Vestibular Evoked Myogenic Potentials for a Pediatric Population
A.K. Fenn, B.D. Corneil and D. Mostafa
Abstract:
People rely on balance and stability throughout activity and in everyday life and the structure to
which we owe our balance and stability to is known as the vestibular system. Vestibular dysfunction
affects individuals of all ages and is frequent in society. Therefore finding a suitable test to help identify
these vestibular dysfunctions in all populations is crucial. Currently, clinical tests for vestibulospinal reflex
(VSR) require an uncomfortable posture, making it difficult for all populations to perform. In this study,
we assess the feasibility of alternative testing methods on a pediatric population. Specifically, we focus
on surface EMG recordings during a series of postures with an applied auditory stimulus in order to drive
vestibular evoked myogenic potentials (VEMPs) as this test provides immediate results that are cost
effective for a clinical setting. Target muscles involve the sternocleidomastoid (SCM) and the splenius
capitis (SPL) as they act as synergists and can therefore be recorded simultaneously for comparison.
Ultimately, the goal is to provide a case for further research into VEMPs regarding which muscles to
targets and what postures are involved, while ensuring the proposed test is cost and time effective. We
have found that SCM and SPL VEMPs can be achieved through seating and standing neck turn postures,
making it easier on the subjects being tested.
Introduction:
Organs within the inner ear provide two main functions for an individual: hearing and balance.
The cochlea transduces our sense of sound from mechanical sound waves produced in the auditory canal
via the malleus, incus and stapes. Important structures for an individual’s balance also lie within the inner
ear that make up the vestibular system. Now imagine for a second you were back in your youthful and far
more physically active days, playing a sport you love, whether it be hockey, soccer, football, etc. No matter
the activity, your youthful self required the stability and balance to perform and maneuver around all
obstacles in your way. However, none of these activities are possible without the ability for your body to
react and provide the stability you need. This rings true in everyday life, as even the simplest tasks such
as walking require a complex amount of sensory and motor signals that help us to maintain balance. In
fact, many individuals experience problems associated with balance and stability, with an estimated 35%
of Americans over 40 years of age indicating some form of vestibular dysfunction (Agrawal et al., 2009).
Therefore it is crucial to understand how an individual is able to sustain balance via the vestibular system
and how to test and individual for a balance deficiency. An individual’s sense of balance relies in large part
on their semicircular canals and their otolith organs, which together form the vestibular system. Three
semicircular canals (anterior, horizontal and posterior) are responsible for sensing angular head rotation,
allowing you to realize the positioning of your head in all three rotational axes. Next, the otolith organs,
consisting of the saccule and utricle, are responsible for sensing linear acceleration and head position
relative to gravity. When the body is disturbed, these vestibular organs lead to powerful stabilizing
reflexes in the body. Say for example, you slip on a patch of ice. Your vestibular organs are quick to respond
by activating neck flexors to tuck your chin to your chest and arm extensors in order to brace your fall
with your arms. An individual with a vestibular deficiency may suffer serious injury in a similar situation,
as their vestibular system may be unresponsive, preventing these reflexes from occurring. Therefore it is
important to test the vestibular systems functionality in all populations, meaning the ideal method of
testing should be cost and time effective but also easy enough for all demographics to perform.
Contemporary means of testing the vestibular system have yet to provide effective and
comfortable testing methods that cater to all demographics. Caloric testing is a means of testing
semicircular functionality by applying water to the auditory canal with temperatures at roughly 30 and
44oC (Teggi et al., 2009). Although effective, patients experience uncomfortable effects throughout the
test similar to that of vertigo, making it extremely uncomfortable and unsuitable for a pediatric
population. Similarly, other tests are designed to test the functionality of an individual’s otolith organs
using surface EMG electrodes (Colebatch et al., 1994). These tests assess a vestibular evoked myogenic
potential (VEMP) that indicate otolith function, in particular saccular function due to its close physical
proximity to the cochlea within the inner ear (figure 1). Again, these tests provide immediate results in a
cost effective fashion, however they lack ease of posture for pediatric, elderly and infirm populations. The
focus of this study is to assess the feasibility of alternative VEMP testing methods.
Originated by Bickford’s et al. research with regards to the inion response and its myogenic origin,
VEMPs have since been discovered and, as mentioned before, are used as a means of testing an
individual’s vestibulospinal reflex (VSR) and predominantly saccular function (Rosengren et al., 2010).
VEMPs were first described by Colebatch et al. (1994), who identified EMG activity of the
sternocleidomastoid (SCM) to be vestibular in origin. More specifically, Colebatch et al. localized a short-
latency myogenic response labelled as the p13 and n23 to be purely vestibular derived (Figure 2,
Colebatch 1994). Further research has shown that VEMPs can be driven by air-conducted (AC) sound,
bone conducted vibration and electrical stimulation and recorded by surface electrodes over a target
muscle (Rosengren et al., 2010). AC sound has been shown to yield the largest VEMPs. This relates back
to the close physical proximity of the saccule to the cochlea, which allows for cross-talk. Cross-talk simply
Figure 1: Inner Ear
Otolith organs (circled in yellow) lie in close
proximity, with the saccule being the closer of the
two. Therefore fluid movement within the inner ear
has the potential to stimulate multiple organs in
close proximity simultaneously.
means that the saccule is stimulated along with the cochlea, as the fluid movement caused by an auditory
stimulus spans out to the saccule as well. Stimulation of the saccule with an adequate amount of
background activity on a muscle lead to initiation of a VEMP. Muscle background activity is needed as the
VEMP is classified as an inhibitory response (Rosengren et al., 2010), which can only be observed providing
there is sufficient muscle contraction. After sufficient background activity has been reached, there is a
linear relationship between response amplitude and sound intensity (Rosengren et al., 2010).
Furthermore, VEMPs are not localized to the SCM which is classified as a cervical VEMP (cVEMP).
Ocular VEMPs can be measured using surface electrodes capturing extraocular EMG activity (Rosengren
et al., 2010). In a similar fashion, muscles throughout the body such as biceps brachii, flexor carpi radialis,
soleus and medial gastrocnemius have been shown to deliver VEMP readings (Naranjo et al., 2015). This
is because many muscles in the body may play a role in balance and stability with regards to a postural
threat.
In this study, we will be focusing on cVEMPs driven by contraction of the SCM and the problems
associated with this muscle as it is currently considered the best for clinical testing. Currently, cVEMPs
recorded from the SCM are common place for clinically testing a patient’s vestibular function. Clinical tests
for the SCM cVEMP can aid in the diagnosis of vestibular neuritis, benign positioning vertigo, Meniere’s
and other vestibular related deficiencies (Rosengren et al., 2010). However, attaining sufficient levels of
tonic activation of the SCM is hard to achieve. Thus, the test requires an extreme posture that is hard for
certain demographics such as children and elderly to sustain (Kelsch et al., 2006). This posture involves
Figure 2: Colebatch et. al. p13 – n23 VEMP
The arrow indicates the moment of stimulus onset.
The x – axis represents the latency of EMG activity
and the Y – axis represents response amplitude.
Measured p13 and n23 peaks are identified
accordingly. Peaks n34 and p44 were determined to
be acoustic in nature.
patients laying down in the supine position and raising their head and shoulders off the table
approximately 30o while turning their head to either side of their body (Figure 3). The difficult nature of
this procedure limits patients’ ability to reach adequate background activity (BGA) for a sufficient duration
of time. Furthermore, the SCM is prone to false-negatives that increase in frequency with age (Camp et
al., 2016). Significant numbers of false negatives may be attributed to the fact that the SCM is a large and
very powerful muscle, making it highly fatigable and causing late recruitment during the most forceful of
head turns/ neck flexions (Corneil et al., 2001). Such a level of forced contraction makes this test
increasingly difficult for children and elderly patients, especially if they do in fact have a vestibular
problem. Therefore, there may be a better suited target than the SCM that allows for easier testing and/or
more accurate results. Recently, studies out of Western University were done to improve current clinical
techniques. These studies involved surface EMG recordings from the current target SCM and a purposed
target, the splenius capitis (SPL). In this instance, SCM and SPL cVEMPs were recorded in a simple standing
posture, with the subjects turning their head to different degrees (90, 60 and 45 degrees). From this, SCM
measured cVEMPs from only the most extreme contraction at a 90 degree turn, making it the most
strenuous for subjects, whereas SPL measured cVEMPs at a lesser degree (Camp et. al., 2016).
Furthermore, with regards to ease of testing, all subjects stated that they experienced no pain or
discomfort. As a logical next step, we introduce similar methods to a pediatric population in order to test
the feasibility of further exploration.
This study will also focus on the SPL using surface EMG electrodes in a pediatric population. SPL
functions as an ipsilateral neck turner, therefore making it a synergist to the contralateral SCM. Choosing
a synergist muscle like the SPL will allow us to collect recordings of both muscles simultaneously for
comparison. SPL contraction has been tested before for ease of contraction compared to the SCM (Gulec
et al., 2013). However, during a seated, head-turned posture, cVEMPs were recorded in both left and right
SPLs. Clearly both SPL muscles would not have been activated, therefore it leads us to believe that cross-
talk may have taken place amongst another dorsal neck muscle such as the trapezius or SCM. Another SPL
experiment had been done using the neck extended in a prone position that suggested an excitatory
reaction (Wu et al., 1999) that leads us to believe it may not be vestibular in origin. The Wu et. al.
experiment also required the subjects to lay down in a prone position and extend their neck to look
forward, which may also be difficult for certain populations (Figure 4). Therefore, our test will provide
simultaneous results for the ipsilateral SCM and the contralateral SPL with regards to the auditory stimulus
to determine the feasibility of conducting cVEMPs in a way that makes it easier for a weaker pediatric
population.
Figure 3: Kelsch et. al. Clinical Posture
Pediatric subjects often propped themselves up
using their elbows in the proposed clinical position.
Subjects were also required to turn their head to
either side while holding this position.
Methods:
Subjects:
3 children participants (2 female, one male ages: 6, 11 and 13) volunteered for study. All individuals were
free from conductive or sensorineural hearing loss and no history of vestibular or neurological disease.
Before the study, the parents of all participants gave consent approved by the human ethics committee
of Western University.
Experimental setup:
Upon consent, subjects were wiped with rubbing alcohol on the skin superficial to the
sternocleidomastoid, splenius capitis muscles and on their forehead. Once the skin has dried completely,
surface EMG electrodes (Delsys Bagnoli Desktop System) were placed so that reading bars lay
perpendicular to the fibres within the sternocleidomastoid and Splenius Capitis muscles and a ground
electrode was placed on the forehead to record background activity. Aligning the surface EMG electrodes
with the orientation of their respective muscle fibres is crucial in this experiment as it minimizes the
potential of cross talk from surrounding muscles. However, we do note that cross talk inherently exists to
a certain degree within the readings, but has no bearing on the results if a positive VEMP is reached. This
is due to the fact that any VEMP response from any muscle shows that a subject’s vestibular system is
Figure 4: Wu et. al. SPL Contraction Posture
This position utilized the SPL as a neck extensor.
intact and would thus provide enough evidence in a clinical setting. The contacting components of the
EMG surface electrodes were dabbed with a conducting gel for to optimize background activity readings.
Vet wrap was then used to secure surface electrodes in place during subject postures, preventing them
from shifting to other muscles of the neck. Auditory stimuli were delivered to the ear using a Er2 insert
earphones (etymotic) and Stewart Audio PA-50B Pa half rack amplifier, playing a sound file created in
MATLab from a Macintosh laptop that consisted of 500Hz tone bursts of 4 ms duration including a 1 ms
rise and 1 ms fall delivered at 7.4 Hz for 30-60 seconds. Subjects were then guided through 3 postures
(seated, standing and clinical) (figure 5) throughout the experiment, with the auditory stimulus delivered
to the ear contralateral to the neck turn. During the posture, readings were focused on the contracted
muscles (the ipsilateral sternocleidomastoid and contralateral splenius capitis, with respect to the
auditory stimulus). Sound intensity was calibrated at 500Hz using a sound pressure level meter (Bruel &
Kjaer, Atlanta, USA) and delivered at a maximum intensity of 118 dB peak SPL. In practice, the volume was
reduced to ~100db, which was sufficient to provoke a cVEMP.
Data Analysis:
All measurements were made with subjects in three postures as previously stated. Recordings were made
simultaneously from bilateral surface SCM and SPL electrodes. A surface EMG test was considered positive
for the cVEMP if the amplitude of the stimulus triggered average contained positive and negative peaks
within 10-30 ms, which is identified as the vestibular response (Colebatch, 1994). Both the positive and
Figure 5: Camp et. al.
Figure shows the placement and
orientation of electrodes in a neck
turned posture.
negative peaks must also exceed 2 standard deviations (SD) from the mean EMG threshold. Latencies for
cVEMPs were measured at the first and second response peaks (p13 and n23). Background EMG activity
on all muscles was measured from the mean rectified activity over the 40 ms preceding stimulus delivery.
All data are presented as mean +/- SD unless otherwise stated. Due to the small scale nature of this
practice, results will be laid out regarding each individual subject.
Results: *contralateral and ipsilateral refer to muscles with regards to the applied auditory stimulus*
Subject 1 (Male, age 11):
Turn – Posture Contralateral Splenius Ipsilateral Sternocleidomastoid
Right – Seated Positive (10,15) Positive (18,23)
Right – Standing Positive (11,15) Positive (16,25)
Left – Seated Positive (14,19) Positive (19,23)
Left – Standing Positive (14,19) Positive (14,17)
Right – Clinical Negative Positive (13,19)
Left - Clinical Negative Positive (13,16)
Turn – Posture
Contralateral Splenius Ipsilateral Sternocleidomastoid
Right – Seated
Right – Standing
Left – Seated
Left – Standing
Right – Clinical
Left – Clinical
Results:
Average VEMP latencies for the SPL p13 and n23 were 12.3 +/- 2.1 and 16.6 +/- 2.0 respectively.
SPL tested positive in all proposed seating and standing postures. SPL recorded a false negative in the left-
turn clinical posture. Average VEMP latencies for the SCM p13 and n23 were 15.5 +/- 2.6 and 20.5 +/- 3.7
respectively. SCM recorded positive VEMPs in all postures. Clinical tests were done for 30 seconds as
opposed to a minute duration for the seated and standing postures due to the difficulty of the test.
Our first subject (male, age 11) gave the best representation of the proposed SPL muscle
recordings due to minimal hair on the back of the neck, yielding maximum skin contact for the surface
electrodes. Furthermore, this subject was able to hold all postures, including the clinical posture,
sufficiently throughout the experiment. What is important to note is that this subject received positive
tests for all seating and standing postures for both the SPL and SCM, where the subject experienced no
discomfort. This clearly shows that although the amplitude of SPL readings are significantly lower than
that of the SCM, they both yield a cVEMP, which indicates the intactness of the VSR with an easier posture
compared to the clinical.
Subject 2, (Female, age 13):
Turn – Posture Contralateral Splenius Ipsilateral Sternocleidomastoid
Right – Seated Positive (18,26) Positive (19,26)
Right – Standing Positive (18,28) Positive (19,26)
Left – Seated Positive (13,24) Positive (21,26)
Left – Standing Negative Positive (17,26)
Right – Clinical Negative Positive (9,17)
Left - Clinical Negative Positive (11,15)
Turn – Posture
Contralateral Splenius Ipsilateral Sternocleidomastoid
Right – Seated
Right – Standing
Left – Seated
Left – Standing
Right – Clinical
Left – Clinical
Results:
Average VEMP latencies for the SPL p13 and n23 were 16.3 +/- 2.9 and 26 +/- 2.0 respectively. SPL
recorded false negatives in the Left-turn standing posture as well as all clinical postures. Average VEMP
latencies for the SCM p13 and n23 were 15.5 +/- 2.6 and 20.5 +/- 3.7 respectively. SCM recorded positive
VEMPs in all postures.
Our second subject (female, age 13) gave best insight to the struggles associated with the SPL as
a target for cVEMPs. Given the fact that majority of females have relatively long hair, contact becomes
difficult for the surface electrode to the skin superficial to the SPL. Regardless, cVEMPs were still present
in majority of the seated and standing postures (when contact with the skin was continually achieved
throughout the 30-60 seconds). SCM cVEMPs were observed in all postures, which is encouraging because
there is potential to achieve positive cVEMPs at an easier posture. Therefore, in practice, it may be
beneficial to start with an easier posture such as seated or standing and progress to clinical method if a
cVEMP is not reached.
Subject 3 (Female, age 6):
Turn – Posture Contralateral Splenius Ipsilateral Sternocleidomastoid
Right – Seated Negative Negative
Right – Standing Negative Negative
Left – Seated Positive (12,16) Positive (16,20)
Left – Standing Positive (12,16) Positive (16,20)
Right – Clinical Negative Positive (13,17)
Left - Clinical Negative Positive (13,20)
Turn – Posture
Contralateral Splenius Ipsilateral Sternocleidomastoid
Right – Seated
Right – Standing
Left – Seated
Left – Standing
Right – Clinical
Left – Clinical
Results:
Average VEMP latencies for the SPL p13 and n23 were 12.3 +/- 0.6 SD and 17 +/- 1.7 SD
respectively. Average VEMP latencies for the SCM p13 and n23 were 14.5 +/- 1.7 SD and 19.3 +/- 1.5 SD
respectively. False negatives were prevalent in all right-turn seated and standing postures as well as the
clinical posture for the SPL. Perhaps barriers such as ear wax or EMG contact existed in the test that
prevented cVEMPs from occurring in right-turn and clinical postures. However, this subject displays the
importance of many recordings and the clinical posture. Had only the clinical procedure been carried out,
this test would have been read as positive immediately. Therefore, similar to subject 2, this test may
require a progression of postures in order to be most effective.
Discussion:
Feasible?
As a preliminary feasibility study, the main point to take away is that cVEMPs were successfully measured
on both the SCM and/or SPL throughout all postures in this small sample of three children. More
importantly, all our subjects had successful cVEMPs in the seated and/or standing postures. This indicates
that although there may be potential barriers that need to be accommodated for throughout the test, it
is possible to record cVEMPs while catering to the ease required to test all demographics. The goal of this
study is to provide a test that is cost and time effective while providing a posture that is plausible for all
demographics to sustain. Therefore, we acknowledge that this study is feasible regardless of
complications involved, as it sets a stage for further research in the field that may be able to one day
develop a new clinical practice to test an individual’s VSR.
Potential future studies:
We know that VEMPs can be recorded throughout the entire body, therefore problems such as hair
superficial to the SPL can be avoided if a more suitable target can be identified. In choosing potential
muscles, it is important to consider the posture that would reach a sufficient level of background activity
and whether or not that posture is feasible for all populations as well. Also note that this experiment was
done with only three children. In which, the ages spanned from 6-13. Without a sufficiently scaled study,
it is impossible to make conclusions about the pediatric population as a whole and therefore studies need
to be carried out with a wider range of subjects for more accuracy. Potential targets in for further research
such as trapezius come to mind as it is superficial and can be contracted by simply shrugging. However, it
is important to imagine trying to instruct a child who is only a few months of age through the same
exercise. The answer then becomes, “how do you get a baby to shrug their shoulders?” For instance, in
our study, a doctor may need to physically aid a baby in the posture or receive help from a parent holding
a toy in order get children to turn their head to a desired side. All these factors combined make it difficult
to propose new targets for VEMPs. Perhaps further research can stay focused on SCM and SPL activity
while accounting for barriers (ex. Shave hair superficial to the SPL). But regardless, the main goal of further
research should be to focus on retaining an effective and easy test that is able to provide adequate results.
As we saw throughout this study, positive VEMPs can be achieved by targeting multiple muscles with a
progression of postures. This suggests that utilizing more target muscles and a wider range of postures
may provide an effective way to test an individual’s VSR in all populations.
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