Acoustic Neuromas: Treatment Options
BACKGROUND
In the early 1900s, the surgical mortality rate for
acoustic neuromas, also known as vestibular schwannomas
(VSs), was approximately 70 percent. Harvey Cushing,
a pioneer in the field of neurosurgery, developed an
intracapsular resection technique to prolong the patient’s
life, palliate symptoms and preserve cranial nerve function.
By the early 1930s, Cushing had achieved a 2 percent
mortality rate, which was unequaled by neurosurgeons
for the next 30 years.
Half of Cushing’s patients with subtotal resections
died from tumor recurrence within five years of the
original operation. The post Cushing generation of neurosurgeons
reported that the morbidity and mortality of surgery
for recurrent tumors were higher than for the initial
resection.
By the 1940s, Neurosurgeon Walter Dandy had developed
a surgical technique that allowed “total expiration,”
but sacrificed the seventh and eighth cranial nerves
and often required opening the internal auditory canal.
Dandy’s perioperative mortality rate for these
patients was 14%, but excepting a few patients with
small tumors, all patients suffered loss of their facial
nerve. Dandy’s approach accepted high perioperative
morbidiity and diminished quality of life to forestall
or eliminate recurrence of a symptomatic tumor.(i)
While the approaches of Cushing and Dandy defined the
limits of two distinct treatment philosophies, they
also provided the foundation upon which acceptable goals
and clinical outcomes have evolved over time. Many years
ago, the objective was simple debulking of the tumor
(which was often large) and relief of regional brainstem
compression and hydrocephalus. The goal was to save
lives. Neurological deficits such as hearing loss, facial
weakness, or balance disorders were tolerated as simply
part of the expected result. In the 1960s, the introduction
of the operating microscope facilitated meticulous dissection
of the tumor, making attempts at cranial nerve preservation
possible. Over the ensuing 20 years, preservation of
facial nerve continuity became more common than not.
During the 1980s, hearing preservation became an achievable
goal in selected cases.(ii)
“Even with the advanced technology we have today,
neurosurgeons and otologists still wrestle with these
issues,” says Dr. Leonard Cerullo, neurosurgeon
and founding member of the Chicago Institute of Neurosurgery
and Neuroresearch. “We can opt to resect a large
tumor, but the patient will without a doubt lose hearing
and could have facial nerve damage. And, recurrence
continues to be a possibility. Using a combination of
stereotactic radiosurgery and microsurgery, we can maintain
hearing and facial nerve functionality, but we may not
be able to remove the entire tumor. We do this now,
knowing that these patients must be followed long-term
anyway, so the strategy becomes one of recurrence management
or tumor control.”
OVERVIEW
Acoustic neuromas histologically are benign tumors
arising from Schwann cells that line the vestibular
branch of the eighth cranial nerve. The most common
tumor of the cerebellopontine angle, these neuromas
grow slowly. Nevertheless, they may be destructive locally,
causing erosion of the internal auditory canal (IAC)
and compression of the fifth and seventh cranial nerves
or, less often, the ninth and tenth cranial nerves either
alone or in various combinations.
Large tumors may be associated with compression of
the brainstem or obstruction of the cerebrospinal fluid
(CSF) pathway, resulting in hydrocephalus. Unilateral
vestibular schwannomas account for approximately eight
percent of all tumors inside the skull. One out of every
100,000 individuals per year develops a vestibular schwannoma.
Unilateral vestibular schwannomas are not hereditary.
Bilateral vestibular schwannomas affect both hearing
nerves and are usually associated with neurofibromatosis
type 2 (NF 2). Half of affected individuals inherit
the disorder from an affected parent and half seem to
have a mutation for the first time in their family.
Each child of an affected parent has a 50 percent chance
of inheriting the disorder.(iii iv)
SYMPTOMS & DIAGNOSIS
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| Acoustic neuroma as seen on MRI scan with contrast. |
The most common presenting feature of acoustic neuromas,
occurring in 90% of patients, is unilateral hearing
loss. When “pure tone audiometry” is used,
the most common finding is high frequency hearing loss.
The hearing loss is progressive in most patients, but
in approximately 12% of cases the hearing loss may occur
suddenly. Other symptoms of the acoustic neuroma include
asymmetric tinnitus, dizziness and disequilibrium. Symptoms
may develop at any age but usually occur between 30
and 60 years.
More than 80% of patients with acoustic neuroma have
tinnitus. In some patients the tinnitus is a pure tone,
and in others it is a noise. Many patients with acoustic
neuroma have combined tinnitus and hearing loss. Acoustic
neuromas typically begin in sites that are “transition
zones” from the central to the peripheral nervous
system along the eighth cranial nerve (the nerve that
subserves hearing and balance function).
As the acoustic neuroma expands, it fills the internal
auditory meatus, thereby compressing the cochlear and
facial nerves. Though hearing loss commonly occurs as
a result of tumor compression of the hearing nerve,
facial weakness often does not occur until acoustic
tumors grow quite large. Symptoms occur as a result
of compression of neural structures or their nutrient
blood supply. Acoustic neuromas do not invade and destroy
tissue as is common with cancerous tumors. Rather, the
major clinical concerns related to acoustic neuroma
growth center on the ability of these tumors to compress
the soft, neural tissue that is confined to the tight
quarters of the posterior fossa and internal auditory
canal. However, the greatest concern regarding acoustic
neuroma growth relates to the risks posed to the brain.
Ultimately if untreated, the acoustic neuroma can compress
the cerebellar peduncles, cerebellum, brainstem and
cranial nerves IX-XI (IX: glossopharyngeal nerve, X:
vagus nerve, XI: accessory nerve). This increased pressure
on key centers of the brain can result in neurologic
impairment.(v)
Acoustic neuromas are most frequently diagnosed by
MRI scan in a patient with unilateral hearing loss.
Important information to be determined from the MRI
scan are the distance the tumor extends laterally in
the auditory canal, the extent to which the tumor expands
in the cerebellopontine angle, and whether or not the
brain stem is contracted or distorted. Other important
diagnostic tests are the audiogram and the recordable
brain stem audio evoked responses (BAERs) because these
will indicate the possibility of saving hearing.(vi)
MANAGEMENT
For the treatment of patients with acoustic neuromas,
there are several options including observation, resection,
stereotactic radiosurgery (SRS), and fractionated stereotactic
radiotherapy (SRT). The goal of treatment for acoustic
neuroma is to maximize tumor removal while minimizing
the risk of complications.
Observational Management
Acoustic neuromas usually enlarge slowly. However,
it has been well documented that some tumors stop growing,
that spontaneous regression may occasionally occur,
and that a rare tumor may unexpectedly grow rapidly.
Bederson et al.vii reported 70 patients who were initially
observed because they did not want surgery or did not
have progressive symptoms. The average follow-up was
36 months (range, 6-84 months). During the first year
29 patients (41%) had no detectable tumor growth and,
of 18 who had a second-year scan, only one showed detectable
growth. In 37 patients (53%) growth ranging from I to
17 mm (average, 3.4 ± 0.5 mm) occurred during
the first year and, of 23 patients with a second-year
follow-up scan, 21 showed further growth. In four patients
(6%) there was regression in tumor size. Rapid growth
rate in seven and clinical deterioration in two other
patients without change in the size of the tumor led
to surgical intervention. There was no relationship
of tumor growth to age, duration of symptoms, or initial
tumor size.
Another study documented that there was no correlation
between tumor growth and the patient’s age and
that, over a period of eight months to over four years,
50% showed no change. The true incidence of cessation
of growth is unknown since these were selected patients,
many of whom had stable symptoms.(viii)
Resection of the Lesion
“Large tumors with significant brainstem compression
usually require resection,” says Dr. Sami Rosenblatt,
neurosurgeon. “Typically, we
consider the size of the tumor, neurological condition,
age, and the wishes of the patient. While there are
treatment algorithms that we follow, the ultimate decision
is really between the neurosurgeon and the patient,”
adds Rosenblatt.
There are three main surgical routes for resection
of an acoustic neuroma. The suboccipital retrosigmoid
approach is the oldest and remains widely used, particularly
when hearing preservation is attempted. The translabyrinthine
approach destroys hearing but provides direct exposure
to the tumor without requiring cerebellar retraction.
Even large tumors can be removed through this route.
The “middle fossa” approach is performed via
a temporal craniotomy but requires elevation of the
temporal lobe and drilling of the temporal bone to expose
the auditory canal from above. Using this route, hearing
preservation can be attempted. It is usually chosen
for patients with intracanalicular tumors.(ix)
In a recent report, Martin, et al.x evaluated quality
of life in patients after tumor resection. They found
a disparity between the patients’ reports and the
physicians’ assessments of function, with decreases
in physical functioning, general health, and social
functioning after surgery.(xi)
“With surgical resection, the trade-offs are considerable,”
says Dr. Cerullo. “Patients and neurosurgeons must
weigh the risks of hearing loss and facial weakness
with the benefits of total tumor removal. Thirty percent
of patients report that hearing loss is their most debilitating
problem following surgery,” adds Dr. Cerullo. “Facial
weakness is the second most debilitating side effect
from surgery.”
Stereotactic Radiosurgery
For patients with small- or medium-sized tumors, stereotactic
radiosurgery (SRS) has become a common treatment. Excellent
long-term results have been reported. Patients must
be comfortable with the concept of tumor control rather
than tumor removal. Most seem to be satisfied with this
concept, if it allows them to avoid brain surgery.(xii)
Most patients with smaller tumors do not have a rapidly
progressive neurologic syndrome yet have persistent
hearing loss, balance problems, tinnitus, vertigo, headache,
or a combination of these symptoms which are not improved
consistently by resection or by radiosurgery. Because
improvement in symptoms is rare after resection of an
acoustic tumor, many patients now prefer to choose a
treatment with minimal risk. For many patients, this
choice is stereotactic radiosurgery.(xiii)
The goals of radiosurgery are the long-term prevention
of tumor growth, maintenance of neurologic function,
and prevention of new neurologic deficits. In a recent
study of the long-term outcomes of radiosurgery for
acoustic neuroma Hasegawa et al.xiv followed 317 acoustic
neuroma patients who underwent radiosurgery for five
or more years. The median follow-up was 7.8 years. Seventy-seven
patients were followed 10 or more years after radiosurgery.
Facial numbness or weakness was 2% or less for patients
given tumor margin doses of 13 Gy or less. Preservation
of “useful hearing” was 68%.
“The beauty of SRS is that it is a non-invasive,
one-time treatment,” says Dr. Rosenblatt. “However,
we don’t see the real results or side effects until
six months down the road. Close follow-up with MRI is
required at three, six and 12 months. We follow these
patients for 10 years to look for possible recurrence,”
says Rosenblatt.
Large tumors, especially those that compress the brainstem
and distort the fourth ventricle, are of special concern
in radiosurgery. SRS is known to cause temporary tumor
swelling, which may result in peritumoral edema. Radiation-induced
peritumoral edema can cause facial weakness, numbness
or pain, and, eventually, gait disturbance. Additionally,
it may cause obstructive hydrocephalus. Although tumor
expansion is most likely to be transient, some patients
with large tumors exhibit neurological deterioration,
such as gait disturbance or severe facial pain, and
consequently, require craniotomy. Hasegawa et al.xv
found that large tumors or tumors causing a distortion
of the fourth ventricle should be completely or partially
removed by craniotomy first. Then, stereotactic radiosurgery
for vestibular schwannomas would justify partial resection
to avoid morbidities.
Hasegawa et al.xvi reported that upon follow-up MRI
scans, most vestibular schwannomas initially treated
with SRS showed central necrosis, followed by subsequent
enhancement of the necrotic area. Afterward, tumor volume
gradually decreased over three years and was followed
by marked regression over 10 years. They observed three
types of tumor expansion. One was tumor expansion with
central necrosis, which usually occurred three to six
months after SRS. In this type, further treatment was
rarely required as long as the tumor was not too large.
Another type was solid expansion, which occurred at
the time of re-enhancement after central necrosis. This
type sometimes required craniotomy or further radiosurgery,
although some patients in this group may not have needed
further treatment. Half of this expansion was transient,
and marked shrinkage occurred over the long term without
any other treatment. The last type was cyst formation
or enlargement. This type was likely to require further
treatment other than radiosurgery.
Fractionated Radiotherapy
In the past several years, a number of groups have
used fractionated radiotherapy to treat patients with
acoustic neuromas. This technique was developed when
surgeons at several centers that used LINAC-based irradiation
technology were not satisfied with the results or accuracy
of their device after single fraction irradiation (SRS).
To decrease the cranial nerve deficits, they began to
deliver radiation over multiple sessions (fractionation).
The goal of this approach is to weaken the effect of
the radiation on the tumor target. There are little
data on this approach in the peer-reviewed literature
that includes diligently documented outcomes and follow-up
data.(xvii) There are inherent contradictions from a
logical point of view with this approach.
Conclusion
Today, patients with acoustic neuromas have a variety
of treatment options. Because these tumors are generally
slow growing, observational management is often a preferred
first course of treatment. Depending on tumor size,
neurologic functionality, patient age, and co-existing
conditions, large tumors can be resected or removed
using a combination of SRS and microsurgery. For patients
with small or medium-sized tumors, SRS has become a
common treatment, with excellent reported long-term
results.
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