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272 Disorders of the Auditory System
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and brainstem. The lower cranial nerves
— which are responsible for swallowing,
the gag reflex, vocal fold function, and
tongue function — are rarely involved.
Once a retrocochlear lesion is indicated by audiometric testing, the most
appropriate diagnostic test is an MRI scan
of the internal auditory canals with gadolinium enhancement. Magnetic resonance
imaging will generally detect even the
smallest tumors (approximately 1–2
in size). Gadolinium is important as a
contrast agent because it is concentrated
by the tumor and shows up as a very
bright spot on the image. Techniques of
MRI using high resolution and avoiding
gadolinium have been used, but the diagnosis is then contingent on the quality of
the scan and the experience of the radiologist. At the present time, gadoliniumenhanced scanning remains the diagnostic gold standard.
An acoustic neuroma will typically
manifest as a bright, pear-shaped lesion
in the internal auditory canal that extends
out into the CPA. The canalicular component is narrower and somewhat triangular, and the extracanalicular component
extends like a mushroom cap to fill the
space between the temporal bone and the
brain (see case studies in this chapter).
Most tumors arise in the internal auditory canal and so the canalicular component is an important diagnostic finding.
The absence of this raises a suggestion of
meningioma or some other type of tumor
of the CPA.
mm
Audiologic Management
Audiologic management of patients with
acoustic neuromas depends to a great
extent on the symptoms of the patient after
surgery. This could include auditory and/
or vestibular management. If hearing has
been partially or totally lost after surgery,
then appropriate audiologic steps should
be pursued. This may include a hearing
aid fitting and counseling. Also, auditory
training may be useful and appropriate in
some cases.
If the patient has balance difficulties
after surgery, then vestibular rehabilitation approaches may be initiated. Vestibular exercises can be of value in helping
to stabilize the patient. It is also useful to
help the patient connect with others who
have had this surgery for support. Patient
groups across the country have been
formed and provide valuable assistance
in this regard.
Medical Management
The treatment options for acoustic neuroma include surgical removal, stereotactic radiation therapy, or serial observation
(see Luxford, 1997). The choice of treatment depends on the size of the tumor,
the age and general health of the patient,
the patient’s hearing status, and (increasingly) the patient’s preference.
Surgical removal of an acoustic neuroma is usually performed as a team
approach between a neurotologist and a
neurosurgeon. The surgery can be done
by one of three approaches: translabyrinthine, retrosigmoid (or suboccipital),
or middle cranial fossa. The choice of approach depends on the size of the tumor,
the desire to preserve hearing, and the
surgeon’s preference.
The translabyrinthine approach is
done through a postauricular incision.
Following a complete mastoidectomy,
the drilling is extended through the semi-
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circular canals and the posterior petrous
bone to access the CPA and the internal
auditory canal. Hearing is always lost
during this approach. The main virtue
of this approach is that the facial nerve
is identified early as it exits the internal auditory canal and the tumor can be
separated easily from the facial nerve in
this location, ensuring safety to the nerve.
Tumors of almost any size and location
can be removed through this approach.
The retrosigmoid or suboccipital approach is through a craniotomy or bony
opening posterior to the sigmoid sinus
and inferior to the lateral sinus behind the
ear. After bone removal, the dura is incised
and cerebral spinal fluid is allowed to
egress. This allows the cerebellum to relax
posteriorly and provides a good view of
the tumor in the CPA, but not in the internal auditory canal. A drill has to be used
to remove part of the temporal bone to
obtain access to the canalicular portion
of the tumor. The tumor is then incised
and its internal contents removed. Once
the tumor is reduced in size, the facial
nerve and the auditory nerve can be identified and dealt with. Tumors that extend
laterally into the internal auditory canal
cannot be fully accessed through this
approach and place the patient at risk for
tumor recurrence. This approach allows
for preservation of hearing in cases where
the tumor can be successfully separated
from the cochlear division of the eighth
nerve; however, in general, the hearing preservation rate is reported to be in
the 30% range (see Glasscock, Bohrer, &
Steenerson, 1997; Kaylie, Gilbert, Horgan,
Delashaw, & McMenomey, 2001).
The middle cranial fossa approach is
appropriate for accessing tumors that are
small and limited to the internal auditory
canal when hearing preservation is a goal.
A 4 cm × 4 cm bony opening is made in
the skull above the ear and the dura of the
temporal lobe of the brain is gently lifted
and retracted. A drill is used on the roof of
the temporal bone to outline the internal
auditory canal. Once the canal is open, the
facial nerve is separated from the tumor
and the tumor is removed. The goal of
surgery is to remove the tumor completely and to preserve the facial nerve
and the auditory branch of the eighth
nerve. Additionally, this approach can be
utilized to decompress the internal auditory canal for patients with tumors in an
only hearing ear in which tumor growth
has been demonstrated and hearing is
declining. Hearing preservation of this
approach ranges from 80% to 90% when
careful patient selection is employed
(Arts, Telian, El-Kashlan, & Thompson,
2006). Preservation of auditory function
in these patients depends on having good
hearing and good ABR waveform morphology preoperatively, as well as being
able to anatomically separate the tumor
from the cochlear division of the auditory nerve without entering the inner ear.
This can only be accomplished in selected
cases. Facial nerve preservation occurs in
the majority of cases of small to medium
size tumors, with success rates of better
than 90% in experienced hands. Total
tumor removal is usually achieved, except
in the case of very large tumors where
adherent tumor may be left on the facial
nerve so as to not sacrifice facial function.
The regrowth rate after successful tumor
removal is in the 3% to 4% range (Dew,
Shelton, & Hitselberger, 1997).
Surgery carries certain risks in addition to facial paralysis and hearing loss.
Patients will routinely experience unilateral loss of vestibular function, which
may result in vertigo and nausea with
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head movement in the first 3 to 7 days
postoperatively, and imbalance lasting
for several weeks following surgery. As
mentioned earlier, larger tumors have
often already resulted in considerable
loss of vestibular function before surgery
and result in preoperative compensation.
This mitigates the vertigo and imbalance
to a large degree in many patients. Some
patients, especially elderly patients, will
require more time to recover and may
need vestibular rehabilitation therapy.
Most will regain their balance function to
near-normal levels with time and can generally return to work in about a six-week
time frame. However, some patients will
continue to experience transient loss of
balance with rapid head movement, but
these patients tend to learn to compensate
for this deficit without much modification
of daily activities of life.
Other complications of surgery include cerebral spinal fluid leak in about
10% of cases, prolonged headaches, meningitis, loss of other cranial nerve function
(fifth and sixth cranial nerves are most
common), stroke, and serious neurologic
problems. The mortality rate in experienced hands is less than 1% and is much
improved over the early days of acoustic
neuroma surgery (see Charpiot, Tringali,
Zaouche, Ferber-Viart, & Dubreuil, 2010).
Stereotactic radiotherapy — also known
as gamma knife, CyberKnife, Linac, and
“radiosurgery” — is a nonsurgical method
that has gained increasing popularity
(Luxford, 1997). Originally introduced in
Denmark in the 1970s, stereotactic radiotherapy has become increasingly prevalent in this country in the last 20 years or
so. The goal is to deliver a highly focused
beam of radiation to a carefully delineated treatment area that conforms to the
tumor while avoiding the surrounding
bone and brain. Acoustic neuromas are
benign tumors and are not radiosensitive
per se; however, radiation may induce
fibrosis and destruction of tumor vasculature, and thus have a direct effect on the
tumor cells. The tumor, therefore, typically does not disappear, but its growth
is arrested. Radiation therapy has been
reported to be successful in a high percentage of cases in regard to tumor control
(Leon et al., 2019). However, Arthurs and
colleagues (2011), in a systematic review
of papers published between 2004 and
2009, found that hearing preservation was
accomplished in only 44% to 66% of cases
undergoing traditional radiosurgery, but
that only 2% to 4% of the patients studied required additional treatment. When
fractionated radiotherapy was performed,
these authors found hearing preservation
rates of 59% to 94%, with 3% to 7% of the
patients requiring additional treatment.
Additional Information
Acoustic neuromas constitute about 85%
of tumors occurring in the CPA and about
90% of tumors involving the internal auditory canal (see Hain, 2019, for review). The
differential diagnosis of lesions occurring
in these locations includes meningiomas,
epidermoid tumors, lipomas, schwannomas of other cranial nerves (facial, trigeminal), and arachnoid cysts. Other rare
lesions may occur, including malignancies (primary or metastatic), chordomas,
vascular tumors, or inflammatory conditions. Meningiomas and other benign
tumors are treated in a similar fashion to
acoustic neuromas (i.e., surgical excision
with attempted preservation of adjacent
neural structures). Smaller lesions can
sometimes be simply observed if they are
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asymptomatic. Arachnoid cysts are generally treated conservatively unless they
become very large and cause compressive
symptoms. Malignant lesions are usually
managed with the assistance of an oncologist and involve multiple modes of treatment, such as surgery, radiation, and/or
chemotherapy. The selection of treatment
is individualized according to tumor
type, grade, stage, and patient status
(Luxford, 1997).
Case 6–1: Acoustic Neuroma
History
A 44-year-old female presented with problems of imbalance, light-headedness, and
vertigo. These symptoms reportedly were
first experienced following a sailing trip
2 months prior to the current evaluation.
The patient’s husband reported concerns
regarding a decrease in his wife’s hearing
sensitivity, and the patient reported some
hypersensitivity to sound. She additionally reported severe headaches and unilateral left-sided tinnitus.
threshold ABR was obtained (Figure 6–1B)
and compared to behavioral thresholds
for the click stimulus, the right ear demonstrated good agreement between the behavioral and electrophysiologic thresholds,
whereas the electrophysiologic threshold
for the left ear was found to be significantly elevated (30 dB) above the behavioral threshold for the same ear, suggesting
retrocochlear involvement. A VNG was
also performed and was unremarkable.
Medical Examination
The otolaryngologic exam revealed an
abnormal Romberg examination with the
patient leaning significantly to the left.
Given the patient’s reported symptoms
and her audiologic test results, an MRI
with contrast was performed to rule out
retrocochlear involvement. However, test
results revealed a small left-sided acoustic
tumor (Figure 6–1C).
Impression
Small left-sided acoustic neuroma.
Audiology
An otoscopic check was unremarkable
bilaterally. A comprehensive audiologic
evaluation indicated essentially normal
peripheral hearing sensitivity and normal word recognition scores bilaterally
(Figure 6–1A). Tympanograms yielded
normal pressure, volume, and compliance
bilaterally, and speech recognition thresholds were in good agreement with puretone averages for both ears. A routine
ABR test was completed with test results
revealing normal absolute and interwave
latencies for both ears. However, when a
Audiologic Recommendations
and Management
Continue to monitor hearing sensitivity.
Medical Recommendations
and Management
Given the small size of the acoustic neuroma, the decision was made in consultation with the patient to continue to monitor for any progression of the tumor. The
patient was to be seen for routine audiologic evaluations and MRIs to monitor for
any changes in the tumor and to reevaluate treatment options in 9 months.
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A
Figure 6–1. Pure-tone thresholds, speech recognition scores, and tympanometry results (A),
an ABR showing better approximation of threshold for the right ear than for the left ear (B), MRI
results showing a small acoustic neuroma on the left side (see arrow) (C) for a 44-year-old female
with a small, left-sided acoustic neuroma (Case 6–1). continues
6. Auditory Nerve Disorders 277
rR
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Left Ea
ight Ear
v
Absolute Latencies
(msec)
Interwave
(msec)
Threshold
Comparisons
(Behavioral—
Electrophysiological)
B
Figure 6–1. continues
Left EarRight Ear
I1.7 1.8
III 3.93 3.9
V6.0 5.75
I-III 2.22 2.1
III-V 2.08 1.85
I-V4.3 3.95
V ILD.25 msec
30 dB 10 dB
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C
Figure 6–1. continued
Case 6–2: Acoustic Neuroma
History
This 41-year-old female was seen for an
audiologic evaluation as part of her work
requirements as a flight nurse. She had
no previous evaluations on record. She
indicated no concerns regarding hearing sensitivity on the right side but noted
reduced hearing and aural fullness on the
left side. These symptoms were reported
as having been experienced for the past
2 years. She denied any other significant
audiologic or otologic history.
Audiology
An otoscopic check was unremarkable
bilaterally. Tympanometry was performed
and revealed normal pressure, volume,
and compliance bilaterally (Figure 6–2A).
A comprehensive audiologic evaluation revealed a mild low-frequency sensorineural hearing loss for the right ear,
and a mild sloping to a moderate highfrequency sensorineural hearing loss for
the left ear (see Figure 6–2A). Word recognition was excellent for the right ear
and poor for the left ear. A significant
asymmetry was noted for both pure-tone
thresholds and word recognition scores.
Medical Examination
The otolaryngologic exam was essentially
normal. Tympanic membranes were normal, extraocular muscle movement was
full, and no nystagmus was observed.
The patient’s Romberg, gait, and tandem
walk were all within normal limits. Facial
nerve function was normal and symmetric. Given the degree of asymmetry in
the hearing thresholds for the two ears
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A
Figure 6 –2. Preoperative (A) and postoperative (D) audiograms for a 41-year-old patient who had an
acoustic neuroma removed (Case 6–2). Also shown are T1-weighted gadolinium-contrasted coronal (B)
and axial (C) MRI images documenting the presence and location of the acoustic neuroma. continues
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B
Figure 6–2. continues
and the age of the patient, no ABR was
done. Rather, the patient was sent directly
for imaging, which revealed a left-sided
acoustic neuroma (Figures 6–2B and 6–2C).
Impression
Left-sided acoustic neuroma.
Audiologic and Medical
Recommendations
and Management
The patient was presented with management options, which included obser-
C
vation, traditional surgery, or gamma
knife radiosurgery, as well as possible
audiologic follow-up — if and when war-
ranted.
Additional Information
The patient chose to undergo traditional
surgical removal of the acoustic neuroma.
The surgery was successful with respect to
the surgery and the patient did well postoperatively. However, no hearing could
be preserved in the left ear as is reflected
in Figure 6–2D.
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D
Figure 6–2. continued
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