Abstract
Acoustic neuromas are benign, slow-growing tumours arising from the Schwann cells that surround the vestibular nerve in the internal acoustic meatus. Therefore, pathologically they are termed vestibular schwannomas. The tumour develops in the nerve sheath and exerts its effects mainly through compression of the nerve rather than invading it. As the tumour grows, it spreads beyond the bony internal acoustic meatus causing progressive compression of other cranial nerves and eventually of the brain itself. Early diagnosis of these tumours improves patient morbidity and mortality, as a wider range of management options can be offered. This article is intended as an overview of acoustic neuroma: covering its diagnosis, investigation and management.
The GP curriculum and acoustic neuroma
Manage primary contact with patients who have a common/important ear, nose and throat (ENT), oral or facial problem, e.g. vertigo or tinnitus Demonstrate empathy and compassion towards patients with ENT symptoms that may prove difficult to manage, e.g. tinnitus, facial pain, unsteadiness Understand how to recognise rarer but potentially serious conditions such as oral, head and neck cancer Demonstrate knowledge of the scientific backgrounds of symptoms, diagnosis and treatment of ENT, oral and facial conditions
Acoustic neuromas represent 8% of all intracranial tumours, and 80% of cerebellopontine angle tumours. They affect men and women equally, with an incidence of 1 per 100,000 per year. Ninety-five per cent of cases are sporadic, with the remaining 5% exhibiting autosomal dominant inheritance and are typically associated with neurofibromatosis-2 (NF-2). The former is unilateral with a typical age of onset around 40–60 years whereas NF-2associated cases are usually bilateral with an average age of onset between 18 and 24 years (Roland, McRae, & McCombe, 2001). Mutations in the NF-2 gene on the long arm of chromosome 22 have been implicated in the development of these neuromas. Although NF-2 is a rare condition, it is an important diagnosis to make, as these patients will be at high risk of developing concomitant tumours, such as meningiomas and ependymomas. These patients may initially present with unilateral disease and develop bilateral acoustic neuromas at a later date, or bilateral disease may be discovered when imaging is performed for the more symptomatic side. If a young patient is diagnosed with an acoustic neuroma, it is important to consider the possibility of NF-2 and to ascertain if any family members have similar symptoms. In these cases, the option of genetic testing can be discussed.
The growth of acoustic neuromas begins in the nerve sheath surrounding the vestibular nerve within the confines of the bony internal acoustic meatus. As it increases in size, it gradually fills the entire meatus and then begins to protrude out of it, causing resorption of bone as it does so. Eventually, the expansion of the tumour causes compression of the brainstem and fourth ventricle. This expansion can also affect cranial nerves V and VI, thereby producing the associated signs and symptoms. Although acoustic neuromas are relatively slow-growing tumours, on occasion haemorrhage can occur into the tumour, causing a rapid increase in size and a marked exacerbation of symptoms.
The growth pattern of acoustic neuromas plays an important role in predicting disease behaviour. This knowledge is vital in offering patients a likely prognosis for their condition, and for the timing of any intervention. A systematic review found that between 18 and 73% of acoustic neuromas exhibit growth, however, 9–75% of these tumours did not grow, and up to 22% may get smaller. A mean growth between 1 and 2 mm/year was reported. There were exceptional circumstances that resulted in tumours showing significant regression or excessive growth (exceeding 18 mm/year) (Nikolopoulos, Fortnum, O’Donoghue, & Baguley, 2010).
This supports the understanding that there is a large variation in growth and the rate of the growth is often unpredictable. A neuroma that appears to be slow-growing initially might suddenly accelerate in size. It is for this reason that diagnosed patients may need indefinite follow-up with interval scanning.
Clinical features
Two clinical phases, otological and neurological, have been described. The otological phase is characaterised by a small tumour that compresses structures within the internal acoustic meatus. The effects observed in the neurological phase are caused by the tumour expanding into the cerebellopontine angle. Therefore, signs and symptoms arise directly from tumour-related expansion and compression of local and surrounding structures.
The most frequent presenting symptoms include:
Gradual and progressive unilateral hearing loss (90%) Tinnitus (70%) Sudden onset hearing loss (10%) Balance disturbance: This may be quite mild as the vestibular nerve compensates in response to the slow destruction of neurons
Less common and later-onset symptoms include:
Trigeminal nerve symptoms in the form of facial pain Paraesthesia and numbness Facial nerve symptoms (uncommon) Ataxia and unsteadiness: Brainstem displacement and cerebellar involvement Diplopia: Pressure on cranial nerve VI Hoarseness with dysphagia: Involvement of cranial nerves IX and X Headaches: Described as a discomfort or a dull ache around the ear and mastoid as a result of an enlarging tumour or due to raised intracranial pressure
Suspicion of acoustic neuroma should be raised in any patient presenting with unilateral or asymmetrical auditory symptoms, either hearing loss or tinnitus in the absence of an obvious cause. The onset of these symptoms might be progressive or acute. It should also be considered in patients with imbalance and impaired facial sensation without any other explanation.
Patients with the clinical features listed above should be referred urgently to the local Otolaryngology Department for further investigation and management.
Investigation
All patients with unilateral or asymmetrical hearing loss should be investigated with a pure-tone audiogram and if available, this should be compared with previous audiograms. The majority of patients will demonstrate a high-frequency hearing loss; however, any pattern of loss may be encountered.
Contrast enhanced magnetic resonance imaging (MRI) is the most accurate modality to detect acoustic neuromas. It has the benefits of using non-ionising radiation and the ability to detect smaller tumours than other modalities. Therefore, it is not only useful in first diagnosis of acoustic neuromas, but also it can be an effective tool for monitoring their progression. The important features of the acoustic neuroma to note on radiological investigation are:
Lateral extension of the tumor within the auditory canal The degree of any extension into the cerebellopontine angle Whether or not the brainstem is compressed
Figures 1 and 2 show axial and coronal sections, respectively, of MRI images taken at the level of the internal acoustic meatus. These are T1-weighted images with gadolinium contrast and show a left-sided acoustic neuroma that has an intra-canalicular portion and also a large extension into the cerebellopontine angle with compression of the brainstem and fourth ventricle.
Axial section of a T1-weighted MRI image with gadolinium contrast showing a left acoustic neuroma spreading beyond the internal acoustic meatus and compressing the brainstem and fourth ventricle. The intra-canalicular portion of the tumour can also be seen. Coronal section of a T1-weighted MRI image with gadolinium contrast showing a left acoustic neuroma compressing the brainstem.
If there are contra-indications to the use of MRI or there is limited availability of this modality, computerised tomography (CT) scanning with contrast can be an alternative. It is more widely available, cheaper than MRI, and allows adequate examination of any bony erosion within the internal acoustic meatus. It has excellent sensitivity (almost 100%) in detecting acoustic neuromas of more than 15 mm diameter. However, this sensitivity falls if the tumour is less than 10 mm and it is unable to detect tumours that are entirely within the internal acoustic meatus.
Koos classification of acoustic neuroma.
However, a tumour found at the cerebellopontine angle raises the possibility of other diagnoses apart from acoustic neuroma. These differential diagnoses are listed below:
Meningioma Cerebellar tumour VIIth nerve tumour Congenital cholesteatoma Aneurysms of the basilar/vertebral arteries
Management
Managing these tumours is divided into three main categories:
Interval scanning Radiotherapy or radiosurgery Surgical excision
These patients are best managed using a multi-disciplinary approach and any surgical intervention involves an experienced neurosurgeon and an experienced otologist. The decision as to which of the above treatment strategies is recommended depends on various factors, such as patient age, co-morbidities, size of tumour, hearing thresholds in the opposite ear, likelihood of hearing preservation, and patient preference depending on a risk/benefit discussion.
The majority of acoustic neuromas will grow slowly and therefore, a ‘watch and wait’ strategy is commonly employed. This is especially true for small tumours in elderly patients or those with multiple co-morbidities who can be monitored using MRI or CT scanning at regular intervals. Routine practice would involve annual imaging to look for neuroma growth. If any growth is detected, consideration should be given to offering the patient either stereotactic radiosurgery or surgical excision.
Stereotactic radiotherapy or radiosurgery can be used for small- and medium-sized neuromas up to 3.0 cm in diameter. These treatment modalities do not remove the tumour, rather they help to control the growth of the tumour by slowing down or stopping an increase in size. Despite treatment, these patients will be subjected to long-term serial scanning to monitor tumour growth; if any increase in size is found, surgical excision would have to be considered. However, the treatment approaches are not without risks and side-effects; there is potential for radiation injury to the brainstem and cranial nerves and of inducing malignant change within the schwannoma or within other tissues in the brain and surrounding structures. Regardless of treatment with stereotactic radiotherapy or radiosurgery, the acoustic neuroma might continue to grow and the decision is likely to be made for surgical management depending on various factors. Surgical excision after radiotherapy or radiosurgery is generally believed to be more technically challenging and associated with poorer outcomes, especially in terms of facial nerve function.
The common surgical approaches used to excise acoustic neuromas.
Key points
Acoustic neuromas are benign tumours arising from the nerve sheath of the vestibular nerve They compress surrounding cranial nerves and as they grow they eventually compress the brain Any patient presenting with unilateral or asymmetrical auditory symptoms (hearing loss or tinnitus) without any obvious cause should be referred to ENT to rule out acoustic neuroma Diagnosis is made by pure-tone audiometry and contrast MRI scans or if contra-indications exist, a CT scan is performed Due to the majority of tumours being slow-growing, a ‘watch and wait’ strategy with annual interval scanning might be offered If any growth, stereotactic radiosurgery or surgical excision could be used
Footnotes
Acknowledgement
We would like to thank Dr Jurgen Brand, Consultant Radiologist at Glangwili Hospital, Carmarthen for his help in obtaining the MRI images included in this article.