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LESSON 143

Vestibular schwannoma

Year 2
Spring semester
Disorders of the Auditory and Vestibular Systems
Inner ear and retrocochlear disorders
Duration: 12:29

We explain where a vestibular schwannoma arises, how it grows and which symptoms reveal it. With examples, we cover asymmetric sensorineural loss, word recognition, acoustic reflex and ABR findings, neurofibromatosis type 2, and the audiologist's role in monitoring.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson, we learn about vestibular schwannoma and tumors of the cerebellopontine angle.
We will cover the symptoms, the audiological findings, neurofibromatosis type 2 and the audiologist's role in monitoring.
A vestibular schwannoma is a benign tumor that grows on the nerve of hearing and balance.
The tumor arises from Schwann cells, which form the myelin sheath around nerve fibers.
It usually starts on the superior or inferior vestibular nerve, inside the internal auditory canal.
An origin on the cochlear nerve is rare; even so, the most common symptom is hearing loss.
Its old name is acoustic neuroma, but the tumor arises neither from the hearing nerve nor from neurons.
It accounts for about eight percent of intracranial tumors and is usually unilateral.
As the tumor grows, it extends out of the internal auditory canal into the cerebellopontine angle.
The cerebellopontine angle is a narrow space between the cerebellum and the pons, where the nerves enter the brainstem.
The facial nerve also runs with the eighth nerve in the internal auditory canal.
This is why the tumor can affect hearing and balance and, less often, the facial nerve.
Pressure on the labyrinthine artery, which supplies the inner ear, is one proposed explanation for sudden hearing loss.
About eighty to ninety percent of cerebellopontine angle tumors are vestibular schwannomas.
Second comes meningioma, which arises from the meninges and is usually benign.
Epidermoid cysts and schwannomas of the facial nerve are less common.
These masses can give similar audiological findings; the physician differentiates them with imaging.
In this lesson, we focus on the most common mass, the vestibular schwannoma.
The tumor is usually diagnosed between the ages of forty and sixty, but it can occur at any age.
Growth rate varies widely; many tumors grow slowly, and some do not grow at all for years.
In growing tumors, the average rate is reported as about one to two millimeters a year.
As magnetic resonance imaging, or MRI, has become widespread, tumors are found smaller and more often.
Most unilateral tumors are not inherited and arise spontaneously.
Bilateral tumors, however, are linked to an inherited disease; we will look at this later in the lesson.
The most common symptom is a unilateral, slowly progressive sensorineural hearing loss, seen in most patients.
Tinnitus, or ringing, in the same ear is the second most common symptom.
Imbalance is often mild; because the tumor grows slowly, the brain largely compensates for the loss.
In some patients the loss begins suddenly, which is why this tumor is also investigated in sudden hearing loss.
Patients often notice that they cannot understand speech when they hold the phone to that ear.
Large tumors can add facial numbness, weakness of the facial muscles and gait problems.
Tumor size shapes both the symptoms and the options; let us think it through step by step.
At first the tumor lies inside the internal auditory canal; at this stage, hearing loss and tinnitus stand out.
As it grows, it extends into the cerebellopontine angle and fills the surrounding space.
Larger tumors can press on the brainstem and the trigeminal nerve; this is the cause of facial numbness.
Very large tumors can block the flow of cerebrospinal fluid and raise intracranial pressure.
In this example, the right ear is within normal limits.
The left ear, however, shows a loss that increases toward the high frequencies.
Masked bone conduction overlaps air conduction, so the loss is sensorineural.
The two ears differ by fifteen decibels or more at several frequencies; this is an asymmetry.
Different criteria are used for asymmetry; what they share is that an unexplained one-sided difference is investigated.
The audiogram alone cannot separate cochlear from retrocochlear loss; for that, we look at the test battery.
When retrocochlear pathology is suspected, several tests are interpreted together with the pure-tone thresholds.
The word recognition score can be clearly lower than expected from the pure-tone thresholds.
A score that falls as presentation level rises, called rollover, is a retrocochlear sign.
Acoustic reflexes may be absent at expected levels, or reflex decay may appear with a sustained stimulus.
In the auditory brainstem response, or ABR, wave V may be delayed, or the waves may not form at all.
Otoacoustic emissions, or OAEs, can still be present despite the loss if the tumor has not affected the cochlea.
The ABR is the most widely used electrophysiological test in this battery.
In the drawing, the dashed line shows a normal recording, and the solid line an example recording from the tumor side.
The interval between waves one and five is prolonged; the signal travels slowly along the nerve.
In a unilateral tumor, one useful measure is the interaural difference in wave V latency.
In most laboratories, this difference is considered significant above about zero point two to zero point four milliseconds.
However, the ABR can be normal in small tumors; the gold standard for diagnosis is contrast-enhanced MRI.
Because the tumor usually arises from the vestibular nerve, vestibular tests are often affected too.
On caloric testing, a weak response on the tumor side is a common finding.
The vestibular evoked myogenic potential, or VEMP, can be abnormal depending on which nerve branch the tumor affects.
The video head impulse test, or vHIT, measures the response at a different speed, so it may be less affected than calorics.
These tests suggest which branch is affected, but they do not make the diagnosis on their own.
A clinical tip: unilateral tinnitus or an asymmetric loss may look like an ordinary complaint, but it should not be overlooked.
A word recognition score too low for the thresholds and absent reflexes strengthen this suspicion.
State these findings clearly in the report and refer the patient to an ENT physician.
A normal ABR, especially in small tumors, does not mean that further investigation is unnecessary.
Neurofibromatosis type 2, or NF2, is an inherited disease with autosomal dominant transmission.
It is caused by changes in the NF2 gene on chromosome twenty two, which codes for a protein called merlin.
Its hallmark is bilateral vestibular schwannomas; meningiomas and spinal tumors can also occur.
Symptoms usually begin in adolescence and young adulthood, earlier than with a unilateral tumor.
In about half of patients, no one else in the family is affected; the change first appeared in that person.
In current nomenclature, this condition is also called NF2-related schwannomatosis.
In NF2, hearing is at risk in both ears, so regular audiological monitoring is especially important.
Serial audiograms, word recognition scores and ABR show the tumor's effect on hearing over time.
A drop in the word recognition score can appear before any change in the pure-tone thresholds.
A communication plan is discussed with the family early, while hearing is still good; visual supports are part of it.
If the cochlear nerve cannot be preserved, options such as an auditory brainstem implant may arise; we will see this in a later lesson.
The process often begins with an unexplained asymmetry noticed on the audiogram.
The audiologist completes the findings with the test battery and reports the results clearly.
The patient is referred to an ENT physician; the decision to image is made by the physician.
Contrast-enhanced magnetic resonance imaging is the main method and can show even small tumors.
After diagnosis, the team decides the next steps based on tumor size and the patient's condition.
The team decides on treatment based on tumor size, growth rate, hearing level, age and the patient's preference.
For small, slowly growing tumors, observation with regular imaging and audiograms is a common approach.
The tumor can be removed with microsurgery; some surgical approaches also aim to preserve hearing.
Stereotactic radiosurgery is a focused radiation treatment that aims to stop the tumor from growing.
Each method carries different risks for hearing, the facial nerve and balance; the choice is shared by physician and patient.
Monitoring hearing and the facial nerve during surgery, called intraoperative monitoring, is an area where audiologists also work.
After treatment, hearing in that ear decreases or is lost completely in many patients.
For severe unilateral loss, CROS systems that route sound from the poorer side to the better ear can be used.
Bone-anchored devices, which carry sound through bone to the opposite cochlea, are another option.
In selected patients whose cochlear nerve was preserved, a cochlear implant may also be considered.
For those with balance loss, vestibular rehabilitation helps the brain adapt to the new situation faster.
A vestibular schwannoma is a benign tumor arising from the Schwann cells of the vestibular nerve.
Its most common symptoms are unilateral sensorineural loss and tinnitus in the same ear.
A low word score, absent reflexes and a delayed ABR support retrocochlear suspicion; diagnosis is made with MRI.
Bilateral vestibular schwannomas are the hallmark of neurofibromatosis type 2.
Test yourself: What is the gold standard method for diagnosing vestibular schwannoma?
The correct answer is B: contrast-enhanced magnetic resonance imaging.
Audiological tests raise the suspicion, but the ABR can be normal in small tumors.
In the next lesson, we will examine how infections such as congenital cytomegalovirus, mumps and meningitis affect hearing.
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Prerequisites

LESSON 137 Asymmetry and schwannoma

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