Download Late components in the compound action potentials (CAP) recorded

75
Hearing Research, 45 (1990) 75-86
Elsevier
HEXRES
01342
Late components in the compound action potentials (CAP)
recorded from the intracranial portion of the human eighth nerve
Aage R. Merller and Hae Dong Jho
Department
of Neuroiogicai Surgery, University of Pittsburgh School of Medicine, Pittsburg& Pennsylvania,
(Received
9 September
1989; accepted
28 October
U.S.A.
1989)
The compound action potential (CAP) that can be recorded from the exposed intracranial
portion of the eighth nerve in man to
stimulation
with broadband
clicks of about 100 dB Pe SPL normally has an initial (small) positivity followed by a sharp negative
peak with a latency of about 3 to 3.5 ms. The negative peak is usually followed by another positive-negative
deflection with a latency
of about 4 ms. Usually, no stim~us-related
potential can be discerned at latencies longer than 5 ms. However, in a few patients we
found a series of waves that occurred between 4 and 12 ms after the stimulation.
The polarity of these waves reversed precisely (180”
phase shift) when the polarity of the sound was reversed. Thus, these waves appeared clearly when the responses to clicks of opposite
polarity were subtracted. These late waves were quasiperiodic
with intervals between 1 and 2 ms. The waveform and duration differed
between patients, but were remarkably
constant in each patient. The timing of these late peaks was nearly independent
of the
stimulus intensity in the range studied (between 105 and 70 dB Pe SPL); in this respect the late waves differ fundamentally
from the
initial peaks, which showed a monotone decrease in latency with increasing stimulus intensity over this intensity range. Although the
origin of these late waves is not known, the similarities between these waves and stimulated otoacoustic
emissions indicate that the
late waves may be the result of active cochlear processes similar to those that produce the cochlear echo.
Human
auditory
nerve; Compound
action potentials;
Active co&ear
Whole-nerve action potentials (known as cornpound action potentials, or CAP), recorded either
from the round window of animals or man or
from the exposed intracranial portion of the eighth
cranial nerve, have been the focus of numerous
studies. The CAP recorded from the human ear
using the so-called ‘electrocochleography
technique,’ in which the recording electrode is placed
on the promontorium (Spoor et al., 1976; Eggermont, 1978) or in the ear canal (Coats and
Dickey, 1970; Elberling, 1976) is dominated by a
single negative peak (N1). It is thus similar to the
CAP recorded from a distal location on the intracranial portion of the human auditory nerve
Correspondence to: A.R. Meller, Department
of Neurological
Surgery, Room 9402, Presbyterian
University
Hospital,
230
Lothrop Street, Pittsburgh,
PA 15213, U.S.A.
0378-5955/90/$03.50
0 1990 Elsevier Science
Publishers
processes
with a monopolar electrode (MGller et al., 1981;
Meller and Jannetta, 1981, 1983; Spire et al.,
1982). In small animals a second negative peak
(Nz) appears after the first negative peak. The
second negative peak (Nz) in the CAP recorded
from the auditory nerve in man (Msller and Jannetta, 1981, 1983) and small animals presumabIy
originates in the cochlear nucleus (Mnller, 1983a).
The human N2 is smaller, particularly when recorded from the promontorium or the distal part
of the intracranial portion of the human eighth
nerve, because it is, anatomically, much farther
away from the cochlear nucleus in man than it is
in small animals.
In the present study we show that in a few
patients there are components in the CAP recorded from the intracranial portion of the human
eighth nerve in response to click sounds that appear with a latency that is longer than that of the
N2, and that these potentials behave rather differently from the initial negative peaks in the CAP.
B.V. (Biomedical
Division)
Methods
axial tomography (CAT) or nuclear magnetic resonance (NMR) scans. All patients included in this
study had normal otological findings with normal
middle ears judged by otoscopic examinations
and
normal
tympanograms.
The mentioned
imaging
tests showed no pathologies
or abnormalities
in
any of the patients.
The anesthesia
and operative technique
(Jannetta, 1981a,b), as well as the methods of recording the CAP from the exposed intracranial
portion
of the eighth nerve, were the same as were used
earlier (see Moller and Jannetta,
1983; Moller,
1988; Moller and Jho, 1989) and, therefore, they
will be described only briefly here.
The patients who were operated upon to relieve
TN and DPV were anesthetized
using a balanced
technique with a strong narcotic (fentanyl) and a
muscle relaxant (pancuronium
or vecuronium),
to-
Recordings
were made from the intracranial
portion of the eighth nerve in patients undergoing
microvascular
decompression
(MVD) operations
for intractable tin&us, disabling positional vertigo
(DPV), trigeminal
neuralgia
(TN), or hemifacial
spasm (HFS). Preoperatively,
all patients underwent complete audiological
evaluation,
including
pure tone audiograms,
determination
of speech
discrimination
scores, recording of acoustic middle ear reflexes and BAEP using 2-kHz tonebursts,
and, occasionally,
click stimuli.
The tympanic
membrane
was inspected
and cerumen was removed, if present, by an experienced
otologist.
Patients with disabling positional vertigo (DPV) in
addition
underwent
complete
otoneurological
evaluation. All patients had routine computerized
IOOps
A
0
CLICK
I
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1
2
3
ms
dS
I
I
100
Fig. 1. Waveform
I
I
IIIIIII
I
1000
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10000
Hz
of condensation
(solid lines) and rarefaction
(dashed lines) clicks (A) and spectrum (B), as measured
slightly acoustically damped cavity using a Bruel and Kjaer l/4-inch
microphone.
in a 2.5-cc
2.5
c
2.0
I
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65
75
85
95
105
dB PeSPL
Fig. 2. (A) Examples of typical CAP recorded from the exposed intracranial portion of the eighth nerve in response to broadband
rarefaction (solid lines) and condensation (dashed lines) clicks presented at different sound intensities (indicated by legend numbers).
These results were obtained in a patient who was operated upon to relieve trigeminal neuralgia. The patient had normal hearing
before and after the operation and BAEP did not change noticeably during the operation. Negativity is shown as an upward
deflection in this and in subsequent graphs. (B) Latency of the positive (lower curves) and negative (upper curves) peaks in the
recordings shown in (A).
gether with nitrous oxide. Patients operated upon
to relieve HFS underwent surgery without the use
of muscle relaxants; they were maintained on isoflurane and nitrous oxide, sometimes supplemented with a small amount of narcotic. None of
these anesthesia techniques is known to affect
short-latency auditory evoked potentials noticeably.
The stimulus sound was generated by applying
lOO+s rectangular waves of either polarity (supplied by a Grass Instrument Co., Type SlO
CTCMA audiostimulator) to miniature stereo ear-
78
Fig. 3. Recordings similar to those shown in Fig. 2(A) but obtained in a patient in whom there were late waves in the CAP. The
patient was operated upon to relieve trigeminal neuralgia. (A) Recorded from a distal location on the exposed eighth nerve. (B)
Recordings similar to those in (A) but from a more proximal location.
phones (Radio Shack, Realistic) (see Moller, 1988)
that were secured in the patients’ ears by plastic
adhesive tape. The sound generated by these earphones, as measured in a 2.5-cc slightly damped
cavity, is shown in Fig. 1.
Needle electrodes (Grass Instrument
Co., Type
E2 subdermal electrodes) were placed on the vertex
and both earlobes for recording brainstem
auditory evoked potentials
(BAEP), with a ground
electrode over the sternum.
A fine, Teflon-insulated, multistrand,
silver wire with a cotton wick
sutured to its uninsulated
tip (Moller and Jannetta, 1983) was placed on the exposed eighth
nerve for monopolar
recording
of the auditory
nerve CAP. The reference electrode was a subderma1 needle electrode placed either on the opposite
earlobe or on the ipsilateral shoulder.
The potentials were amplified using differential
amplifiers with filter settings at ~-HZ highpass (6
dB/octave
rolloff) and 3-kHz lowpass (24 dB/octave rolloff) (Grass Instrument
Co., Model 12).
The recorded potentials
were averaged using an
LSI 11/73 processor, which also stored the data
on computer disks for later processing. Potentials
from the exposed eighth nerve were usually recorded from the time that the eighth nerve was
exposed until the end of the intracranial
part of
the operation.
BAEP to the same stimuli were
recorded during the entire procedure,
from the
time the patient was anesthetized
until closure of
the wound.
To detect changes in auditory
function
as a
result of surgical manipulation,
BAEP obtained
after the patients had been anesthetized but before
79
the operations were begun were used as a baseline.
BAEP recorded during the operation
were then
compared to each patient’s baseline.
The recordings
on which this study is based
were obtained
in the course of intraoperative
monitoring
of BAEP that is done routinely
in
patients undergoing
such neurological
operations
to minimize the risks of hearing loss due to manipulations
of the eighth nerve. The monitoring
procedures have been approved by the Biomedical
Institutional
Review Board of the University
of
Pittsburgh
School of Medicine, and the patients
gave informed consent to the procedure.
Results
Typically,
compound
action
recorded from the most distal
potentials
(CAP)
point on the in-
,,“’\
,_,----
-i^--.._-,
_:I
__^
__-\_T
tracranial portion of the uninjured
eighth nerve in
response to stimulation
with broadband
clicks in
patients with normal hearing have an initial small
positive deflection,
followed by a large negative
peak with a latency of 3 to 3.5 ms (Fig. 2A). (A
large difference between the response to rarefaction and condensation
clicks, as seen at 95 dB, is
present occasionally
and it was reproducible
in
this patient.) When recordings
are made at locations that are closer to the brainstem,
the negative
peak is often followed by a small positive deflection, and often by another negative deflection of
smaller amplitude than the first one.
In patients with normal hearing in whom dissection of the eighth nerve did not result in any
detectable
changes
in the farfield
potentials
(brainstem
auditory evoked potentials, BAEP), the
latency of the initial positive peak and that of the
^, -
“5
-‘,_
\-
‘;
,“‘
--
*____,s
----\-
2PV
Fig. 4. Same data as shown in Fig. 3(B). (A) Sum of the responses to condensation
and rarefaction
responses to condensation
and rarefaction
clicks.
clicks. (B) Difference
between
the
80
L
I
0
2
II
4
Fig. 5. Recordings
Compound
action
I
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6
8
10
(2
14
16
18
20
U-d
ms
0
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2
4
6
from the same patient whose results are shown in Figs. 3-4, obtained
potentials
recorded in response to condensation
and rarefaction
clicks.
shown in (A).
main negative peak in the CAP recorded from the
intracranial portion of the eighth nerve increased
monotonically when the stimulus intensity was
decreased. Conversely, the polarity of the click
stimuli in these cases had little effect on the
waveform or latency of the CAP (Fig. 2).
A rather different pattern of responses was seen
in another patient (Fig. 3). While the waveform of
the initial portion of the potentials shown in Fig. 3
is similar to the waveform of the potentials shown
in Fig. 2, in the patient whose results are shown in
Fig. 3 a series of waves followed the initial components of the CAP. The initial portion of the CAP
changed little as a result of reversing the click
polarity, but the polarity of these late waves reversed when the stimulus polarity was reversed.
When the stimulus intensity was decreased these
waves were still discernible.
These later components of the potentials recorded from the exposed eighth nerve cancelled
when the responses to clicks of opposite polarity
*
10
I
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I
I
12
14
16
18
20
ins
at three different repetition rates. (A)
(S) Difference between the recordings
were added (Fig. 4A), but they were seen clearly
when the responses to opposite click polarity were
subtracted (Fig. 4B). These later waves can be
seen to have the form of oscillations, with several
waves being recognizable over a period ranging
from 4 to 10 ms after the presentation of the
stimulus. The timing of these waves (Fig. 4B)
showed no noticeable dependence on the stimulus
intensity in the range that was studied, in contrast
to the increase in latencies of the early components of the CAP that occurred when stimulus
level was decreased.
Increasing the rate of stimulus presentation
from 19 to 49 or 99 pulses per second (pps) (Fig.
5) resulted in a decrease in amplitude of the initial
negative peak in the CAP as well as in the amplitude of the late potentials (Fig. 5). The latency of
the initial negative peak in the CAP increased
slightly as a result of increasing the repetition rate
of the stimulus, while there was no noticeable
difference in the timing of the late potentials.
81
PREOP
A
CAP
INTRAOP
SKIN
B
125
250
CLOSURE
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Dx:
TRIGEMINAL
NEURALGIA
Fig. 6. (A) BAEP obtained before and during the operation
on
recorded from the eighth nerve. Vertex negativity is shown as an
(Meller, 1983~). Solid lines: the responses to rarefaction
clicks;
audiograms
obtained before
the patient whose results are shown in Fig. 3, together with CAP
upward deflection and the BAEP were subjected to digital filtering
dashed lines: the responses to condensation
clicks. (B) Pure tone
and 6 days after the operation.
82
120
in Fig. 3. Examples of the individual
variability
are further illustrated in Fig. 9, which shows CAP
from 5 other patients.
Of a total of 77 patients operated upon consecutively
to relieve
vascular
compression
of
cranial nerves V, VII, or VIII over a period of 2
years and 8 months in whom we recorded intraoperatively
from the exposed nerve, 18 showed
late potentials
such as those illustrated
in Figs.
2-8.
Discussion
Fig. 7. Sums and differences of the responses to rarefaction
and condensation
clicks recorded from locations on the eighth
nerve near the porus acusticus
(solid lines) and near the
brainstem (dashed lines), in the same patient whose results are
illustrated in Figs. 2-6. The difference between the responses
to rarefaction
and condensation
clicks was highpass filtered
using a zero-phase digital filter.
During the operation, the BAEP of the patient
whose results are shown in Figs. 3-5 changed
slightly (Fig. 6A). The patient had a slight hearing
loss preoperatively
(Fig.6B), which was unchanged
after the operation.
It is important
to ensure that the long-latency
components
we are describing result from propagated neural activity in the auditory nerve and are
not a result of volume-conducted
potentials
that
originate
from sources at a distance
from the
recording site. Comparison
of the responses recorded from two different locations on the eighth
nerve (Fig. 7) shows that the latencies of the early
components
of the CAP increase when the recording electrode is moved from a distal to a more
proximal position, and that these late potentials
are shifted about the same amount.
The waveforms, durations, and latencies of these
late potentials varied considerably
from patient to
patient, as seen from Fig. 8, which shows results
similar to those in Fig. 3 but from two other
patients who had late waves. It is seen that the
amplitudes of the waves are smaller in the patient
illustrated in Fig. 8 than in the patient illustrated
We have shown in this study that the response
from the exposed eighth nerve in a few individuals
contains late components
consisting of a series of
waves that appear between about 4 and 12 ms
after the stimulus. This is considerably
later than
the commonly
seen components,
which typically
are dominated
by a negative deflection
with a
latency of 3-3.5 ms. The polarity of these late
waves is reversed when click polarity is reversed,
and their timing is nearly independent
of the
stimulus intensity in the range 75-105 dB PeSPL.
The fact that the latency of these late components shifts when the recording electrode is moved
along the exposed eighth nerve indicates that these
potentials
are a result of propagated
neural activity in the auditory nerve. This, in turn, seems to
rule out the possibility that they are generated by
any stationary
source such as the cochlea or the
cochlear nucleus, or that they are picked up by the
reference electrode.
Considerable
experimental
evidence has been
accumulated
to show that the neural discharges of
single auditory nerve fibers in response to broadband transient
sounds are phase-locked
to the
damped
oscillations
of the basilar
membrane
(Pfeiffer and Kim, 1972; Kiang et al., 1965) in
nerve fibers with characteristic
frequencies up to 5
or 6 kHz. It is generally assumed that the gross
potentials that can be recorded from the auditory
nerve are a convolution
between the summed distribution of discharges in all the nerve fibers from
which the recording is made and the waveform of
a single neural discharge (Goldstein,
1960). The
finding that the late components
in the response
from the human auditory nerve reverse in polarity
when the polarity of the stimulus sound is reversed
83
DIFFERENCE
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Dx:
TRIGEMINAL
NEURALGIA
Fig. 8. (A) Compound action potentials recorded in response to condensation (dashed lines) and rarefaction (solid fines) clicks. (B)
The differences between the responses to condensation and rarefaction clicks. The results are from another patient who showed late
waves in the response from the eighth nerve. The patient was operated upon to relieve t~ge~n~ neuralgia. The audiogram of the
patient is also shown (C).
84
:
‘,
I!
,,
.-.
j2PV
,,’
-/
‘.
--.
,---
Fig. 9. Compound action potentials recorded in response to condensation and rarefaction clicks obtained in five patients (A-E) who
displayed late waves. The audiograms of these patients are also shown.
85
thus indicates
that the late components
are the
results of neural activity that is phase-locked
to a
low-frequency
oscillation of the basilar membrane.
If we assume that click stimuli excite the basilar
membrane relatively uniformly and that sensitivity
is relatively uniform along the basilar membrane,
then the gross response would not bear any resemblance
to any specific oscillation
because it
will be a sum of damped oscillations
of different
frequencies.
Therefore, only the initial portion of
the damped oscillations of the basal portion of the
basilar membrane will result in synchronization
of
neural activity in many nerve fibers. This probably
explains
why, in the majority
of the patients
studied, the response from the auditory nerve is a
single positive-negative
wave resembling
the response from a long nerve to a brief excitation
(Lorente de No, 1947).
Sharp discontinuities
in the mechanical properties of the basilar membrane could result in overrepresentation
of the damped
oscillations
produced by fibers that are tuned to frequencies
within a narrow range. This conceivably
could
leave enough residual oscillations
in the summed
neural activity to be noticeable in the CAP, which
could be one explanation
for the late components
seen late in the CAP. However, even though these
late potentials might originate from a point on the
basilar membrane that is tuned to a frequency that
is equal to that of the oscillations
(around
500-1000 kHz), the observed latencies seem too
long to explain the travel time on the basilar
membrane
to that point on the basis of available
data on travel times in the normal human cochlea
(von BekkCsy, 1960). These data are from human
cadaver ears. Nonlinearities
that are known to
exist in the normal cochlea may, however, result in
a much longer travel time. Results from human
electrocochleography
show latency times of 4 ms
in the 1-kHz region (Eggermont,
1976). When
recorded from the intracranial
portion of the auditory nerve, about 1 ms of neural conduction
time
should be added, which means that values of
about 5 ms could be expected. This seems to be in
agreement with the results obtained in the present
study.
The late components
in the CAP, described in
the present paper, bear some resemblances
to
evoked otoacoustic
emissions.
Both late compo-
nents in the CAP and otoacoustic
emissions have
relatively long latencies and a quasiperiodic
nature
(Kemp, 1978, 1979, 1980). Otoacoustic
emissions
have been explained on the basis of inhomogeneities of the basilar membrane,
and they are generally assumed to be due to co&ear
nonlinearities
(see Kemp, 1979). An important
difference
between otoacoustic
emissions and late components
in the CAP, however, is that the former seem to be
constant phenomena
that can be demonstrated
in
nearly all people, while we have only seen the late
components
in the CAP in a small percentage of
the patients we have studied. Also, late components in the CAP have been seen in patients with
shown
that
hearing
loss, while it has been
otoacoustic
emission is generally not detectable
when the hearing loss exceeds 15 dB (Rutten,
1980; Kemp et al., 1986). This indicates that either
the phenomenon
described in the present paper is
not identical
to otoacoustic
emission or that recording of CAP is a more sensitive method to
detect this phenomenon.
The finding that the latency of the late components does not change noticeably when the stimulus intensity is altered is in agreement with known
properties of phase-locked
activity in single auditory neurons in response to transients
(Kiang et
al., 1965) or continuous
noise (Moller,
1977,
1983b). The small decrease in latency with increasing stimulus intensity that has been shown to exist
in the results from single nerve fibers may be
explained by the shift in maximal excitation of the
basilar membrane
towards the base of the cochlea
when the stimulus intensity
is increased (Moller,
1977, 1983b), but this may not have been detectable in all of our recordings from the human auditory nerve.
The fact that these late potentials are only seen
in a small proportion
of the patients that were
studied indicates that it is not a normal phenomenon. However, we have studied too few patients to
date to show definitely
that these long-latency
components
are uniquely associated with pathologies in the ear or the auditory nerve.
Acknowledgements
This study was supported by a grant from the
National
Institutes
of Health (Grant No. 1 ROl-
86
NS213’78). The author is grateful to Peter Jannetta, M.D., for making the patients under his
care available for this study, and to Margareta B.
Msller, M.D., Dr. Med Sci., for audiolo~cal and
otological evaluation of the patients in this study.
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