An acoustic neuroma or vestibular schwannoma is a benign (non-cancerous), usually slow-growing tumour that originates on the balance nerve in the inner ear. About one to two cases per 100 000 people is reported every year. The cause is not known. While there has been speculation that cellphones might be responsible, this has not been proven.
Some patients present with acoustic neuromas on both sides as a result of a genetic condition known as Neurofibromatosis type 2 (NF2). These patients also often have other benign tumours of the central nervous system.
An acoustic neuroma is a noncancerous growth that may present with hearing loss in one ear.
Patients with an acoustic neuroma often complain of hearing loss on one side, tinnitus (a noise in the ear) and dizziness. Their balance may also be poor. Depending on the size of the acoustic neuroma, patients may also exhibit signs of brain compression.
The tumour is usually picked up on an MRI. While MRI imaging is invaluable in this regard, the diagnosis can only be confirmed by a microscopic examination of the tumour tissue by a pathologist. Often other tests such as hearing and balance tests may be necessary.
Management and Treatment
Management could range from observation/watchful waiting to surgery and radiation. Your doctor should discuss all possible management options with you.
What is an acoustic neuroma?
An acoustic neuroma, also called a vestibular schwannoma or neurilemoma, is a benign (non-cancerous) growth or tumour. It usually grows slowly. With the current management options available in South Africa, the survival rate for patients with an acoustic neuroma is excellent.
Acoustic neuromas originate from the schwann cells (protective insulation cells) of the auditory and balance nerve (8th cranial nerve). The 8th cranial nerve is also called the vestibulocochlear nerve. This nerve is located in the internal auditory canal (inner ear canal) of the temporal bone.
Acoustic neuroma is the common term used but is in fact a misnomer. It should more correctly be referred to as a vestibular schwannoma as it is by far more common to originate from the vestibular (balance) portion of the vestibulocochlear nerve than the auditory (hearing) portion. It is also far more often a schwannoma than a neuroma (nerve fibre tumour). To avoid confusion the term acoustic neuroma will be used.
What causes acoustic neuroma?
Acoustic neuroma is a rare condition, affecting only one to two people per 100 000 of the population per year. It is not yet known exactly what causes acoustic neuroma. Cellphones, viruses and exposure to loud sounds and noise have been blamed, but there is no scientific proof of this. There is a genetic condition called Neurofibromatosis type 2 (NF2) which is associated with acoustic neuromas.
Acoustic neuromas are more common in females with a female to male ratio of around 60:40. It also usually affects people aged between 50 and 60 years. It is less common in the elderly and rare in children. If diagnosed in a child, NF2 should be considered. In 95% of cases there will only be one acoustic neuroma. If there is more than one, or another type of tumour present, NF2 should also be considered.
Where exactly is it located?
The acoustic neuroma usually originates inside the internal auditory canal (IAC) of the temporal bone of the skull near the inner ear. From there it can grow outside the canal into the space surrounding the brain until it reaches the brain itself at a point called the cerebellopontine angle (CP angle). The CP angle refers to the angle formed between the cerebellum and the pons of the brain stem. (The pons is a portion of the brain stem, located above the medulla oblongata and below the midbrain). With further growth, it can press and distort the brain stem and cerebellum.
Acoustic neuromas grow slowly and growth is considered if more than 2 mm per year as measured on MRI. In some cases it may grow faster. As it is not cancerous, it does not spread to other parts of the body (metastasize). 10% of all brain tumours are acoustic neuromas. When tumours occur in the region of the inner ear canal and CP angle, they are usually (90%) acoustic neuromas.
Can it be something else?
Due to the deep location of these tumours, it is not always practical to take a biopsy and the diagnosis is most often made on the MRI findings. However, the only way to confirm a diagnosis is to have a piece of the tumour analysed under a microscope by a pathologist. This is usually done after surgery. As mentioned earlier, 90% of tumours occurring in this location are acoustic neuromas. Other tumours and growths in this area include meningioma, schwannoma and neuroma of other cranial nerves such as a facial nerve neuroma, cholesteatoma, haemangioma and lipoma. There is a less than 2% risk of a tumour in this area being malignant (cancerous).
Neurofibromatosis type 2 (NF2)
Neurofibromatosis type 2 (NF2) is a genetic condition that is associated with acoustic neuromas. The acoustic neuromas usually occur on both sides. They also develop meningiomas, gliomas and other tumours of the central nervous system. The risk of NF2 is one per 44 000 births. There is often a family history of the condition. Not all patients with NF2 will develop acoustic neuromas. Genetic testing can be offered for family members concerned about developing the condition but this is not very sensitive. Between 2 and 4% of patients diagnosed with an acoustic neuroma suffer from NF2.
How fast do acoustic neuromas grow?
Acoustic neuromas usually grow slowly. Growth is considered if there is enlargement on MRI of more than 2mm per year. In some cases and in NF2 they can grow faster. Sometimes they may not grow for a year and then grow again at a later stage. It is therefore important that acoustic neuromas should be followed up regularly for several years. Because an acoustic neuroma did not grow in one year, does not mean that it will not grow in the future. The chances that an acoustic neuroma will grow in the first year after diagnosis is on average 60%. Very small acoustic neuromas may stop growing all together. The reason why some grow faster than others is not known. Any sustained growth after radiotherapy is a bad sign and should be assessed immediately.
What are the possible symptoms of an acoustic neuroma
With smaller acoustic neuromas:
- Unilateral (one sided) hearing loss
- Sudden hearing loss
- Difficulty following a conversation on a phone
- Tinnitus (noise in the ear)
- Balance problems and gait disturbance
- Ear fullness and pressure
- Numbness in the face
- Weakness of the facial muscles of expression
Although these symptoms occur with acoustic neuroma, the majority of patients experiencing these symptoms will not have an acoustic neuroma. Patients with small acoustic neuromas may not have any symptoms at all and this is often the case where an acoustic neuroma is picked up incidentally – for example, if a MRI of the brain is performed for another reason.
Other symptoms (when large acoustic neuromas press on the cerebellum and brainstem):
- Swallowing disorders
- Other lower cranial nerve deficits such as voice impairment, swallowing problems, shoulder and tongue weakness
- Poor balance and gait disorders
- Weakness and numbness in the limbs
- Raised intracranial pressure and eventually death
Clinical examination and findings
The clinical examination by the doctor may not reveal much. With the neurotological examination, emphasis will be placed on examining the cranial nerves, eyes, ears, hearing and balance function. The doctor will check to see whether the patient’s gait is disturbed and whether the patient is off balance. The examination may reveal hearing loss and some patients may have nystagmus (abnormal involuntary repetitive jerking eye movements).
With larger tumours other neurological signs such as weakness and numbness in the arms or legs may be present. As the facial nerve function is usually normal, even with large acoustic neuromas, weakness of the facial muscles may be more likely to indicate a facial nerve neuroma, cholesteatoma and ossifying fibroma. The cerebellum will be evaluated for involvement.
Diagnosis of an acoustic neuroma
A MRI with gadolinium (contrast) injection is the gold standard investigation required. Since the development of MRI, acoustic neuromas are nowadays picked up earlier and smaller – even as tiny as 2mm. Despite this, the incidence of acoustic neuromas is not much higher than 40 years ago, and it is therefore probably safe to assume – at least until new evidence is found – that cellphones are not to blame.
Other tests that may also be required include:
- Diagnostic hearing tests including speech discrimination scores
- Immittance and reflex testing of the ear.
- Auditory brainstem response audiometry (ABR).
- Balance and vestibular testing (VNG, DVA and VHIT).
- Vestibular evoked myogenic potentials (VEMP).
- Computed tomography (CT scan) of the temporal bone.
How to manage your acoustic neuroma.
Currently there are three options available for acoustic neuroma: observation/ watchful waiting; surgery; and radiation. Chemotherapy is still under investigation and is not currently used. The options depend on many factors and may not all be applicable to each patient. Keep the following in mind:
- You should be managed as part of a multidisciplinary team. When surgery is indicated, this means the collaboration of an ENT surgeon (neurotologist) and a neurosurgeon. The team may also consist of neurologists, audiologists, physiotherapists, physicians, radiotherapists, oncologists, radiologists and general practitioners.
- Seek the opinions of and treatment from experienced teams only.
- Your doctors should discuss all treatment options and modalities, including their benefits and complications, with you.
- Always enquire about the personal experience and results of the team. A team should have its own audited results on outcomes and complications.
- Check that the surgical team offers all three surgical approaches as outlined below, namely the middle cranial fossa (MCF) approach, the translabyrinthine (TL) approach and the retrosigmoid (RS) approach. Hearing (where applicable) and facial nerve monitoring are mandatory.
- It is advisable to use the same facility for all follow up MRI as well as hearing and vestibular testing.
- Choose a centre where vestibular and balance, hearing and facial nerve rehabilitation can be offered.
- Think very carefully about opting for surgery overseas as all skills, experience and proven options are available in South Africa. Besides the costs involved, it is always a problem if you develop a problem or complications after returning to South Africa. Appropriate follow up treatment may not be practical.
Surgery for acoustic neuroma
Depending on the size and location of the acoustic neuroma, age of the patient, hearing and balance function one of three surgical operations can be performed. The different operations are described by the approach utilised to reach the acoustic neuroma (indicated in blue) . The acoustic neuroma can be removed through the following approaches:
- Middle cranial fossa (MCF) approach
- Translabyrinthine (TL) approach
- Retrosigmoid (RS) approach
Primary inner ear schwannoma
A primary inner ear schwannoma (PIES) is a tumour that originates in the inner ear. The importance is that this tumour has up till now been underdiagnosed, even missed in some cases, in South Africa. Although it is considered benign it causes a lot of problems.
A magnetic resonance image (MRI) of a cochlear schwannoma
Hearing loss is very common and occurs in nearly all patients with a primary inner ear schwannoma. In 2013, a large literature review of 234 patients reported a 99% rate of hearing loss. Hearing loss may be progressive or sudden.
Disequilibrium and balance disorders are more common than vertigo (a sensation of motion) which occurs in 60% of patients. Tinnitus is nearly universally present and it is therefore not strange for PIES to mimic Meniere’s Disease (MD).
Vestibular schwannomas(VS) are the most common observed abnormality seen on magnetic resonance imaging (MRI) in patients who present with sudden sensori neural hearing loss (SSNHL). It is found that 4,5% of patients with SSNHL has a vestibular schwannoma and that primary inner ear schwannomas represent 10% of all temporal bone schwannomas. 50% of these tumours will actually grow. They can originate in different parts of the inner ear and may spread outside the inner ear.
MRI of the inner ear is the examination of choice and administration of gadolinium (intravenous contrast injection) is obligatory, otherwise it may be missed.
Regarding the management of these tumours they may be observed or removed with microsurgery, depending on the circumstances. Radiotherapy is indicated to a lesser degree than microsurgery. Cochlear implantation (CI) can restore hearing in some cases if the cochlear nerve is intact. For those patients with no hearing a CI can often be performed at the same time that the tumour is removed.
It should be emphasized that MRI with gadolinium contrast administration is indicated in all patients with unexplained sensorineural hearing loss (SNHL) and asymmetrical mixed hearing loss.
Dr Hofmeyr’s experience
Dr Hofmeyr and his team have managed more than 350 patients with acoustic neuromas. He shares his experience mainly with Dr Tommy Bingle (neurosurgeon). Dr Hofmeyr has surgical experience in the middle cranial fossa (MCF), translabyrinthine (TL) and retrosigmoid (RS) approaches.