A
vestibular schwannomaA schwannoma is a benign nerve sheath tumor composed of Schwann cells, which normally produce the insulating myelin sheath covering peripheral nerves....
, often called an
acoustic neuroma, is a
benignA benign tumor is a tumor that lacks the ability to metastasize. Common examples of benign tumors include moles and uterine fibroids.The term "benign" implies a mild and nonprogressive disease. Indeed, many kinds of benign tumors are harmless to human health...
primary intracranial tumor of the
myelinMyelin is a dielectric material that forms a layer, the myelin sheath, usually around only the axon of a neuron. It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a type of glial cell. The production of the myelin sheath is called myelination...
-forming cells of the
vestibulocochlear nerveThe vestibulocochlear nerve is the eighth of twelve cranial nerves, and is responsible for transmitting sound and equilibrium information from the inner ear to the brain...
(CN VIII). The term "vestibular schwannoma" involves the vestibular portion of the 8th cranial nerve and arises from
Schwann cellSchwann cells or neurolemmocytes are the principal glia of the peripheral nervous system . Glial cells function to support neurons and in the PNS, also include satellite cells, olfactory ensheathing cells, enteric glia and glia that reside at sensory nerve endings, such as the Pacinian corpuscle...
s, which are responsible for the myelin sheath in the
peripheral nervous systemThe peripheral nervous system consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system to the limbs and organs. Unlike the CNS, the PNS is not protected by the bone of spine and skull, or by the blood–brain...
. Approximately 3,000 cases are diagnosed each year in the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
with a prevalence of about 1 in 100,000 worldwide. It comprises 5-10% of all intracranial neoplasms in adults. Incidence peaks in the fifth and sixth decades and both sexes are affected equally.
Signs and symptoms
The earliest symptoms of acoustic neuromas include ipsilateral
sensorineural hearing lossSensorineural hearing loss is a type of hearing loss in which the root cause lies in the vestibulocochlear nerve , the inner ear, or central processing centers of the brain....
/deafness, disturbed sense of balance and altered gait,
vertigoVertigo is a type of dizziness, where there is a feeling of motion when one is stationary. The symptoms are due to a dysfunction of the vestibular system in the inner ear...
with associated nausea and vomiting, and pressure in the ear, all of which can be attributed to the disruption of normal vestibulocochlear nerve function. Additionally more than 80% of patients have reported
tinnitusTinnitus |ringing]]") is the perception of sound within the human ear in the absence of corresponding external sound.Tinnitus is not a disease, but a symptom that can result from a wide range of underlying causes: abnormally loud sounds in the ear canal for even the briefest period , ear...
(most often a unilateral high-pitched ringing, sometimes a machinery-like roaring or hissing sound, like a steam kettle).
Large tumors that compress the adjacent brainstem may affect other local cranial nerves. Paradoxically, the 7th cranial nerves are rarely involved pre-operatively; involvement of the
trigeminal nerveThe trigeminal nerve contains both sensory and motor fibres. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing. Sensory information from the face and body is processed by parallel pathways in the central nervous system...
(CN V) may lead to loss of sensation in the involved side's face and mouth. The
glossopharyngealThe glossopharyngeal nerve is the ninth of twelve pairs of cranial nerves . It exits the brainstem out from the sides of the upper medulla, just rostral to the vagus nerve...
and
vagus nerveThe vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...
s are uncommonly involved, but their involvement may lead to altered gag or swallowing reflexes.
Larger tumors may lead to increased
intracranial pressureIntracranial pressure is the pressure inside the skull and thus in the brain tissue and cerebrospinal fluid . The body has various mechanisms by which it keeps the ICP stable, with CSF pressures varying by about 1 mmHg in normal adults through shifts in production and absorption of CSF...
, with its associated symptoms such as headache, vomiting, and altered consciousness.
Hearing loss
Patients with a severe or profound unilateral hearing loss following the removal of an acoustic neuroma tumour are significantly disabled in a number of situations such as hearing sounds from the deaf side, hearing in the presence of background noise (both in quiet and noisy surroundings) and localising sounds.
The perceived hearing handicap may even be greater in unilateral losses than in bilateral. It has also been reported that patients with unilateral hearing loss experience difficulties in group discussions and dynamic listening situations where there is limited possibility to compensate for the handicap by changing listening position.
It is recommended that the hearing difficulties after tumour removal should be thoroughly examined with each patient and rehabilitative options discussed.
Pathogenesis
Acoustic
neuromaA neuroma is a growth or tumor of nerve tissue. Just as the Latin word for swelling is now restricted to neoplasias, the equivalent Greek suffix -oma has shared in that fate. Thus, the typical modern usage of neuroma is for nerve tumors...
s may occur sporadically (meaning the cause is unknown), or in some cases occur as part of
von Recklinghausen neurofibromatosisNeurofibromatosis type I , formerly known as von Recklinghausen disease after the researcher who first documented the disorder, is a human genetic disorder. It is possibly the most common inherited disorder caused by a single gene...
, in which case the neuroma may take on one of two forms.
- In Neurofibromatosis type I, a schwannoma may sporadically involve the 8th nerve, usually in adult life, but may involve any other cranial nerve or the spinal root. Bilateral acoustic neuromas are rare in this type.
- In Neurofibromatosis type II, bilateral acoustic neuromas are the hallmark and typically present before the age of 21. These tumors tend to involve the entire extent of the nerve and show a strong autosomal dominant inheritance. Incidence is about 5 to 10%.
The usual tumor in the adult presents as a solitary tumor, originating in the nerve. It usually arises from the vestibular portion of the 8th nerve, just within the internal auditory canal. As the tumor grows, it usually extends into the posterior fossa to occupy the angle between the cerebellum and the pons (
cerebellopontine angleThe cerebellopontine angle is a structure at the margin of the cerebellum and pons.Breast cancer can metastasize to the cerebellopontine angle.It can be a site for lipoma.It is where acoustic neuromas are usually found.-External links:...
). Because of its position, it may also compress the 5th, 7th, and less often, the 9th and 10th cranial nerves. Later, it may compress the pons and lateral medulla, causing obstruction of the
cerebrospinal fluidCerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...
and increased
intracranial pressureIntracranial pressure is the pressure inside the skull and thus in the brain tissue and cerebrospinal fluid . The body has various mechanisms by which it keeps the ICP stable, with CSF pressures varying by about 1 mmHg in normal adults through shifts in production and absorption of CSF...
.
Schwannomas can occur in relation to other cranial nerves or
spinal nerve roots, resulting in
radiculopathyRadiculopathy is not a specific condition, but rather a description of a problem in which one or more nerves are affected and do not work properly . The emphasis is on the nerve root...
or spinal cord compression. Trigeminal neuromas are the second most common form of schwannomas involving
cranial nervesCranial nerves are nerves that emerge directly from the brain, in contrast to spinal nerves, which emerge from segments of the spinal cord. In humans, there are traditionally twelve pairs of cranial nerves...
. Schwannomas of other
cranial nervesCranial nerves are nerves that emerge directly from the brain, in contrast to spinal nerves, which emerge from segments of the spinal cord. In humans, there are traditionally twelve pairs of cranial nerves...
are very rare.
Diagnosis
Contrast-enhanced
CTX-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...
will detect almost all acoustic neuromas that are greater than 2.0 cm in diameter and project further than 1.5 cm into the cerebellopontine angle. Those tumors that are smaller may be detected by MRI with
gadoliniumGadolinium is a chemical element with the symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with...
enhancement. Audiology and vestibular tests should be concurrently evaluated using air conduction and bone conduction threshold testing to assess for sensorineural versus conduction hearing loss.
Treatment
Indicated treatments for acoustic neuroma include surgical removal and radiotherapy. About 25% of all acoustic neuromas are treated with medical management consisting of a periodic monitoring of the patient's neurological status, serial imaging studies, and the use of hearing aids when appropriate.
Conservative treatment
Because these neuromata grow so slowly, a physician may opt for conservative treatment beginning with an observation period. In such a case, the tumor is monitored by annual MRI to monitor growth. This route is common among patients over 70 years old. Records suggest that about 45% of acoustic neuromata do not grow detectably over the 3–5 years of observation. In rare cases, acoustical neuromata have been known to shrink spontaneously. Often people with acoustic neuromata
dieDeath is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....
of other causes before the neuroma becomes life-threatening. This is especially true of elderly people possessing a small neuroma.
Since the growth rate of an acoustic neuroma rarely accelerates, annual observation is sufficient. Acoustic neuromata may cause either gradual or—less commonly—sudden
hearing loss-Definition:Deafness is the inability for the ear to interpret certain or all frequencies of sound.-Environmental Situations:Deafness can be caused by environmental situations such as noise, trauma, or other ear defections...
and
tinnitusTinnitus |ringing]]") is the perception of sound within the human ear in the absence of corresponding external sound.Tinnitus is not a disease, but a symptom that can result from a wide range of underlying causes: abnormally loud sounds in the ear canal for even the briefest period , ear...
.
Surgery
Removal of acoustic neuromas may be performed using several approaches. Each approach has its advantages and disadvantages. Microsurgery for acoustic neuroma is the only technique that removes the tumor. Radiation treatment (discussed in another section) does not remove the tumor, but has the potential to slow or stop its growth. Surgery is the only treatment that will definitively treat balance symptoms associated with tumor growth, as the vestibular nerves are removed at surgery.
Surgery cannot repair damage that has already occurred to the facial or hearing nerves.
Even after surgery, there is a small chance that the neuroma will grow back and follow-up MRI scans are necessary.
Choice of surgical approach is based on the patient's age, medical condition, size of tumor, and preoperative hearing thresholds and speech discrimination, as well as other tests such as electronystagmography, imaging, and auditory brainstem response testing. The patient's and surgeon's preferences also play a significant role.
During removal of the tumor, the tumor along with the superior and inferior vestibular nerves are removed. This results in an acute loss of vestibular input to the brain from the operated side. However, vestibular function improves rapidly due to compensation by the other ear and other balance mechanisms.
Surgery carries risk to the
facial nerveThe facial nerve is the seventh of twelve paired cranial nerves. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity...
which may therefore be monitored during the procedure. Best results (normal or near normal facial function) are more likely with small acoustic neuromas.
Three surgical approaches are commonly used. The first is the
translabyrinthine approachThe translabyrinthine approach is a surgical approach to the cerebellopontine angle, or CPA. It is used in the surgical extirpation of lesions of the cerebellopontine angle, including acoustic neuroma....
, which destroys hearing in the affected ear. Thus, it is often employed in patients who already have poor speech discrimination in the affected ear. Any size tumor may be removed with this approach. There is no brain retraction with this approach, so it is often considered the safest route to remove the tumor. In patients with neurofibromatosis type 2 who undergo auditory brainstem implantation, this technique is used as it provides the most direct path of access to the
lateral recessThe lateral recess is a projection of the fourth ventricle which extends into the inferior cerebellar peduncle of the brainstem. The lateral aperture, an opening in each extremity of the lateral recess, provides a conduit for cerebrospinal fluid to flow from the brain's ventricular system into the...
and cochlear nucleus, where the device is placed.
The two other approaches (suboccipital retrosigmoid and middle fossa) are hearing preservation approaches, which have a chance of preserving some or all of the hearing in the affected ear. Neurosurgeons often prefer the retrosigmoid approach, as they are frequently more familiar with it from training.
The middle fossa approach is used for tumors typically less than 2 cm in greatest dimension, where hearing conservation is to be attempted. This approach has the advantage over the retrosigmoid approach in its direct access to the lateral end of the internal auditory canal. Multiple reports have shown that the retrosigmoid approach cannot reach the lateral end of the internal auditory canal without violating the posterior semicircular canal, and hence destroying the hearing.
A less common approach is minimally invasive endoscopic surgery. This approach is available in specialized centers.
Acoustic neuroma surgery is highly technically demanding. It may be performed by neurosurgeons or otolaryngologists, alone or together.
Radiation therapy
Radiation therapyRadiation therapy , radiation oncology, or radiotherapy , sometimes abbreviated to XRT or DXT, is the medical use of ionizing radiation, generally as part of cancer treatment to control malignant cells.Radiation therapy is commonly applied to the cancerous tumor because of its ability to control...
is done in a variety of ways, but mainly by four methods:
CyberKnifeThe CyberKnife is a frameless robotic radiosurgery system used for treating benign tumors, malignant tumors and other medical conditions. The system was invented by John R. Adler, a Stanford University Professor of Neurosurgery and Radiation Oncology, and Peter and Russell Schonberg of Schonberg...
, gamma knife
radiosurgeryRadiosurgery is a medical procedure that allows non-invasive treatment of benign and malignant tumors. It is also known as stereotactic radiotherapy, when used to target lesions in the brain, and stereotactic body radiotherapy when used to target lesions in the body...
, fractionated stereotactic radiotherapy, with a linear accelerator (linac), or
proton therapyProton therapy is a type of particle therapy which uses a beam of protons to irradiate diseased tissue, most often in the treatment of cancer. The chief advantage of proton therapy is the ability to more precisely localize the radiation dosage when compared with other types of external beam...
. In the gamma knife approach, 201 beams of gamma radiation are focused on the tumor in a single session. The damage to the tumor at the convergence point may cause it to stop growing but usually does not cause it to shrink in the long term. It may cause short-term shrinkage due to necrosis in the tumor. The damage may be to the tumor cells and/or to the tumor vasculature.
It is not clear what percentage of tumors are controlled by this method for long periods. In earlier times when higher radiation doses were used, the failure rate was about 12% (which then required surgery). Most surgeons feel that these tumors are much more difficult to remove after radiation treatment. Radiation does not remove the tumor, and when irradiated tumors are surgically removed, it is often found that they have growing tumor cells in them.
Two risks of radiation treatment are carcinogenic progression of the acoustic neuroma (conversion from
benignA benign tumor is a tumor that lacks the ability to metastasize. Common examples of benign tumors include moles and uterine fibroids.The term "benign" implies a mild and nonprogressive disease. Indeed, many kinds of benign tumors are harmless to human health...
to
malignantMalignancy is the tendency of a medical condition, especially tumors, to become progressively worse and to potentially result in death. Malignancy in cancers is characterized by anaplasia, invasiveness, and metastasis...
) or induction of other tumors (such as glioblastoma) in the nearby irradiated brain tissue. The incidence of these events appears to be low, and it is often said to be one in one thousand or less (however, the incidence is markedly higher in patients with neurofibromatosis Type 2). This calculation is done by dividing the number of obvious cases of tumorigenic progression or secondary tumor reported in the medical literature by the estimated number of gamma knife procedures done in the world to date. This is not a scientifically valid method of estimating the carcinogenic risk of medical radiation exposures, and involves a list of very questionable assumptions.
The proper and scientifically valid way to estimate such risks can be found at the web site of the
Health Physics Society, where estimates of the risks of CT scans and other procedures can be found. These calculations have never been made for gamma knife radiosurgery.
Due to the possibility of regrowth and the possibility of tumorigenic progression or secondary tumors, it is essential that radiation treatments for acoustic neuromas be followed by yearly MRI for the rest of the patient's life. MRIs at this time (2007) cost between $1,500-$3,000. Long-term secondary effects (for instance cognitive effects) on a scale of 10–20 years are not yet established for gamma knife surgery.
Fractionated stereotactic therapy involves a beam of ionizing radiation focused on the tumor from a moving
gantryMedical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...
. The beam is wider and less accurate than that of the gamma knife. The total dose is also much higher than that used in gamma knife radiosurgery, but the fractionation of the dose (done on many different days) spares normal tissue. This method has not been done on as many patients as gamma knife procedures and there have not been as many years of follow-up study. This means that the tumor control by this method is not yet established, and the incidence of secondary effects of the radiation are not yet known. There are a number of variations on the linac machine, which can confuse patients. The best known are the "Peacock" which is essentially a modified collimator, and the
CyberknifeThe CyberKnife is a frameless robotic radiosurgery system used for treating benign tumors, malignant tumors and other medical conditions. The system was invented by John R. Adler, a Stanford University Professor of Neurosurgery and Radiation Oncology, and Peter and Russell Schonberg of Schonberg...
which uses a miniature linac machine attached to a robot arm which is guided using x-ray imaging to check the position of the patient between each treatment shot.
A
proton therapyProton therapy is a type of particle therapy which uses a beam of protons to irradiate diseased tissue, most often in the treatment of cancer. The chief advantage of proton therapy is the ability to more precisely localize the radiation dosage when compared with other types of external beam...
machine uses a beam of protons to kill the tumour and a
cyclotronIn technology, a cyclotron is a type of particle accelerator. In physics, the cyclotron frequency or gyrofrequency is the frequency of a charged particle moving perpendicularly to the direction of a uniform magnetic field, i.e. a magnetic field of constant magnitude and direction...
is used to generate the beam. This is preferable to the x-rays used by the linac and gamma knife machines as the protons can be stopped before they exit the tumor, thus reducing damage to normal tissue.