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Narrow Internal Auditory Meatus
An Idiopathic Case Confirming the Origin and Pathway of Vestibular Evoked Myogenic Potentials in Humans
Ken Ito, MD;
Shin-ichi Ishimoto, MD;
Toshihisa Murofushi, MD
Arch Otolaryngol Head Neck Surg. 2001;127:275-278.
ABSTRACT
Objective To confirm the origin and pathway of vestibular evoked myogenic potentials
(VEMPs) in humans.
Design Case study.
Setting University hospital.
Patient A patient with a narrow internal auditory meatus (IAM).
Main Outcome Measures Imaging studies and functional studies concerning the seventh and eighth
cranial nerves.
Results Of the 4 nerves in the IAM, all but the cochlear nerve had normal function
and normal courses, despite the pronounced narrowing of the IAM. The facial
nerve had a normal diameter, but the vestibular nerves were thinner. Imaging
revealed that the cochlear nerve was absent or extremely thinned. Both the
cochlea and the cochlear nerve showed no function in the affected ear, although
the VEMPs were evoked normally.
Conclusion Our results definitively support the vestibular origin of VEMPs in humans.
INTRODUCTION
A CONGENITALLY narrow internal auditory meatus (IAM) is rare, especially
as an isolated finding without inner, middle, or external ear anomalies.1, 2, 3, 4, 5
Cases with thorough neurological and physiological examinations have not been
found in the literature. Acquired stenoses of the meatus, such as osteomas,
exostoses, and fibrous dysplasias, were more frequently reported.6, 7, 8 Herein, we present the
findings of imaging and functional studies in a rare case of a unilateral
narrow IAM. Of the 4 nerve bundles in the IAM, only the cochlear nerve was
atrophic and showed no measurable function, mimicking the conditions of cochlear
deafferentation. Therefore, the results of functional studies in this case
serve as direct evidence for the origin and the pathway of the vestibular
evoked myogenic potentials (VEMPs) in humans.
REPORT OF A CASE
A 31-year-old woman with a hearing impairment in her right ear presented
with questions about whether she could regain hearing in her affected ear
and whether she might lose the normal hearing in her left ear in the future.
Neither she nor her family members noticed her hearing loss until it was indicated
during a routine school medical examination when she was 7 years old. She
had not noticed exacerbation of the hearing loss since then. Except for her
hearing impairment, her medical history was unremarkable. No one in her family
had a history of hearing impairment. Physical examination findings in the
head and neck were normal, including the tympanic membranes of both sides.
Temporal bone x-ray films suggested a narrowing of the right IAM. On the x-ray
films, the IAM was 3 mm in diameter on the right and 9 mm in diameter on the
left.
RESULTS
IMAGING FINDINGS
High-resolution computed tomography revealed no abnormalities in the
external auditory meatus or in the middle ear on either side (Figure 1). The IAM, cochlea, vestibule, and semicircular canals
were normal on the left side. The inner ear structures were also normal on
the right side. However, the right IAM was very narrow, and small, branched
canals for the 4 nerves were identified at the periphery. The roof of bone
overlying the superior canal was intact on both sides.
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Figure 1. Axial computed tomographic scans
of the internal auditory meatus (IAM). Compared with the normal left IAM (L),
the right IAM (R) is much narrower, and shows the branching for each nerve
in the IAM.
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T2-weighted magnetic resonance images were more informative than T1-weighted
images (Figure 2). On the left side,
the inner ear structures were normal, and the 4 nerves in the IAM were clearly
identified. On the right side, the cochlea, vestibule, and semicircular canals
were normal. At the cerebellopontine angle, where the seventh and eighth cranial
nerves leave the brainstem, the right eighth nerve was obviously smaller in
diameter than the other side. On T2-weighted images, routes for the facial,
superior vestibular, and inferior vestibular nerves were not identified. This
was probably because of the little fluid space between the stenotic canal
wall and the nerve. However, the route for the cochlear nerve was clearly
identified to its entrance to the cochlea, which, paradoxically, suggests
the atrophy of the nerve, because of the sufficient fluid space in the stenotic
canal. On T1-weighted images, the right eighth cranial nerve was thin, and
the 3 branches were not clearly identified, indicating the thinning of these
branches or the absence of certain branches.
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Figure 2. Sagittal T2-weighted magnetic
resonance imaging scans of the narrow right internal auditory meatus (IAM)
(R) and the normal left IAM (L). A-B, C-D, and E-F correspond to the levels
at the cerebellopontine angle, in the middle of IAM, and at the entrance into
the cochlea, respectively. On the left side, 4 nerves in the IAM are identified
in the high fluid intensity (D). The small arrowheads indicate the routes
for the cochlear nerve; large arrowheads, the basal turns of the cochlea;
ant, anterior direction; and pst, posterior direction.
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A summary of the imaging studies is shown in Table 1.
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Summary of Imaging and Functional Test Results
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NEURO-OTOLOGICAL FINDINGS
Auditory
Pure-tone audiometry revealed a profound sensorineural hearing impairment
in the right ear and normal hearing in the left ear. Thresholds for sound
sensation were unmeasurably high in the right ear. In the frequency range
of 500 to 1000 Hz, at higher tone levels above 100- to 105-dB hearing level
(HL), the patient had a feeling that her whole head was shaking as though
she were experiencing an earthquake. Repeated distortion product otoacoustic
emission and transiently evoked otoacoustic emission studies confirmed normal
responses in the left ear, but very poor or no responses in the right ear,
indicating severe cochlear impairment. On click evoked electrocochleography,
action potentials were not recorded in the right ear, whereas they were normally
evoked in the left ear. Auditory brainstem response tests also showed a normal
response in the left ear but no response in the right ear. Promontory stimulation
tests9 were performed on the right ear, using
a needle electrode placed on the promontory mucosa through the ear drum. When
the electric current was applied, the patient had a sensation of vibration
but not sound, which indicated severe to total atrophy of the cochlear nerve.
Vestibular
Caloric responses were normal on both sides. Vestibular evoked myogenic
potentials were evoked by click stimuli and recorded from the sternocleidomastoid
muscles ipsilateral to the stimulation. Rarefaction clicks of 95-dB normal
HL were used. The stimulation rate and the analysis time were 5 Hz and 50
milliseconds, respectively. Two hundred responses were averaged to obtain
1 recording. The VEMPs were normal both in wave latencies and in amplitudes
(Figure 3). Although the P13-N23
amplitudes10 of the VEMPs were higher on the
left, the difference (26%) was within the normal range (<34%).11
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Figure 3. Recordings of the vestibular evoked
myogenic potentials. Stimulus: rarefaction clicks of 95-dB normal hearing
level, ipsilateral; stimulation rate: 5 Hz; analysis time: 50 milliseconds
(ms); and average, 200 traces. Two traces are shown for each side (L, left;
R, right). The amplitudes of the first positive-negative peak (P13-N23) were
74.8 and 44.3 µV for the left and right sides, respectively, which gives
the evoked potential ratio of 26%. The time scale of the abscissa is 5 milliseconds
per division (div), and that of the ordinate is 20 µV per div.
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Other
Type A tympanograms were recorded on both sides. Stapedial reflexes
were normally recorded in the right ear when the tones were delivered to the
left ear, indicating normal facial nerve function on the right side. No responses
were found in the left ear with stimulation of the right ear, owing to the
profound deafness in the right ear. The results of physical examination of
facial nerve functions were normal. Other cranial nerve tests also revealed
no abnormalities. The patient had no nystagmus, dysequilibrium, or ataxia.
Nystagmus was not evoked, even when an intense sound was delivered to the
right ear.
A summary of the function of each nerve and sense organ is shown in Table 1.
COMMENT
Vestibular primary afferent neurons of certain mammals respond to high-intensity
sounds.12, 13, 14, 15
Part of these neurons were also shown to respond to natural vestibular stimuli.14, 16 The counterpart of these phenomena
in humans is believed to be VEMPs, which are the potentials recorded on the
tonically contracting sternocleidomastoid muscle when loud monoaural clicks
are supplied to the ipsilateral ear.10, 11, 17
In humans, it has been suggested that VEMPs are of vestibular origin because
of (1) the disappearance of the responses after vestibular deafferentation
surgery despite the preserved hearing in 1 patient,10, 17
(2) the preservation of the responses in patients with severe sensorineural
hearing loss (>80-dB HL),10, 18
and (3) the independence from the pure-tone hearing results in vestibular
neurolabyrinthitis.19 However, there are objections
to each theory: (1) because pure-tone thresholds remain normal until more
than 80% of spiral ganglion cells, ie, cochlear nerve fibers, have disappeared,20 the integrity of the cochlear nerve is not guaranteed;
(2) as the sound intensity delivered to the ear during VEMP recordings is
very high (usually >90-dB normal HL), responses could have been improved because
of the loudness recruitment phenomenon if the primary cause of deafness resides
in the cochlea and not in the cochlear nerve; and (3) different patterns of
the results can serve only as indirect evidence. To confirm the vestibular
origin of VEMPs, cochlear deafferentation, or section of the cochlear nerve,
would be essential, which could not be done in human subjects. Substitution
for cochlear deafferentation could be found in congenital anomalies in the
IAM, in combination with the nerve atrophy.
With the exception of the cochlear nerve, the nerves in our patient's
IAM had normal function and normal courses, although the superior and inferior
vestibular nerves were thinner on the right side. The functional defects in
hearing were not restricted to the nerve, but involved the sensory organ,
ie, the cochlea. Therefore, our patient simulated the state of selective and
complete cochlear deafferentation in that no signals could be transmitted
through the cochlear partition of the inner ear. This state is rarely found
among human subjects, because it is very unlikely that an acquired disease
process can impair only the cochlear nerve and because no treatment cuts the
cochlear nerve selectively. If only the cochlear nerve were impaired, eg,
by a viral infection, it would be impossible to diagnose this condition by
imaging. The normal VEMPs that were recorded in our patient support the hypothesis
that the origin of this response resides outside the cochlea and that the
afferent pathway for this response runs in the vestibular nerve in humans.
This confirmation reinforces the importance as well as the validity of using
VEMPs to test vestibular function in humans.
Because of the smooth outlines of the narrow bony canals in our patient,
the narrowing was considered to be congenital, formed by the excessive bone
proliferation around the atrophic nerves in the course of ontogeny, because
in cases of acquired stenosis, such as fibrous dysplasia or osteoma, the outlines
of the stenotic canal are irregular, being constricted only at the portion
where the lesion exists.6, 7 The
normally shaped fluid space in the cochlea revealed by the imaging studies
indicates that the cochlea was normally formed at least once during ontogeny.
If that was the case, the cochlear nerve also might have been formed once,
since the developing cochlea, the otic vesicle, releases an analog of a nerve
growth factor, which is essential for the differentiation of the afferent
nerve fibers.21 The existence of the bony route
corresponding to the cochlear branch, connecting to the cochlea, supports
this hypothesis. Therefore, a question remains as to the onset of our patient's
hearing loss. The possibility is that the once-formed cochlea and the thinner
cochlear nerve might have been damaged postnatally, for unknown reasons.
The results of pure-tone audiometry in our patient are of particular
interest. A sensation of vibration, rather than sound, was invoked. One hypothesis
is that this sensation may have been formed by the saccule, conducted via
the vestibular nerve.14, 22 A fact
that may support this hypothesis is that the frequency range (500-1000 Hz)
that invokes this sensation during pure-tone audiometry corresponds with the
best frequency of the acoustically responsive vestibular afferent neurons
in cats13 as well as with the optimum frequency
of the Tullio phenomenon.23, 24, 25
AUTHOR INFORMATION
Accepted for publication August 11, 2000.
This study was supported in part by a fellowship from the Canon Foundation,
Leiden, the Netherlands (Dr Ito).
From the Department of Otolaryngology, Faculty of Medicine, University
of Tokyo, Tokyo, Japan.
Corresponding author and reprints: Ken Ito, MD, Laboratoire EMI 99-27
INSERM, CHU Hôpital Pellegrin, Bâtiment PQR entrée 3, 2ème
étage, Place Amélie Raba Léon, 33076 Bordeaux, France
(e-mail: itoken-tky{at}umin.ac.jp).
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