Portable Stimulus Generator for Obtaining High-Frequency (8

J Am Acad Audiol 3: 166-175 (1992)
Portable Stimulus Generator for Obtaining
High-Frequency (8-14 kHz) Auditory
Brainstem Responses
Stephen A. Fausti*
Richard H. Frey t
James A. Henryt
Perry G. Robertsont
Robert S. Hertert°
Currently, the most useful application of high-frequency (?8 kHz) auditory evaluation is for
serial monitoring of patients receiving potentially ototoxic agents . Many individuals, however,
are unable to respond to behavioral auditory test techniques . An objective evaluation method
such as the auditory brainstem response (ABR) is valuable with difficult-to-test individuals .
Laboratory instrumentation has been demonstrated to evoke high-frequency-specific (8-14
kHz) ABRs with reliable intrasubject latencies over time . This instrumentation is limited,
however, because it cannot be transported to the patient confined to a hospital room . A
portable device has now been constructed to deliver high-frequency (8-14 kHz) tone-burst
stimuli comparable to the lab system . This digital/analog high-frequency tone-burst stimulus
generator weighs less than 5 pounds . It can be utilized with any ABR signal averager capable
of generating a positive (condensing) click at = 4.8 volts. Case studies are presented to
demonstrate the frequency-specific responses obtained with these high-frequency toneburst stimuli.
Key Words:
Auditory brainstem response (ABR), high-frequency tone-burst stimuli,
ototoxic agents, hearing loss
t is well known that therapeutic drugs such
as the aminoglycoside antibiotics, and the
chemotherapeutic agent cisplatin, carry
with their administration the risk ofototoxicity .
Serial behavioral auditory monitoring during
treatment with these agents has been utilized
to identify hearing loss within the standard
frequency range (<-8 kHz), with ototoxic threshold changes reported to occur initially at the
highest frequencies tested (Jackson and Arcieri,
1971 ; Fee, 1980 ; Kobayashi et al, 1987 ; Skinner
et al, 1990) .
*U .S . Department of Veterans Affairs Medical Center,
Portland, Oregon (PVAMC), Oregon Health Sciences University, Portland, Oregon ; + Auditory Research Laboratory,
PVAMC, t Biomedical Engineering Section, PVAMC, and
§Biomedical Engineering Section, PVAMC, Portland, Oregon
Reprint requests : Stephen A . Fausti, VA Medical Center
(M/S 151J), P .O . Box 1034, Portland, OR 97207
Studies evaluating high-frequency (>8 kHz)
thresholds have revealed that hearing loss as a
result of ototoxicity typically begins in, and is
often limited to, the high-frequency region
(Fausti et al, 1984b, 1984c; Kopelman et al,
1988 ; van der Hulst et a1,1989) . Thus, a protocol
designed to detect hearing loss at the earliest
possible time during treatment with potentially ototoxic agents should include monitoring of frequencies in the high-frequency range.
If monitoring is limited to the conventional
frequency range, detection of hearing loss could
indicate that the loss may have already progressed to the range critical for verbal communication . With high-frequency monitoring used
as an early warning tool, such progression can
be prevented (or its effects reduced) with selection of alternative treatment procedures .
The reliability and validity of behavioral
auditory monitoring techniques depend on the
active cooperation and attention of the patient.
Based on a 3-month survey at the U.S . Depart-
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ment of Veterans Affairs Medical Center, Portland, Oregon (PVAMC), however, it was estimated that 30 percent of hospitalized patients
who are receiving treatment with aminoglycosides and cisplatin are too ill to provide
reliable voluntary responses. Because of their
reduced ability to report ototoxic symptoms,
these unresponsive patients can be more susceptible to hearing loss than more responsive,
less ill patients . For such individuals, a hearing
evaluation technique that does not require a
voluntary response would be valuable in monitoring the effects of ototoxic agents on the auditory system . The auditory brainstem response
(ABR) is one such technique .
Click-evoked ABR has been utilized with
infants (Bernard et al, 1980), comatose adults
(Piek et al, 1985), and with young burn-wound
patients (Hall et al, 1986) to evaluate latency
changes resulting from aminoglycoside antibiotic treatment . With click stimuli commonly
used to elicit the ABR, the greatest acoustic
energy concentration is seen between 2 and 4
kHz (Mitchell et al, 1989). Correlations between click-evoked ABR threshold and hearing
thresholds at 2 to 4 kHz have been reported by
Jerger and Mauldin (1978) and Gorga et al
(1985) . Therefore, although click-evoked responses provide valuable diagnostic information, data obtained with this stimulus limit the
prediction of thresholds to frequencies between
2 and 4 kHz. Thus, hearing loss can progress to
the point of communicative impairment before
being detected with the click-evoked ABR.
Measurement of early auditory evoked
potentials with more frequency specificity than
that offered by the click is important for a more
accurate estimation of hearing sensitivity in
patients unable to respond to conventional
behavioral test methods. Previous investigations dealing with frequency-specific ABRs have
primarily utilized stimuli at frequencies within
the conventionally tested range (<8 kHz) of
hearing (Purdy and Abbas, 1987 ; Gorga et al,
1988 ; Fjermedal and Laukli, 1989 ; Purdy et al,
1989). In order to identify significant shifts in
high-frequency (8-20 kHz) thresholds produced
by ototoxic agents before the speech frequency
range is affected, stimuli specific to these frequencies are needed .
It has been demonstrated that high-frequency (>_8 kHz) tone bursts are capable of
eliciting measurable responses in normal-hearing persons (Fausti et al, 1984a, 1991a;
Rappaport et a1,1985; Gorga et al, 1987). Initial
high-frequency-specific ABR data were gath-
ered in this laboratory using a rack-mounted
instrumentation system (Portland Auditory
Research/Veterans Affairs-Tone Burst
[PARVA-TB]). Intra- and intersession reliability of ABRs to high-frequency tone bursts produced by the PARVA-TB was demonstrated as
comparable to click-evoked response reliability
(Fausti et al, 1991b) . Additionally, evaluation
on responsive hospitalized patients receiving
ototoxic agents has been performed using the
PARVA-TB (Fausti et al, 1984a) . The PARVATB unit utilized in these documentation studies, however, is limited to laboratory usage and
cannot be transported to hospital wards . Collaboration between this laboratory and Biomedical Engineering Section at PVAMC has
resulted in the design of the Portland Auditory
Research/Veterans Affairs-Portable Tone Burst
(PARVA-PTB), a light-weight high-frequency
tone-burst generating device .
A population of unresponsive patients receiving ototoxic medications is found in virtually every major inpatient medical care center .
A means of objectively monitoring hearing in
the high-frequency range may be the only way to
prevent or reduce socially and vocationally
handicapping hearing loss in these patients . It
is this population that is ultimately targeted by
the portable instrumentation and methodology
developed and validated for the purpose of objectively detecting high-frequency hearing loss .
he power for the PARVA-PTB is provided
T by a Tektronix TM 501 Power Supply module, which measures 3-3/4 inches in width, 5
inches in height, and 17 inches in length and
weighs less than 5 pounds . A portable evoked
potential signal averager (Bio-logic Traveler)
was acquired for the purpose of interfacing with
the PARVA-PTB . A combination of digital and
analog circuits, the PARVA-PTB was designed
to maximize the benefits of each of these signal
processing methods . Digital circuits have advantages in precision timing, zero offset control, logic and gating flexibility, and programmability . Analog circuits offer advantages in
low distortion sine-wave generation, compact
and efficient active filtering, and linear power
amplification. Thus, while digital circuits allow
for programmable output functions in the time
and amplitude domains, analog circuits provide quality high-frequency stimuli, which could
not easily be obtained with digital components .
The portable stimulus generator is composed of nine (five digital and four analog)
Journal of the American Academy of Audiology/Volume 3, Number 3, May 1992
System Clock
Trigger Input
Signal Conditioner,
Stimulus from
Signal Avenger
Frequency Selection from
Stimulus Generator
Atanuator Selection ~ '
from Stimulus Generator
Low Distortion
Sine We, . Generator
8,10,12614 kHz
Passive Filter
Ramp Wavoform
Figure 1 Block diagram of components (modules) contained within the
PARVA-PTB portable stimulus generator.
Multiplying 12 Bit
Active Butterwortir
Flit. .. for 8, 10, 12
8 14 kHz
Fixed Gain
Power Amplifier
Output to Headphones
modular sub-systems, or function blocks (Fig .
1) . Each section performs a specific task in
stimulus generation .
Digital Circuits
Trigger Input and Signal Conditioner
This module AC-couples a square-wave
stimulus from the signal averager to the stimulus generator. The square-wave is clipped and
shaped by resistive, capacitive and semiconductor network components to generate a digitally compatible start-pulse .
System Clock
This section provides digital pulses for all
logic and Digital/Analog (D/A) devices. When a
start-pulse is received, the clock is gated-in to a
binary counter, which is wired to sequentially
address an Erasable Programmable Memory
(EPROM) device . This digitally shapes the analog-generated sinusoid within desired trapezoidal parameters (in the current case, 0.2 msec
rise-fall times with a 1.6 msec plateau) .
In addition to the above gating functions,
logic sequences also reset the system to a quiescent state after each stimulus delivery . Thus,
the system is essentially turned off between
trigger impulses, eliminating any potential for
system noise (bleed-through) occurring during
the averaging period.
Erasable Programmable Memory
This is a programmable device that allows
user-specified binary values to be stored at each
of its many addresses . By programming sequential addresses with precalculated values,
these data can be passed to a D/A converter to
generate a particular waveform, or envelope .
Multiplying DigitallAnalog Converter
This 12-bit module function (Analog Devices DAC7541AKP) offers full four-quadrant
multiplication and 1/2 dB differential linearity.
EPROM values are used to multiply the reference voltage input to the D/A converter. This
design uses the precision sine wave as the input
voltage reference. When binary values are zero,
output voltage is zero . At values other than
zero, the output is a product of the binary value
and the input sinusoid .
Analog Circuits
Sine-Wave Generator
This resistor-programmable, sine/cosine
wave oscillator (Burr-Brown 4423) has a range
from 0.002 Hz to 20 kHz. Up to 5 kHz, frequency
distortion is <_ 0.2 percent, and from 5 to 20 kHz,
frequency distortion is <_ 0 .5 percent. Amplitude
stability is rated at 0.05 percent per degree of
temperature (centigrade) . Output frequency is
front-panel controlled .
In this module, Butterworth filters are constructed from ultra-low noise operational amplifiers (Burr-Brown OPA-27FN). These amplifiers are designed for professional audio equipment and feature low noise (3 .8 nV/Hz at 1 kHz),
low offset (25 VV), low drift (0 .6 pV/°C), high
open-loop gain (>- 120 dB), high common-mode
rejection (>-114 dB), and high power-supply
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Portable Generator for High-Frequency ABRs/Fausti et al
rejection (>_ 110 dB ). The high-pass range of the
filters is coupled to tone-burst frequency so that
the filter range corresponds appropriately at
each test frequency. The use of filtration introduces a stimulus onset delay constant . From
stimulus triggering (electrical onset) to stimulus arrival at the earphone transducer (acoustic
onset), a constant time delay will occur from
passing the stimulus through a filter . In this
unit, the delay has been documented at a mean
0.33 msec across frequency (standard deviation
= 0.02 msec). Latency data can be corrected for
this time delay.
This is a resistor network that attenuates
from maximum output in 10-dB calibrated increments . The attenuator is front-panel controlled . Output from the attenuator is directed
to the power output amplifier.
Output Amplifier
This is a fixed-gain, low-distortion, linear
audio amplifier (Burr-Brown OPA-27FN, with
external transistor drivers) . Output is impedance matched to the earphones for maximum
performance and is front-panel switched between left and right earphones .
The logic system retains the PARVA-PTB
in a normally quiescent state, awaiting a startpulse from the signal averager . Once a pulse is
received, the system clock and counter are gated
ON to digitally shape the sinusoid . This shaped
wave is then directed through the high-pass
filter section to the power amplifier and out to
the earphones.
The trigger source for this external stimulus generator is an electrical square-wave stimulus (which generates the acoustic click) output
from any signal averager capable of generating
a positive (condensing) click at = 4.8 volts (± 0.2
V) . This voltage level on the Bio-logic Traveler
occurs at 91 dB nHL. The averager is set to
either right or left channel, as the stimulus
generator itself provides for earphone selection . Frequency and output amplitude are controlled at the front panel of the PARVA-PTB.
This unit provides four frequencies, 8, 10, 12,
and 14 kHz, and six attenuation steps from
maximum output to -60 dB in 10-dB increments. At the level of the measuring microphone in the coupler, average maximum acoustic output across frequencies was approximately
126 dB peak-equivalent SPL (peSPL). Meas-
urements were obtained as described in Fausti
et al (1991a). Presentation rate is controlled by
the averager.
Spectra for 8 and 14 kHz tone-burst stimuli
created by the PARVA-PTB are shown in Figure 2. These spectra were plotted from traces
captured on a Hewlett-Packard (H-P) 3561A
Dynamic Signal Analyzer . In previously reported studies (Fausti et al, 1984a, 1991a;
Rappaport et al, 1985), it was observed that
certain stimulus parameters provided clear and
repeatable response waveform information.
Based on these studies, stimulus parameters
were set for the stimulus generator unit at 0.2
msec rise-fall time and 1.6 msec plateau, for a
total of 2.0 msec duration (between zero voltage
points). EPROM capability allows resetting
these parameters if desired. Acoustic output in
dB peSPL is measured using the flat-plate
coupler calibration technique of Fausti et al
(1979) and the continuous tone oscilloscope
displacement matching technique described in
Fausti et al (1991a).
he following case studies are presented
to demonstrate various cause and effect
factors in resulting ABRs to the high-frequency
1 00
Frequency (kHz)
Figure 2 Spectra for 8 and 14 kHz tone bursts delivered
from the PARVA-PTB at 115 dB peSPL.
Journal of the American Academy of Audiology/Volume 3, Number 3, May 1992
tone-burst stimuli delivered by the PARVAPTB . Four examples are included to show resulting ABRs from : (1) a normal-hearing person; (2) an individual with normal hearing in
the frequencies below 4 kHz, sloping to a severe
loss at 8 kHz ; (3) a person with a severe hearing
loss in one ear, and a severe loss in the other ear
except for a narrow range of improved hearing
centered at 9 kHz where hearing is essentially
normal; and (4) a patient receiving long-term
gentamicin treatment with consequent highfrequency hearing loss . Taken together, these
examples support the frequency-specific nature of the high-frequency tone bursts generated by the PARVA-PTB . The fourth subject
illustrates the ability of the ABR evoked by
these high-frequency tone-burst stimuli to reveal a latency/morphology change, which correlates with a change in behavioral hearing threshold at the corresponding frequency.
All subjects were tested in an Acoustic
Systems 19701A double-walled, RF-shielded
sound-treated booth . Pure-tone thresholds were
obtained with a Virtual 320 audiometer (Fausti
et al, 1990a) using the clinically acceptable,
modified Hughson-Westlake ascending-descending technique of C arhart and Jerger (1959) .
The audiometer is calibrated to ANSI (1989)
standards. Threshold responses (audiograms)
for case study subjects are shown on a linear
scale with all tested frequencies spaced equally
on the abscissa . ABRs were obtained from subjects reclined to 30 degrees from horizontal with
the PARVA-PTB and the Bio-logic Traveler
using a standard 2-channel electrode montage
(forehead/common, vertex/non-inverting, and
ipsilateral and contralateral mastoid/inverting
placements). A complete run for each ABR
condition consisted of 1000 stimulus presentations, consecutively repeated, at a rate of 11 .1
per second . Bioamplifier filter settings were at
100 and 1500 Hz . Wave identification techniques used with ABR click stimuli (Chiappa et
al, 1979 ; Beattie et al, 1986 ; Picton et al, 1988)
were employed as a guideline for peak identification with high-frequency tone-burst stimuli.
Responses shown in the following case studies
are the added ipsilateral waveforms ofrepeated
runs .
Subject 1
The right-ear audiogram for an 18-year-old
male with normal conventional frequency hearing is shown in Figure 3. Immittance screening
revealed normal tympanograms and acoustic
.2s .5 1 2 3 4 6 6 9 10 11 12 14 16 16 20
Frequency (kHz)
Figure 3 Right-ear behavioral thresholds (in dB SPL)
for an 18-year-old normal-hearing male .
reflexes (Wiley et al, 1987 ; Shanks et al, 1988).
His ABRs for click stimuli and for 8 and 14 kHz
tone bursts at = 60 dB SL (95 dB peSPL) in the
right ear are seen in Figure 4. This subject was
selected as representative of normal-hearing
individuals tested with the PARVA-PTB . Note
that, although not as morphologically well de-
Latency (ms/div)
Figure 4 Auditory brainstem responses for clicks and
for 8 and 14 kHz tone bursts from the right ear of an 18year-old normal-hearing male . Each trace represents the
added ipsilateral waveforms of repeated runs . This subject demonstrates that, although not as morphologically
well defined as click-evoked responses, ABRs to highfrequency tone bursts clearly identify peak latencies for
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Portable Generator for High-Frequency ABRs/Fausti et al
threshold analysis revealed a normal threshold
for 8 kHz in the left ear (10 dB HL). Also,
unusual for an individual with a hearing loss
this severe, language and prosody were normal,
although her fundamental speaking pitch was
exceptionally high .
Tympanograms were normal bilaterally .
High-frequency sensitivity evaluation revealed
threshold responses at high sound pressure
levels (80-120 dB SPL) through 14 kHz for the
right ear (Fig . 7) . An island of improved hearing
Frequency (kHz)
Figure 5 Behavioral thresholds (in dB SPL) for a 57year-old male with bilateral sensorineural hearing loss .
fined as click-evoked responses, his high-frequency tone-burst ABR waveforms are easily
marked .
Subject 2
A 57-year-old male presented hearing
thresholds within normal limits bilaterally for
frequencies below 4 kHz and with elevated
thresholds at 6 and 8 kHz to 70 to 85 dB SPL
(Fig. 5) . High-frequency evaluation revealed
thresholds of 90 to 110 dB SPL at 9 to 12 kHz.
Immittance screening revealed normal tympanograms and acoustic reflexes . No behavioral pure-tone responses were obtainable above
12 kHz for either ear.
Click-evoked ABRs were well defined at 80
dB nHL (Fig . 6A). Tone-burst-evoked ABRs
were obtained for 8 and 10 kHz at 115 dB peSPL
(Fig . 6B) revealing poorly defined waveforms
with limited evidence for marking peak
latencies . These results are consistent with the
elevated pure-tone thresholds obtained at these
frequencies. It is important to note that this
individual's conventional frequency hearing
sensitivity appears to have had no influence on
the high-frequency tone-burst-evoked ABR. That
is, the high-frequency specificity of the toneburst stimuli is supported.
Subject 3
A 24-year-old female with severe bilateral
congenital (maternal rubella) hearing loss in
the conventional frequency range (<_ 8 kHz for
the right ear and <- 6 kHz for the left ear) was
referred to this facility because conventional
Latency (ms/div)
Figure 6 Auditory brainstem responses for clicks (A)
and for 8 and 10 kHz tone bursts (B) from a 57-year-old
male with bilateral sensorineural hearing loss . Each trace
represents the added ipsilateral waveforms of repeated
runs . Abiological response (i .e ., stimulus absent) is shown
for comparison . This subject demonstrates that although
normal hearing thresholds below 4 kHz contribute to a
normal click-evoked ABR, they do not appear to contribute to responses to high-frequency tone bursts in the
frequency range where hearing thresholds are impaired .
This supports the contention that high-frequency tone
bursts are stimulating specific basal areas of the cochlea
corresponding with those frequencies.
Journal of the American Academy of Audiology/Volume 3, Number 3, May 1992
screening revealed normal tympanograms and
contralateral acoustic reflexes bilaterally . Baseline ABRs were obtained to click stimuli at 80
dB nHL. Baseline ABRs to 8,10,12, and 14 kHz
tone-burst stimuli were obtained at 105, 105,
110, and 115 dB peSPL, respectively . For all
waveforms, wave V was clearly identified . Beyond baseline, 14 hearing evaluations were
performed every 2 to 3 days during treatment .
Additionally, immediate and 6-month posttreatment follow-up testing was accomplished .
The only changes in behavioral hearing thresholds were seen in the right ear for frequencies
.25 .5
9 10 11 12 14 16 18 20
Frequency (kHz)
Right Ear
Figure 7 Behavioral threshold responses (in dB SPL)
for a 24-year-old female with severe bilateral sensorineural
hearing loss and with left unilateral island of improved
in kHz
high-frequency hearing sensitivity.
sensitivity from 8 to 11 kHz was seen for the left
ear, with threshold responses obtained through
20 kHz . Because speech reception threshold
(SRT) for the right ear could not be obtained at
maximum output level (approximately 105 dB
HL), speech detection threshold (SDT) was obtained at 80 dB HL . Word recognition ability for
the right ear was 0 percent. The left ear SRT
was 65 dB HL . Word recognition ability for the
left ear was 54 percent at a most comfortable
listening level (MCL) of 75 dB HL improving to
78 percent at 90 dB HL . Uncomfortable listening level (UCL) was at 95 dB HL .
ABRs to high-frequency tone-burst stimuli
at 8, 10, and 12 kHz (approximately 115 dB
peSPL) revealed no scorable waveforms for the
right ear and clearly definable waveforms for
the left ear at 95 dB peSPL (Fig. 8) . These
results demonstrate that ABRs can be obtained
from specific high-frequency tone-burst stimulation in an individual whose poor conventional
frequency thresholds prohibit significant neural contribution to the high-frequency ABR.
Subject 4
This 43-year-old male hospital patient was
treated with gentamicin over a period of42 days
for a pancreatic infection. Conventional baseline hearing test results were within normal
limits, except for a slight 4 to 6 kHz "dip" in
pure-tone air-conduction thresholds (35-40 dB
HL) in the left ear. Baseline threshold responses
to high-frequency pure-tone stimuli were within
one standard deviation of the mean reported by
Schechter et al (1986) bilaterally . Immittance
Figure 8 Auditory brainstem responses for 8, 10, and 12
kHz tone bursts for right (A) and left (B) ears from a 24year-old female with severe bilateral sensorineural hearing loss but with a left unilateral island of improved highfrequency hearing sensitivity. Each trace represents the
added ipsilateral waveforms of repeated runs . No ABRs to
clicks or tone bursts were obtainable in the severely
hearing-impaired right ear. With hearing restricted to the
8 to 11 kHz range, clearly definable peaks were obtained
in the left ear in response to high-frequency tone bursts at
8, 10, and 12 kHz.
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ations (Fig . 10). This case report demonstrates
the high-frequency-specific sensitivity of this
objective measure as well as the ability to detect
relatively small changes in threshold sensitivity.
Figure 9 Right ear baseline and final threshold responses (in dB SPL) for a 43-year-old male who received
prolonged treatment with gentamicin .
above 11 kHz (Fig . 9) . Initial change was noted
on the 12th test (35th treatment day) and progressed through the final treatment evaluation
on the 42nd day. ABR click results remained
unchanged, as did ABRs for 8, 10, and 12 kHz
tone bursts . However, ABRs to the highest toneburst test frequency, 14 kHz, revealed increasing latency and declining morphology corresponding to pure-tone changes at that frequency.
ABRs revealed no markable response to 14 kHz
tone bursts at 115 dB peSPL in either the final
treatment evaluation (day 42) or follow-up evalu-
Latency (ms)
Figure 10 Auditory brainstem responses to 14 kHz
tone bursts at baseline and at final evaluation (an elapsed
period of 42 days) for a 43-year-old male who received
prolonged treatment with gentamicin . Each trace represents the added ipsilateral waveforms of repeated runs .
The definable peaks seen at baseline were not present in
the final test, corresponding to hearing loss at that frequency . This is a further demonstration of the frequency
specificity and potential sensitivity of these high-frequency tone bursts and exemplifies the intended clinical
application of the PARVA-PTB.
his laboratory has been involved in the
T study of high-frequency hearing in patients receiving potentially ototoxic medications. Behavioral monitoring of high-frequency
(8-20 kHz) audition is utilized to detect hearing
loss sooner than with tests limited to the conventional frequency range (0 .25-8 kHz) (Fausti
et al, 1990c) . While instrumentation is available for behavioral high-frequency pure-tone
testing, approximately one third of patients at
risk for ototoxicity are unable to respond reliably . Monitoring such patients with high-frequency ABR tone bursts can provide an objective indication of change in high-frequency hearing. Portability of monitoring instrumentation
is essential in order to evaluate patients at
Reliability of high-frequency-specific ABRs
obtained from normal-hearing individuals
(Rappaport et a1,1985; Fausti et al, 1991b) and
from those with hearing impairments (Fausti
et al, 1990b) has been demonstrated . Repeatable, interpretable results are the primary requirement for serial monitoring of a patient
throughout a period of treatment with a potentially ototoxic agent. Changes in latency of the
evoked response should be sufficient indicators
of changing audition . Another indicator ofhearing loss might include waveform morphology
when considering an initially observed repeatable response that degenerates to a consistently unmarkable response . Therefore, changes
in the high-frequency tone-burst-evoked responses, without concomitant changes in clickevoked responses, should provide evidence for
early identification of ototoxic effects in individuals unable to respond to traditional
behavioral methods of hearing assessment .
It has been shown here that individuals
demonstrate predictable responses to the highfrequency-specific stimuli presented by the
PARVA-PTB . That is, taking individual audiometric configurations into consideration, the
case studies revealed ABRs that would be expected from high-frequency-specific stimuli .
Subject 1 showed that a normal-hearing (see
Fig. 3) individual displayed morphologically
well-defined high-frequency ABRs with repro173
Journal of the American Academy of Audiology/Volume 3, Number 3, May 1992
ducibility (see Fig. 4) . Subject 2 had hearing
within normal limits in the frequency range up
to 3 kHz with a precipitous sensitivity slope
thereafter (see Fig. 5) . While click-evoked ABRs
were well defined (see Fig. 6A), identifiable
peaks could not be obtained to high-frequency
tone bursts (see Fig. 6B) . This subject demonstrated frequency specificity for the high-frequency tone bursts inasmuch as his reasonably
good lower frequency hearing (below 4 kHz) did
not contribute meaningfully to ABRs to highfrequency stimuli. Subject 3 presented with a
severe hearing loss bilaterally except for a unilateral island of improved hearing from 8 to 11
kHz (see Fig. 7) . Identifiable ABR waveforms to
click or high-frequency tone-burst stimuli could
not be obtained from the ear with the severe loss
(see Fig. 8A). Recognizable ABRs were, however, obtained from the ear with the island of
hearing (see Fig. 8B) . This subject demonstrated
that, with essentially normal hearing in the 8 to
11 kHz range (Schechter et al, 1986), clearly
definable ABRs could be obtained to tone-burst
stimuli in this frequency range. Subject 4, with
relatively normal hearing, revealed a change in
hearing thresholds at 14 and 16 kHz after
prolonged treatment with an aminoglycoside
antibiotic (see Fig. 9) . His ABRs to the highest
frequency tone-burst stimuli available, 14 kHz,
revealed latency and morphology changes (see
Fig. 10), demonstrating the sensitivity of this
test method .
The significant advantages of the stimulus
generator developed in this laboratory are those
of portability and compatibility with commercially available portable signal averagers . This
lightweight unit can be taken to the hospital
bedside without difficulty . Experience has shown
that, even in limited-space surroundings such
as those seen in most critical care facilities, the
PARVA-PTB can be utilized efficiently . Additionally, the PARVA-PTB can be utilized with
any evoked potential signal averager, requiring
only a positive (condensing) square-wave (click)
signal of sufficient magnitude (- 4.8 volts) to
drive it . The PARVA-PTB provides multiple
frequency selection, a 60-dB range of attenuation, and ear selection.
The PARVA-PTB has the potential for allowing the clinician to assess high-frequency
auditory function in patients receiving ototoxic
drugs who are too ill to respond to behavioral
audiometry . High-frequency tone-burst ABR
monitoring is being conducted in this laboratory in conjunction with behavioral high-frequency pure-tone testing in responsive patients
receiving potentially ototoxic agents . Preliminary results with these patients have shown
that while behavioral changes occurred initially or solely at the high frequencies (91%), a
large majority of those individuals demonstrating high-frequency behavioral threshold changes
also demonstrated high-frequency ABR latency
change (84%). These latency changes were seen
to correlate well with the frequency-specific
changes noted behaviorally . Thus, the ABR to
high-frequency-specific tone bursts may prove
to be a sensitive measure for the early detection
of ototoxicity . Further results of these investigations will determine the clinical utility of this
high-frequency tone-burst stimulus generator
in the early detection of ototoxicity in unresponsive patients .
Acknowledgment. The authors acknowledge significant contributions to this manuscript made by Dr. Curtin
R. Mitchell of the Oregon Hearing Research Center and
Deanna J. Olson, Heidi I. Schaffer, and Pamela S. Ritchie,
Research Audiologists at PVAMC.
Funding for this study was provided by the Medical
Research Service, U.S . Department of Veterans Affairs.
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