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394 BAlAnCE FunCtion AssEssmEnt And mAnAgEmEnt
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15. COMPUTERIZED DYNAMIC POSTUROGRAPHY: METHODOLOGY AND INTERPRETATIONS 395
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Appendix 15–A
Table 15–A1. Normal Subject Posture-Evoked Response Latencies*
Reference Number of Subjects Short Latency Medium Latency Long Latency
2 10 42.9 ± 3.3 86.6 ± 6.4 128.6 ± 10.1
8 50 43.5 ± 4.2 89.5 ± 10.0 125.3 ± 17.8
21t 26 34 ± 3 86 ± 6 114 ± 16
t and :j: 74 30.0 ± 4.6 73.3 ± 11.2 104.2 ± 17.0
* All studies used 50-deg/s 4-degree toes-up rotations.In these studies, time was determined by the actual platform movement
onset rather than the movement command, reducing latencies by 10 to 15 ms.
Source:
Data provided to NeuroCom International, Inc. by Chris Diener, Universitat Essen, Germany.
Table 15–A2. Mean (+1.67 SO) Latency Scores as Functions of Translation Size, Direction,
Subject Age
and
Population Latency Scores (ms)
20–59 Years
Movement
Medium back 156 (182) 160 (187) 168 (200)
Large back 137 (168) 148 (171) 155 (178)
Medium forward 164 (194) 164 (184) 170 (196)
Large forward 153 (167) 155 (173) 159 (177)
Table 15–A3. Mean (±2 SD) Active Force Strengths as Functions of Translation Direction,
Size, and Subject Age*
Movement
Small back 03.4 ± (1.4) 04.1 ± (4.6) 04.2 ± (4.5)
Medium back 07.1 ± (5.8) 07.8 ± (5.5) 07.9 ± (7.0)
Large back 08.7 ± (5.2) 09.9 ± (5.5) 10.4 ± (7.4)
(n = 58)
Population Strength Scores
20–59 Years
(n = 29)
60–69 Years
(n = 54)
60–69 Years
(n = 54)
70–79 Years
(n = 28)
70–79 Years
(n = 28)
Small forward 03.6 ± (3.9) 05.1 ± (4.5) 05.2 ± (4.2)
Medium forward 08.4 ± (4.9) 09.0 ± (5.0) 08.6 ± (4.2)
Large forward 10.0 ± (5.7) 10.1 ± (5.3) 09.7 ± (6.0)
*All values are in units of angular momentum normalized for differences in body mass.
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Table 15–A4. Mean (+1.67 SO) Sway Energy Scores as Functions of Toes-Up and
Toes-Down Trial Numbers and Subject Age*
Population Adaptation Scores
20–59 Years
Motion
Toes up
1 85 (160) 76 (125) 75 (132)
2 67 (109) 68 (97) 74 (118)
3 62 (99) 62 (91) 74 (103)
4 54 (76) 59 (81) 72 (111)
5 53 (75) 60 (83) 66 (91)
Toes down
1 76 (134) 75 (120) 72 (113)
2 45 (66) 59 (88) 66 (112)
3 40 (58) 52 (81) 61 (95)
4 37 (54) 49 (77) 60 (99)
5 36 (50) 49 (76) 60 (99)
*All values are in units of sway energy normalized for differences in body mass.
(n = 64)
60–69 Years
(n = 54)
70–79 Years
(n = 28)
Table 15–A5. Mean (Fifth Percentile) Equilibrium Scores as Functions of Age and
Sensory Test Condition
Population Equilibrium Scores
20–59 Years
Condition
1 94 (90) 94 (90) 89 (70)
2 92 (85) 91 (86) 86 (63)
3 Average 91 (86) 89 (80) 88 (82)
4 Average 82 (70) 85 (77) 78 (69)
5 Average 69 (52) 65 (51) 61 (45)
6 Average 67 (48) 65 (49) 53 (27)
Composite 798 (704) 776 (676) 729 (638)
(n = 112)
60–69 Years
(n = 54)
70–79 Years
(n = 28)
15. COMPUTERIZED DYNAMIC POSTUROGRAPHY: METHODOLOGY AND INTERPRETATIONS 397
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Table 15–A6. Mean(±2 SO) COG Alignment Scores as Functions of Sensory Test
Conditions and Age*
Population Alignment Scores
20–59 Years
Condition
Initial
dynamic
1 0.2 ± (2.0) −0.3 ± (1.9) −0.2 ± (1.9)
2 0.3 ± (1.6) −0.1 ± (2.0) 0.0 ± (1.7)
3 0.3 ± (1.6) 0.0 ± (2.1) −0.2 ± (1.7)
4 0.3 ± (1.6) −0.1 ± (2.1) −0.1 ± (1.7)
5 0.7 ± (1.8) 0.1 ± (2.2) 0.2 ± (2.2)
6 0.8 ± (1.9) 0.4 ± (2.3) 0.3 ± (1.7)
*All values are in units of degrees from the center position.
(n = 77)
0.3 ± (1.8) −0.1 ± (2.0) 0.0 ± (1.8)
60–69 Years
(n = 54)
70–79 Years
(n = 28)
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16
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Vestibular-Evoked Myogenic
Potentials (VEMPs)
Devin L. McCaslin and Gary P. Jacobson
history and signifiCanCe
of VestibularmyogeniC Potentials
It is now well known that the vestibular system can be
stimulated using sound. In 1929, Pietro Tullio undertook the original investigations demonstrating the vestibular system’s sensitivity to auditory stimuli. Tullio’s
experiments consisted of fenestrating the bony labyrinth of pigeons, subjecting them to sound produced
by a flute, and observing the motion of the labyrinthine fluids (the frequency of the notes produced by
the flute matched the frequency of the movement of the
endolymph) and eye movements. From these experiments Tullio was able to determine that the vestibular
system could be stimulated using sound (Tullio, 1929).
Von Békésy (1935) built on the findings of Tullio and
observed that when human subjects were exposed to
a high-intensity stimulus (e.g., 122 to 134 dB sound
pressure level [SPL]) there was a corresponding head
displacement toward the stimulated ear. Von Békésy
argued that this reflexive movement of the head in
response to an auditory stimulus was likely due to
movement of the endolymph stimulating the vestibular system.
In the mid-1960s, Bickford, Jacobson, and Cody
(1964) reported the presence of a sound-evoked electrical potential (consisting of a negative waveform peaking at approximately 30 ms) that could be recorded
eVoked
from an active electrode placed on the inion. Initially,
this response was believed to be “neurogenic.” However, the investigators demonstrated that the earlier
components of the response were significantly reduced
in amplitude following administration of a muscle paralyzing agent. Additionally, it was noted that the response
grew exponentially in amplitude with increases in the
level of the tonic electromyography (EMG) in the neck
extensors. This evoked-potential response was subsequently named the inion potential. Further studies by this
group of investigators led to the theory that the vestibular system (i.e., the saccule) was the peripheral origin of
the inion potential (Cody, Jacobson, Walker, & Bickford,
1964; Townsend & Cody, 1971). Finding no good clinical
application for these responses, they were abandoned
for almost 30 years. Then, in 1992 Colebatch and Halmagyi reported the presence of another short-latency large
amplitude myogenic potential recorded in response to a
loud click with an electrode (non-inverting) placed over
the belly of a contracted sternocleidomastoid muscle
(SCM). This response was characterized by a positive
wave (P1) followed by a negative wave (N1) occurring
ipsilateral to the ear that received the stimulus (Colebatch & Halmagyi, 1992; Colebatch, Halmagyi, & Skuse,
1994). There were several lines of evidence suggesting
that the vestibular system, and more specifically the saccule, was the peripheral origin of this evoked response
(Colebatch & Halmagyi, 1992; Colebatch et al., 1994).
As with the inion potential, this sonomotor response
represented a sound-evoked attenuation of tonic EMG
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activity in the SCM (Bickford et al., 1964; Colebatch &
Halmagyi, 1992). It has been suggested that the soundsynchronized decrease in the tonic level of EMG that
occurs during the response is interpreted by the central
nervous system as a sudden loss in postural tone. The
reflexive response from the vestibular system to compensate for this perceived decrease in muscle activity
is to increase extensor muscle tone and decrease flexor
muscle tone (Carey & Amin, 2006). Therefore, the
amplitude of this inhibitory muscle response represents an interaction between the level of tonic EMG and
the size of the inhibitory postsynaptic potential (IPSP)
initiated at the end organ (Colebatch & Rothwell, 2004;
Figure 16–1). Colebatch and Halmagyi (1992) referred
to this response as a vestibular-evoked myogenic potential
(VEMP). Since this response is recorded from the SCM
it is now referred to as the cervical VEMP (cVEMP). This
early work led to the description of the ocular VEMP
(oVEMP), which will be discussed in the second half of
this chapter. Our objective for this chapter is twofold.
First, we will describe how the cVEMP and oVEMP are
recorded and measured. Second, we will present how
different recording and stimulating parameters effect
the responses.
DESCRIPTION OF THE cVEMP RESPONSE
The cVEMP is characterized by a biphasic waveform
that begins with a primary positive waveform followed
by a negative waveform (Figure 16–2). The mean peak
latency of the positive deflection in response to a click
stimulus is ~13 ms and is referred to in the literature as
either P1 or p13. The negative deflection occurs at ~23
ms and is commonly referred to as N1 or n23 (Colebatch & Halmagyi, 1992). The direction of the initial
deflection (i.e., positive) suggests that the cVEMP is an
inhibitory response (i.e., moving from a state where
there is increased EMG activity to a state where there
is decreased EMG activity) (Colebatch & Rothwell,
2004; Wit & Kingma, 2006). The response is primarily
ipsilateral.
CERVICAL VEMP PATHWAY
The sacculo-collic response is a reflexive adjustment of
the musculature in the neck triggered by activation
of the saccule. A reflex consists of an afferent limb (i.e.,
Figure 16–1. The figure shows surface and single motor unit (intramuscular-sternocleidomastoid muscle) recordings that have been
recorded simultaneously in response to a click stimulus. The initial
response is a decrease in single unit activity and is represented by “il.”
From Colebatch, J. G., and Gothwell, G. (2004). motor unit excitability
changes mediating vestibulocollic reflexes in the sternocleidomastoid
muscle. Clinical Neurophysiology, 115(11), 2567–2573. Used with permission from the International Federation of Clinical Neurophysiology.
16. VESTIBULAR-EVOKED MYOGENIC POTENTIALS (VEMPS) 401
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Figure 16–2. Left and right cervical evoked myogenic potential waveform evoked using
a 95 dB nH
L 500 tone-burst.
representing the activation of the peripheral end organ
and ending at the brain center where these signals
are received), central processing (i.e., where the afferent activity is routed to the central origin of the efferent
limb), and an efferent limb (i.e., where the efferent
activity terminates at an end muscle). The afferent limb
of the cVEMP extends from the saccule (i.e., receptor
organ) to Scarpa’s ganglion where neural projections
course through the inferior branch of the vestibular
nerve (McCue & Guinan, 1995; Murofushi & Curthoys, 1997; Figure 16–3). The inferior vestibular nerve
becomes part of the VIIIth cranial nerve and the fibers
projecting from the saccule terminate on interneurons
within the medial and lateral vestibular nuclei. The
efferent limb of the cVEMP reflex descends from the
vestibular nucleus through the vestibulo-spinal tract to
the motonucleus of CN XI. From there the activity is
routed through CN XI to terminate on the SCM (Fitzgerald, Comerford, & Tuffery, 1982). CN XI originates in
the anterior horn of the first five cervical segments
of the spinal column and is the solitary motor input
to the SCM (Krause, Bremerich, & Herrmann, 1991)
(Figure 16–4). This neural pathway has been confirmed
by attenuation or ablation of the VEMP response following selective neurectomies, neural pathologies, or
local anesthetic delivered to SCM (Colebatch & Halmagyi, 1992; Murofushi, Matsuzaki, & Wu, 1999).
STIMULUS VARIABLES
Type of Stimuli
A summary of cVEMP stimulus parameters is presented in Tables 16–1 and 16–2.
Air-Conducted Stimuli
An acoustical stimulus (click or tone burst) is the
stimulus most commonly used in the clinic (Papathanasiou, Murofushi, Akin, & Colebatch, 2014). When a
high-intensity, low-frequency (e.g., 500 Hz), acoustical transient is introduced to the ear canal, the sound
pressure is routed through the middle ear system to
the oval window into the vestibule (Lysakowski et al.,
1998). This creates a situation where the endolymph
in the vestibule is moved and the vestibular hair cells
(type I and type II) are sheared, resulting in transduction (Murofushi et al., 1995). It is worth reiterating
that since sound is being used as a hydromechanical
force to activate the vestibular end organ the patient
need not have hearing to stimulate the vestibulocollic
reflex (VCR) and generate a cVEMP. A requirement for
recording a robust response is that the sound conducting system (i.e., middle ear mechanism) be intact. When
figure 16–3. Anatomy of the vestibular labyrinth.
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figure 16–4. Presumed pathways responsible for
generating the cervical and ocular vestibular-evoked
myogenic potential. Note: IO = inferior oblique; MLF =
medial longitudinal fasciculus;
toid muscle.
SCM = sternocleidomas-
402
16. VESTIBULAR-EVOKED MYOGENIC POTENTIALS (VEMPS) 403
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table 16–1. cVEMP Air-Conducted Stimulus Protocol
Frequency 400–700 Hz or click
Level 120 dB pSPL or greater, caution above
140 dB p
Gating Blackman-weighted, 7 ms maximum
duration
Rate 5 per second
table 16–2. cVEMP Bone-Conducted Stimulus Protocol
Frequency 100–500 Hz
Level 31.6 newtons peak
Gating Blackman-weighted, 7 ms maximum
duration
Rate 5 per second
SPL
a conductive impairment is present, the magnitude of
the stimulus reaching the vestibule may be attenuated
to a degree where the response is eliminated. In situations such as this, the preferred stimulus is vibratory or
mechanical (i.e., vibration produced by a Bruel & Kjaer
4810 Mini-shaker or a mechanical stimulus produced
by a specially outfitted tendon hammer). Calibration
of acoustic stimuli for cVEMP testing should be accomplished using dB peak SPL (Papathanasiou et al., 2014).
Bone-Conducted Vibration
lateral response (i.e., bone vibrator placed on one side
of the head) being larger. The authors also reported
that the contralateral response occurred, on average,
1 ms later than the ipsilateral cVEMP. Thresholds for
cVEMPs generated using the bone conduction stimulus
were better than those obtained using air-conducted
stimuli (i.e., 114 dB SPL versus 97.5 dB SPL; Welgampola et al., 2003). The authors also reported that the
optimal placement for delivery of BCV was 3 cm posterior and 2 cm superior to the external auditory canal.
In a similar study investigating BCV, Sheykholeslami
and associates (2000) recorded cVEMPs using BCV
applied to the mastoid. Tone bursts (i.e., 100, 200, 400,
800, 1600, and 3200 Hz) were used with a stimulus
intensity of 70 dB nHL. The investigators reported that
the largest amplitude VEMPs were in response to 200Hz stimuli. Reliable cVEMP responses were unable
to be obtained when 1600- and 3200-Hz stimuli were
used. Recently, a number of clinical bone oscillators
capable of generating enough output to consistently
generate VEMPs have been made available to clinical
investigators. Håkansson et al. (2018) recorded ocular
and cervical VEMPs using the B81, B250, and Bruel
and Kjaer 4810 Mini-shaker and compared the results
with air-conducted responses. Their findings showed
that for cervical VEMPs, thresholds were significantly
lower with bone conduction than with air conduction
(30–40 dB). The authors also reported that responses
obtained to both ipsilateral and contralateral stimulation at 250 Hz were similar. They suggested that this
finding indicated that it might not be necessary to
switch the bone oscillator from one side to the other
when obtaining responses.
Impairments in the middle ear conductive mechanism
can reduce the amplitude or abolish the cVEMP (Bath,
Harris, McEwan, & Yardley, 1999). Bone-conducted
vibration (BCV) is an alternative stimulus to airconducted stimuli that has been used successfully to
stimulate the otolith organs and subsequently activate
vestibular afferents (i.e., irregular neurons; Goldberg,
2000). Vibratory stimuli can be delivered to the skull
with an audiometric bone vibrator. The stimuli may be
either a click or a tone burst (McNerney & Burkard,
2011). In this regard, bone-conducted tone bursts have
been shown to be capable of generating consistent
cVEMP responses (Sheykholeslami et al., 2000; Yang
& Young, 2003). Interestingly, cVEMPs in response to
BCV have larger amplitudes when lower-frequency
stimuli are used (i.e., 200–250 Hz) (Sheykholeslami
et al., 2000; Welgampola et al., 2003). Welgampola and
colleagues (2003) showed that cVEMP responses using
BCV stimulation were present bilaterally, with the ipsi-
Skull Taps
The otolith mass also can be translated by lightly tapping the head. In this regard, it has been shown that
cVEMP responses can be initiated using a commercially available tendon hammer that connects to the
trigger input of a clinical evoked potential machine
(Brantberg, L
öfqvist, Westin, & Tribukait, 2008; Jacobson & McCaslin, 2007; Figure 16–5). The hammer (e.g.,
from Nicolet Biomedical, Madison, Wisconsin) contains an inertial trigger. Each time the hammer strikes
the patient’s head, a trigger pulse is initiated and this
pulse commands the evoked potential system to signal
average one sweep of, for example, 100 ms. Halmagyi,
Yavor, and Colebatch (1995) reported that cVEMPs
could be consistently recorded by tapping the head in
the frontal region using a similarly outfitted tendon
reflex hammer. Recording the cVEMP in response to
mechanical stimulation requires a slightly different
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