[All Fields] OR (“semicircular”[All Fields] AND “canal”[All Fields]) OR “semicircular canal”[All Fields]) AND dehiscence[All Fields]) AND (vestibular[All Fields] AND evoked[All Fields] AND myogenic[All Fields] AND potential[All Fields]) AND (“2012/01/01″[PDat] : “2016/12/31″[PDat])

Strategy 2:((“semicircular canals”[MeSH Terms] OR (“semicircular”[All Fields] AND “canals”[All Fields]) OR “semicircular canals”[All Fields] OR (“semicircular”[All Fields] AND “canal”[All Fields]) OR “semicircular canal”[All Fields]) AND dehiscence[All Fields]) AND ((“neck”[MeSH Terms] OR “neck”[All Fields] OR “cervical”[All Fields]) AND vestibular[All Fields] AND evoked[All Fields] AND myogenic[All Fields] AND potential[All Fields]) OR (ocular[All Fields] AND vestibular[All Fields] AND evoked[All Fields] AND myogenic[All Fields] AND potential[All Fields]) AND (“2012/01/01″[PDat] : “2016/12/31″[PDat])

Strategy 3: ((“semicircular canals”[MeSH Terms] OR (“semicircular”[All Fields] AND “canals”[All Fields]) OR “semicircular canals”[All Fields] OR (“semicircular”[All Fields] AND “canal”[All Fields]) OR “semicircular canal”[All Fields]) AND dehiscence[All Fields]) AND (ocular[All Fields] AND vestibular[All Fields] AND evoked[All Fields] AND myogenic[All Fields] AND potential[All Fields]) AND (“2012/01/01″[PDat] : “2016/12/31″[PDat])

Strategy 4: ((“semicircular canals”[MeSH Terms] OR (“semicircular”[All Fields] AND “canals”[All Fields]) OR “semicircular canals”[All Fields] OR (“semicircular”[All Fields] AND “canal”[All Fields]) OR “semicircular canal”[All Fields]) AND dehiscence[All Fields]) AND ((“neck”[MeSH Terms] OR “neck”[All Fields] OR “cervical”[All Fields]) AND vestibular[All Fields] AND evoked[All Fields] AND myogenic[All Fields] AND potential[All Fields]) AND (“2012/01/01″[PDat] : “2016/12/31″[PDat])

Appendix 2

Process of selecting evidences