Advances in medicine,
especially imaging technology have made the identification of small Acoustic
Neuromas (AN) possible. After routine auditory tests reveal loss of hearing and
speech discrimination (i.e. "I can hear sound in that ear, but can't understand
what's being said") a special test for hearing which records responses from the
brain-stem called the auditory brainstem response test (ABR, BAER, BSER) maybe
done. The results of this test detect the cause of a poorly functioning 8th
nerve. If an abnormality in the ABR test suggests an AN, imaging is done to
confirm the diagnosis. I do
not perform the ABR test in all patients to diagnose an acoustic neuroma because
imaging techniques (MRI/CT scans) are the gold standard for diagnosis. CT scan
has proven to be a powerful tool in locating AN's. The only drawback is that
small tumors confined to the internal auditory canal (IAC) may not show on plain
CT scan. Such cases require air or contrast materials to be introduced into the
body in order to enhance the tumor. Therefore, the MRI, a more recently
developed diagnostic test, has become the gold standard for diagnosis of AN.
Gadolinium is the contrast material used to define & enhance the tumor.
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Small tumors A small tumor is also called intracanalicular because it is confined within the bony internal auditory canal (figure). A patient with such a tumor may have hearing loss, ringing in the ear or ear noise, and vertigo or dizziness. |
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Medium tumors A medium sized acoustic neuroma is one that has extended from the bony canal into the brain cavity, but has not yet produced pressure on the brain itself (figure). Patients with such tumors have worsening of their hearing, difficulty in balance, in addition to dizziness, and occasionally, the onset of headaches due to irritation of the lining of the brain called dura. Some patients may experience numbness of the mid-face or diminished sensation in the eye during the later stages. |
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Large tumors A large tumor is one that is extended out of the internal auditory canal in to the brain cavity and is sufficiently large to produce pressure on the brain and disturb vital centers in the brain (figure). During this stage, all previous symptoms worsen; facial twitch and weakness may occur, and finally patient may develop hydrocephalus due to the blockage of the cavity which contains CSF-the resultant symptoms are headache, visual loss and double vision. |
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This picture shows the microscopic appearance of a normal vestibular
(8th) nerve passing through the internal auditory canal (IAC) to supply
the organ of balance. The facial (7th) nerve runs along with the 8th
nerve in the IAC. The organ of hearing (cochlea) is also seen in this
picture. |
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This picture shows an AN tumor arising from the 8th nerve, within the
IAC. |
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This is a higher magnification of the above picture showing the junction
of the tumor and the VII nerve. The arrows indicate the sheath (covering
layer) of the tumor. |
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This picture is a high magnification of the same tumor showing the
arrangement of the fibers within the AN. The arrow indicates a whorled
appearance of the fibers while the upper part of the tumor shows loosely
packed (Antoni B) fibers. |
ACOUSTIC
NEUROMA-THE BASIC FACTS
Origin and Cause
What is an acoustic neuroma?
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An acoustic
neuroma (sometimes also termed a neurinoma or vestibular schwannoma) is
a benign or non-cancerous growth that arises from the 8th or vestibulo-cochlear
nerve. The 8th nerve is actually 2 separate nerves, the vestibular nerve
and the cochlear nerve. The vestibular nerve is responsible for balance
while the cochlear nerve is responsible for hearing. The vestibular
nerve has 2 parts-the superior vestibular nerve (SVN) and the inferior
vestibular nerve (IVN). These nerves lie adjacent to each other as they pass through a bony canal, from the inner ear to the brainstem. This bony canal is called the internal auditory canal (IAC) and it varies in length from 0.4 to 1.2 cm. We have two figures of a temporal bone (that part of the skull which has the ear in it) dissection to the right. The first figure is a view from the top showing the middle ear and the internal auditory canal (IAC) with the nerves passing through it. The organ of hearing (cochlea) and the dura lining the IAC can be seen clearly. The second figure is a magnification of the IAC region showing the different nerves passing through it. This figure also demonstrates clearly, the cochlear nerve supplying the cochlea. Acoustic neuromas usually arise from the cells of the VIII nerve within the internal auditory canal (third figure). The third figure is a schematic drawing showing an acoustic neuroma arising from the vestibular nerve within the IAC. The facial or 7th nerve that is responsible for facial movement, along with important blood vessels, also passes with the 8th nerve in the canal (figures). The cause of acoustic neuroma is unknown. A small percentage of individuals have a hereditary condition called neurofibromatosis type 2 (NF-2). These patients may have an acoustic neuroma on both sides with an aggressive growth pattern and often involve adjacent nerves. |
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What is the growth pattern?
Acoustic neuromas usually grow very slowly over a period
of many years. Once the tumor fully occupies the internal auditory canal, it
often begins to erode the walls of the canal and enlarges it. This bony erosion
however, does not always occur. They typically remain within their capsule or
lining and displace the surrounding nerves and brain tissue very slowly. This is
why the body has ample time to accommodate the abnormal growth. The tumor first
distorts the 8th nerve, and then presses on the adjacent 7th nerve. The 7th
nerve is gradually stretched into a ribbon like structure over the enlarging
tumor (figure; cross section of the 7th nerve is shown in the right half of the
figure). As the tumor slowly enlarges towards the brain, it protrudes from the
internal auditory canal into an area of the skull called cerebello-pontine
angle. The tumor is now pear or mushroom shaped with the smaller end within the
canal and the larger part towards the brain (figure). It is at this stage that
the tumor presses adjacent nerves like the trigeminal or 5th nerve responsible
for facial sensation. Ultimately, with increasing tumor size, it can press on
the brainstem which can be life threatening.
How often do acoustic neuromas occur?
Acoustic neuromas have been known to occur in all areas of the world without any predilection for individuals of any ethnic background. Small AN's without any symptoms, have been found on autopsy in 2.5% of the general population. Estimates of symptomatic AN range from 1 in every 3,500 to 5 in every million people. It appears that women are more affected than men and most AN's are diagnosed between the ages of 30 & 60 years.
For more information, you may visit the Acoustic Neuroma Association Web site
Symptoms
Early symptoms of AN
can occur in other conditions of the ear that can be easily overlooked. Early
diagnosis of AN is quite challenging because there is no typical pattern.
However, there are symptoms that act as indicators to the possibility if an AN.
Patients with "inner ear" problems should be completely evaluated to rule out AN
as a cause of these symptoms. It is possible that Meniere's disease or hardening
of the bone of the middle ear (otosclerosis) could be causing these symptoms.
Patients with AN may present the following symptoms:
HEARING LOSS
In over 90 percent of the patients with AN, the first symptom is a reduction in
hearing in one ear due to involvement of the VIII nerve. This is usually
accompanied by ringing in the ears or ear noise-also called "tinnitus". The
hearing loss is usually subtle and worsens very slowly over a period of time. In
some cases, the hearing loss may be sudden. Some patients may experience a sense
of fullness in the affected ear. Unfortunately, since hearing loss is often mild
and there is no pain, patients tend to ignore the change in hearing and merely
shift the phone to the opposite ear or make other compromises for the one-sided
hearing loss rather than seek medical attention.
VERTIGO & IMBALANCE
The tumor usually arises from the vestibular or balance nerve.
As a result, unsteadiness or balance problems may be one of the earlier
symptoms in the growth of the tumor. Since the remainder of the balance system
compensates for this loss, balance problems may be forgotten after some time.
If the tumor grows
larger in size it may start to press on other nerves, mainly the trigeminal
nerve, causing facial sensation to become affected.
Patients may then experience constant or intermittent numbness and facial
tingling. Patients may also have facial tics or spasms. If the tumor grows
larger and presses on the brainstem raised intracranial pressure may cause
headaches, facial weakness, vertigo and an unsteady gait to ensue.
There are 3
treatment options available for AN
1) Observation
2) Microsurgical
removal (partial or total)
3) Stereotactic
radiation therapy (radiosurgery)
Observation
AN are occasionally discovered incidentally while evaluating another problem
or when the tumor is very small with subtle symptoms. Since AN are benign tumors
and produce symptoms due to pressure on surrounding structures, careful
observation over a period of time may be appropriate for some patients. For
instance, a small tumor diagnosed in an elderly patient may only require
observation to study the growth rate of the tumor if acute symptoms are not
present. If it appears that the tumor will not need to be treated during the
patient’s normal life expectancy, treatment and its potential risks and
complications maybe avoided. In these patients, MRI is performed periodically to
monitor growth of the tumor. If there is no growth, observation is continued. On
the other hand, if the tumor shows increase in size, treatment may become
necessary. Another group of patients for whom observation is preferred is in
patients who have a tumor in their only or better hearing ear, particularly if
it is a size where hearing preservation is unlikely. In such cases, periodic MRI
is done to monitor growth and surgery is considered only if the hearing is lost
or the tumor size becomes life threatening.
Microsurgical removal
At the present time, the only treatment that can cure the patient is removal
of the tumor by surgery. Within the last 2 decades, microsurgical techniques
have been pioneered and refined. Use of the operating microscope, finely scaled
surgical instruments, alternate cutting & tumor reducing tools, and better
anesthesia, have reduced the death rate extremely. In addition, results have
improved as surgeons have gained experience in the delicate removal process of
the tumor.
Three main surgical
approaches are used depending upon the location, tumor size and hearing level of
the patient. They are- middle fossa (MF), sub-occipital (SO), and the
trans-labyrinthine (TL) approach. Surgery for AN's is done under general
anesthesia using an operating microscope. Postoperatively, one to several days
may be spent in the intensive care with careful monitoring. Problems that may
develop in the immediate postoperative period including headache, dizziness,
imbalance, vomiting and decreased mental alertness due to the development of a
blood clot causing obstruction to the flow of cerebrospinal fluid (CSF).
Other early
complications may include cerebrospinal fluid leak and meningitis, an infection
controlled with antibiotics that will require a longer hospitalization. Some
patients and their surgeons prefer incomplete removal of an AN in order to
reduce the risk of complications, realizing that further surgery maybe needed in
the future. Occasionally in cases with large tumors, disturbances in the vital
brain centers during surgery require ending the surgery prior to complete tumor
removal. In these cases, the tumor which was left behind is followed with MRI
scans and if tumor growth is demonstrated, further surgery maybe necessary to
remove the growing tumor. On the other hand, if the tumor shows no growth,
observation is continued. Partial tumor removal maybe also be required in a
patient with an only hearing ear such as a Neurofibromatosis-2 (NF 2) patient.
Unfortunately, partial removal may result in substantial hearing loss in these
patients and this risk must be considered.
Small tumor
If
the hearing is still preserved in such tumors, a middle fossa approach, incision
for which is in front of the ear (figure) may be considered. A small square
piece of bone from the side of the skull is then removed (blue shaded area in
the figure). The tumor is removed completely in most cases. On rare occasions,
partial removal is possible. This approach attempts to preserve the hearing in
all cases while removing the tumor. In about half of the patients, the tumor
involves the hearing nerve or the artery supplying the inner ear and in such
cases, total loss of hearing occurs in the operated ear.
In addition, the risk to the facial nerve is far greater in this
approach/
Medium tumor
The
operation for medium sized tumors is performed by the sub-occipital and/or the
trans-labyrinthine approach. The incision for these approaches is behind the
ear, overlying the mastoid, the bony projection felt behind the ear (figures).
The mastoid and the inner ear structures are removed to expose the tumor, and
remove it completely. The opening created in the mastoid bone is closed with fat
taken from the abdomen. The translabyrinthine approach sacrifices the hearing
and balance mechanism since the inner ear is entered. Consequently, the ear is
made permanently deaf. In such cases, the balance mechanism of the opposite ear
compensates for the non-functioning operated ear and provides stabilization for
the patient within few weeks to months.
Large tumor
Surgery
for large tumors requires extensive removal of bone to properly expose the tumor
and control the large blood vessels that make access to the tumor difficult. For
this reason, special studies of the arteries (arteriograms) may be required in
addition to the other investigations, in order to diagnose and establish the
size of the acoustic tumor. The operation for large tumors is performed by the
TL-SO approach as described for medium tumors. The figure to the right shows the
area of the skull approached via the TL and the SO approaches. In these
patients, total removal is attempted unless changes in vital signs occur. If
there are changes in blood pressure, pulse rate, or respiratory rate, the
surgery must be terminated even if the tumor has not been totally removed. The
opening in the mastoid is closed with abdominal fat. For large tumors, it is
often necessary to monitor the patient’s general status by inserting a small
tube (arterial line) into an artery in the arm or leg. In these cases,
occasionally a blood clot may form in the artery following surgery. In case this
complication occurs, further surgery maybe required to remove the blood clot. A
very rare complication of this arterial line monitoring is the loss of a finger,
toe, or even a hand or a foot.
Stereotactic Radiation Therapy (Radiosurgery):
This is a technique based on the principle that a single relatively high
dose of radiation delivered precisely to a small area will arrest or kill the
tumor while minimizing injury to the surrounding nerves & brain tissue. The
source of radiation is from either radioactive cobalt (called gamma ray) or a
linear accelerator (LINAC). The treatment team consists of a neurosurgeon,
radiophysicist and a radiation oncologist working together to develop a
treatment plan based on the size & shape of the tumor. Radiation, even at
relatively high doses such as those used in radiosurgery, does not kill or
injure cells immediately. Some tumor cells die in weeks while others die more
gradually over 6-18 months after radiation. This treatment usually arrests
growth of the tumor and some tumors shrink, but they rarely disappear.
Follow-up of these
patients is important because approximately 20% of tumors continue to grow after
radiosurgery or at some time in the future. A tumor that has been irradiated and
grows may be more difficult to remove than an un-radiated tumor. Symptoms such
as dizziness & disturbances in balance typically improve earlier after
microsurgical tumor removal than after radiosurgery. This is because effects of
radiosurgery may require up to 18 months. Residual dizziness & imbalance may be
less after microsurgical treatment. The side effects of radiosurgery may be
headache, dizziness, nausea, facial numbness, or rarely, cranial nerve
paralysis. In the long term requires follow-up MRI's over the years and there is
a potential for additional treatment in cases of continued growth or later
re-growth.
Microsurgery requires
follow-up MRI's suggested at perhaps 1 and 5 years if the tumor has been
completely removed. Radiosurgery may be considered in selected patients in whom
the risk of surgery is excessive because of advanced age or pre-existing health
problems, patients having small to moderate sized tumors or patients with tumors
on both sides, or in the only hearing ear.
Microsurgery of an AN
is a complex and delicate procedure. The smaller the tumor at the time of
surgery, the fewer the chances are for complications. As the tumor size
increases, the chances of complications become greater. Thus, there may be
problems with the cranial nerves affected by the tumor (like facial paralysis or
hearing loss) following surgery that may or may not have been present before
tumor removal. Here is a list of
some of the more common post-operative issues and problems encountered.
Residual problems
This period is the days or perhaps weeks following surgery. There is a possibility of fatigue or tiredness and increased drowsiness, although some patients may experience "survival euphoria" and a renewed sense of energy and vigor. A period of emotional lows is common as the patient adjusts to physical changes. One symptom that may occur after discharge is a nasal drip of clear colorless fluid, which is particularly noticeable when bending over. This may indicate a cerebrospinal fluid leak and should be reported to the surgeon right away due to the risk of infection.
Follow-up period
:After being discharged from the hospital, patients operated for an AN are
followed up regularly (every 2-3 months for the first year, every 6 months for
the 2nd year, and every year thereafter). These follow up visits are important
to monitor the hearing (in patients operated by the MF or SO approach), facial
nerve paralysis if any and for recurrence of tumor.
HEARING LOSS
With small tumors, it may be
possible to save hearing. In larger tumors, especially those that have extended
into the brain cavity, the hearing has usually been partially or totally lost
and cannot be restored. This loss means the patient will continue having
problems locating sound, hearing on the deaf side and understanding speech over
high background noise. Consultation with an audiologist is required for these
patients for amplification options like traditional hearing aids or a CROS
hearing aid (a device which crosses sound over from the operated ear to the
opposite ear) or a BAHA.
TINNITUS
Ear noises usually remain the
same as before surgery, though in a few cases noises may increase or begin after
surgery. A masking device may help some people affected by tinnitus.
FACIAL WEAKNESS OR
PARALYSIS
Since the facial nerve which
controls muscles of facial expression is in close proximity with the AN, it is
usually necessary to manipulate and at times remove the portion of the nerve. In
some cases however, even though the nerve is intact after surgery, nerve damage
or swelling may cause temporary or in some cases permanent facial paralysis.
Regrowth of the nerve is a slow process that may take up to a year for recovery
to be noticeable. If recovery is not observed by 1 year, a second operation may
be required to connect the healthy portion of the facial nerve to a nerve in the
neck usually the one supplying one side of the tongue. This procedure is called
the hypoglossal-facial nerve anastamosis and can restore some but not all facial
movement. Spontaneous movements like laughing are asymmetric. There may be loss
of tongue function. There are some other procedures that adapt available muscles
and nerves to help in toning or reanimating the sagging face. If it becomes
necessary to remove a portion of the facial nerve during surgery, the facial
nerve may be reconnected directly or by inserting a nerve graft. Usually, the
result is asymmetric but will provide some spontaneous movement.
EYE PROBLEMS
Studies
have shown that at least half of those who have had an acoustic neuroma removed
develop long term eye discomfort and other eye problems, particularly if the
tumor was medium or large. Loss of eyelid function and/or altered tear
production can cause irritation and scratchiness in the eye because it is dry &
unprotected. To deal with this problem, there are various surgical procedures
that can be done to protect the cornea. They include canthoplasty (bringing
together tendons in either or both corners of the eye), a spring implantation in
the upper lid, an elastic prosthesis secured around the upper and lower lids, a
gold weight implant in the upper lid; and a tarsorapphy (sewing the lids
together). Artificial tears or eye lubricants maybe needed for a short time or
permanently. Taping part of the lids together, using protective glasses and
moisture chamber, using bandage contact lenses and avoiding eye irritants may be
helpful. In a few patients, double vision may be present due to pressure on the
6th cranial nerve that controls the muscles that move the eyes.
TASTE DISTURBANCE
AND MOUTH DRYNESS OR EXCESSIVE SALIVATION
There maybe some changes in
taste and amount of saliva secretion for a short time following surgery. In some
cases this may be prolonged. In the others, increased salivation occurs while
chewing or there maybe increased tearing while eating. The appetite maybe
affected for some time.
SWALLOWING, THROAT
AND VOICE PROBLEMS
In a small number of
patients, AN surgery affects the nerves which control the throat, swallowing and
voice production leading to hoarseness & difficulty in swallowing. These
symptoms usually improve slowly over time.
BALANCE PROBLEMS
The vestibular portion of the
VIII nerve is almost always removed during surgery. Usually this part of the
nerve is non-functional and has already been destroyed because of the AN.
Dizziness is common following surgery and maybe severe for a time. After a
while, the balance apparatus of the opposite or normal ear compensates for this
loss, and balance improves. This compensation may not be perfect, particularly
in darkness, when the patient is fatigued, when there is a sudden change in body
position, or while walking on uneven surfaces. Maintaining a good general
physical health through proper diet and moderate exercise, can improve balance &
general vitality to a great extent.
FATIGUE
Fatigue sometimes remains a
prolonged problem for some patients after some of the other symptoms have
subsided. It is important in such patients to adjust their pace of life in
harmony with their energy level.
HEADACHE
Headaches
can be a problem for some patients while still in the hospital. This maybe
related to tension from holding the head rigidly, changes in intracranial
pressure, muscle spasm, or anxiety. Headaches are almost never related to tumor
recurrence. Treatment is with analgesics & muscle relaxation. If severe
headaches persist after hospital discharge, medical help should be sought.
DENTAL CARE
If the patient has facial
paralysis, food tends to get lost in the mouth on the affected side and can lead
to dental problems. Washing and rinsing the mouth is therefore necessary, as
well as brushing & flossing the teeth several times a day is important.
PROTECTING THE
OTHER EAR
It is important to provide
sensible protection to the opposite or good ear that has the remaining hearing
apparatus. This is done by avoiding extreme or sudden exposure to loud noises
like firearms or some cordless phones near the good ear. Some physicians suggest
follow-up MRI scans and/or audiograms for some time following AN removal.
PSYCHOLOGICAL COPING
For some patients, adjustment to a new self after AN removal can be a
challenging task. This is because in addition to changes in hearing, the
appearance may now be altered along with the presence of other impairments.
Return to normal activity may be slow. Concentrating on strengths rather than on
weaknesses will help such patients to return to all former activities and also
expand their abilities in new areas.
