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
Transcript
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*
