Acoustic neuroma, more accurately termed vestibular schwannoma, is diagnosed in approximately 1 to 2 per 100,000 people each year worldwide. In Australia, this translates to roughly 250 to 500 new diagnoses annually, based on data from the Australian Institute of Health and Welfare and state cancer registries that track benign intracranial tumours. The incidence has appeared to rise over the past two decades, though researchers attribute this largely to the widespread adoption of magnetic resonance imaging, which detects tumours that would previously have gone unnoticed. Despite being classified as a brain tumour, vestibular schwannoma is benign and grows slowly. When identified early and managed appropriately, the prognosis is excellent for the vast majority of patients. This article explains what acoustic neuroma is, how it presents, the diagnostic pathway, and the treatment options available to patients in Melbourne and across Australia.
What Is Acoustic Neuroma
Acoustic neuroma is a benign tumour that arises from the Schwann cells surrounding the vestibular portion of the eighth cranial nerve, also called the vestibulocochlear nerve. This nerve has two branches. The cochlear branch transmits sound signals from the inner ear to the brain, and the vestibular branch transmits balance and spatial orientation information. The tumour almost always develops on the vestibular branch, which is why the preferred medical term is vestibular schwannoma rather than acoustic neuroma, though both names are used interchangeably in clinical practice.
The eighth cranial nerve travels from the inner ear through a narrow bony channel called the internal auditory canal to reach the brainstem. When a vestibular schwannoma begins growing, it starts within this canal. As it enlarges, it can extend beyond the canal into a region called the cerebellopontine angle, which is the junction between the cerebellum and the pons. Because this area contains several critical structures, including the facial nerve, the trigeminal nerve, and the brainstem itself, a growing tumour can produce a range of symptoms beyond hearing loss.
Vestibular schwannomas account for roughly 6 to 8 per cent of all intracranial tumours and are the most common tumour of the cerebellopontine angle. They are classified as World Health Organization Grade I tumours, meaning they are slow-growing and non-malignant. The vast majority occur sporadically and affect only one ear. Bilateral vestibular schwannomas, meaning tumours on both sides, are characteristic of a genetic condition called neurofibromatosis type 2, which accounts for approximately 5 per cent of cases.
Early Symptoms of Acoustic Neuroma
The symptoms of vestibular schwannoma develop gradually because the tumour grows slowly, typically at a rate of 1 to 3 millimetres per year. This gradual onset means that many patients do not notice changes immediately, and the condition is often discovered incidentally during investigations for other issues. The early symptoms are driven by the tumour compressing the nerves within the confined space of the internal auditory canal.
One-Sided Hearing Loss
Unilateral, or one-sided, sensorineural hearing loss is the most common presenting symptom of acoustic neuroma and is present at diagnosis in approximately 90 per cent of patients. The hearing loss typically develops gradually and may be subtle at first, often noticed as difficulty understanding speech in noisy environments or when using the telephone on the affected side. Some patients describe a sense of fullness or blockage in the ear. In roughly 10 to 15 per cent of cases, hearing loss can develop suddenly, mimicking sudden sensorineural hearing loss, which underscores the importance of investigating any unexplained change in hearing with a comprehensive hearing test. The pattern of hearing loss varies, but higher frequencies are often affected first, and speech discrimination scores are frequently disproportionately poor relative to the pure-tone thresholds, meaning the patient struggles to understand words even at volumes where they can detect sound.
Tinnitus
Tinnitus, or the perception of sound without an external source, is the second most common symptom and is present in roughly 60 to 70 per cent of patients at diagnosis. The sound is typically described as a high-pitched ringing, hissing, or buzzing in the affected ear, though some patients report lower-pitched sounds. In the context of acoustic neuroma, tinnitus usually occurs on the same side as the tumour and may precede noticeable hearing loss. It results from the tumour compressing the cochlear nerve fibres. Any tinnitus that is consistently one-sided, persistent, or accompanied by hearing asymmetry should be evaluated by an audiologist or otolaryngologist to determine whether further investigation is warranted.
Balance Disturbance
Because vestibular schwannoma grows on the balance nerve, it is somewhat counterintuitive that severe vertigo is not a common presenting symptom. The explanation lies in the tumour's slow growth rate, which allows the brain to adapt gradually to the decreasing vestibular input from the affected side through a process called central compensation. Instead of dramatic vertigo episodes, most patients experience mild unsteadiness, a sense of disequilibrium, or difficulty walking in the dark or on uneven surfaces. Some patients report a vague feeling of being slightly off balance or that the environment occasionally shifts. True rotational vertigo is more common with small tumours that cause acute vestibular irritation or with tumours that undergo rapid growth or internal changes such as cyst formation.
How Acoustic Neuroma Is Diagnosed
The diagnostic pathway for vestibular schwannoma begins with a thorough clinical evaluation and proceeds through audiometric testing to imaging. The goal is to identify the tumour as early as possible while avoiding unnecessary investigations in patients whose symptoms have more common explanations.
Audiometry
Pure-tone audiometry is the first-line investigation for any patient presenting with unilateral hearing loss or asymmetric tinnitus. The audiogram typically reveals a sensorineural hearing loss on the affected side, meaning the problem lies in the cochlea or the auditory nerve rather than the outer or middle ear. A key finding that raises suspicion for acoustic neuroma is a significant asymmetry between the two ears, particularly when the speech discrimination score is notably worse than the pure-tone average would predict. This discrepancy occurs because the tumour disrupts the neural transmission of complex speech signals even before it substantially elevates pure-tone thresholds. A comprehensive audiometric evaluation also includes tympanometry to confirm middle ear function is normal and acoustic reflex testing, which may show abnormal reflex decay in the affected ear. When audiometric findings suggest a retrocochlear pathology, meaning a problem beyond the cochlea, the clinician will refer the patient for imaging.
Magnetic Resonance Imaging
Magnetic resonance imaging of the internal auditory canals with gadolinium contrast is the gold standard for diagnosing vestibular schwannoma and has a sensitivity exceeding 98 per cent. On MRI, the tumour appears as a well-defined mass within the internal auditory canal that enhances brightly with contrast administration. The scan allows the radiologist to measure the tumour size precisely, determine whether it is confined to the canal or extends into the cerebellopontine angle, assess its relationship to the brainstem and facial nerve, and identify any cystic components. MRI also distinguishes vestibular schwannoma from other cerebellopontine angle masses such as meningioma or epidermoid cyst. For patients who cannot undergo MRI, high-resolution computed tomography of the temporal bones can identify bony expansion of the internal auditory canal, though it is less sensitive for detecting small tumours.
Additional Investigations
In some cases, auditory brainstem response testing may be used as a screening tool. This test measures the electrical activity along the auditory pathway from the cochlea to the brainstem. In patients with vestibular schwannoma, the waveform typically shows delayed conduction on the affected side. However, auditory brainstem response has largely been superseded by MRI for definitive diagnosis because of its lower sensitivity for small tumours. Vestibular function tests, including videonystagmography and calorics, may show reduced vestibular function on the affected side but are not diagnostic on their own. They are more useful for documenting the degree of vestibular deficit before treatment and establishing a baseline for post-treatment rehabilitation.
Treatment Options for Acoustic Neuroma
Treatment for vestibular schwannoma is not one-size-fits-all. The appropriate approach depends on the tumour size, growth rate, the patient's age and overall health, the degree of hearing remaining, and patient preference. All three main treatment strategies have strong evidence supporting their use, and the decision is best made by a multidisciplinary team including a neurotologist, neurosurgeon, and radiation oncologist.
Observation With Serial MRI
Active surveillance, also called watchful waiting or expectant management, is the recommended approach for many patients with small tumours, particularly those that are confined to the internal auditory canal and are not causing significant symptoms. Multiple long-term studies have demonstrated that 40 to 60 per cent of vestibular schwannomas do not grow over periods of five years or more. For these patients, immediate treatment offers no advantage over observation while exposing them to procedural risks. The surveillance protocol typically involves an MRI scan at six months after initial diagnosis, then annually for several years, with the interval extending if the tumour remains stable. If the tumour demonstrates growth exceeding 2 millimetres per year, causes progressive symptoms, or begins to compress the brainstem, active treatment is initiated. Observation is particularly favoured in older patients, those with contraindications to surgery or radiation, and those with tumours in their only hearing ear.
Stereotactic Radiosurgery
Stereotactic radiosurgery, most commonly delivered using the Gamma Knife or CyberKnife platform, uses precisely targeted radiation to stop tumour growth without making an incision. Despite the name, radiosurgery does not involve cutting. It delivers a high dose of radiation to the tumour in a single session or a small number of sessions, damaging the DNA of the tumour cells and their blood supply so that growth ceases. Tumour control rates, defined as the absence of further growth or the need for additional treatment, are approximately 90 to 97 per cent at ten years following radiosurgery. The tumour does not disappear but typically shrinks gradually or stabilises.
The advantages of radiosurgery include a short recovery time, no surgical risks such as infection or cerebrospinal fluid leak, and a lower risk of facial nerve damage compared to surgical removal for medium-sized tumours. The main disadvantage is that it is generally not suitable for large tumours exceeding 3 centimetres, as the radiation dose required would pose too great a risk to surrounding structures. Hearing preservation rates after radiosurgery range from 50 to 75 per cent at five years, depending on the initial tumour size and the radiation dose delivered. There is also a small risk of delayed malignant transformation, estimated at less than 0.1 per cent. Radiosurgery is performed at specialist centres in Melbourne and other Australian capital cities.
Surgical Removal
Microsurgical removal is the traditional treatment for vestibular schwannoma and remains the preferred option for large tumours, tumours causing significant brainstem compression, cystic tumours that are growing rapidly, and patients who prefer definitive removal. Surgery is performed under general anaesthesia by a team comprising a neurosurgeon and a neurotologist. Three main surgical approaches are used, each selected based on the tumour size, location, and the patient's hearing status.
The translabyrinthine approach goes through the inner ear and provides excellent exposure of the tumour and the facial nerve. It sacrifices all remaining hearing in the operated ear, so it is used when hearing is already poor or absent. The retrosigmoid approach accesses the tumour through an opening behind the ear, preserving the inner ear structures and offering the best chance of hearing preservation. The middle fossa approach accesses the tumour through the skull above the ear and is used primarily for small tumours confined to the internal auditory canal in patients with good remaining hearing.
Complete tumour removal is achieved in greater than 95 per cent of cases. Facial nerve function is a primary concern during surgery, and modern microsurgical techniques and intraoperative nerve monitoring have significantly improved outcomes. At high-volume centres, 80 to 95 per cent of patients maintain normal or near-normal facial nerve function after surgery for small to medium tumours. Recovery from surgery typically requires a hospital stay of 3 to 7 days, with gradual return to normal activities over 4 to 8 weeks.
Prognosis and Recovery
The overall prognosis for patients with vestibular schwannoma is excellent. Because the tumour is benign and slow-growing, it does not shorten life expectancy in the vast majority of cases. The key factors that influence outcomes are the size of the tumour at diagnosis, the treatment modality used, and the experience of the treating team.
For hearing, the prognosis depends on the tumour size and the treatment approach. Patients with small tumours and good hearing at diagnosis have the best chance of preserving functional hearing, regardless of whether they undergo observation, radiosurgery, or hearing-preservation surgery. Once significant hearing loss has occurred, it rarely recovers because the damage to the cochlear nerve fibres is typically permanent. Patients who lose hearing in one ear may benefit from CROS hearing aids, bone-anchored hearing devices, or, in selected cases, a cochlear implant if the cochlear nerve remains intact.
Balance recovery after treatment follows the principle of vestibular compensation. When a tumour is removed or irradiated, the remaining vestibular function on that side may be reduced or absent. The brain compensates for this loss by relying more heavily on vision, proprioception, and the vestibular input from the unaffected ear. Targeted vestibular rehabilitation exercises accelerate this compensation and are recommended for all patients experiencing unsteadiness after treatment. Most patients return to their normal activities within several weeks to months.
Long-term follow-up is standard practice regardless of the treatment chosen. Patients who undergo observation need continued MRI surveillance. Patients treated with radiosurgery require periodic imaging to confirm tumour control. Surgical patients are monitored to ensure no residual or recurrent growth develops. This follow-up is typically managed jointly between the treating specialist and the patient's audiologist, who monitors hearing function over time.
Frequently Asked Questions
Is acoustic neuroma cancerous?
No. Acoustic neuroma, also called vestibular schwannoma, is a benign (non-cancerous) tumour that grows on the vestibular portion of the eighth cranial nerve. It does not spread to other parts of the body. However, because it grows inside the skull near critical structures including the brainstem and facial nerve, it can cause significant problems if left untreated. The tumour is classified as World Health Organization Grade I, which is the least aggressive tumour grade. Regular monitoring and appropriate treatment ensure that the vast majority of patients achieve excellent outcomes.
How fast does an acoustic neuroma grow?
Growth rates vary considerably between patients. Approximately 40 to 60 per cent of acoustic neuromas show no measurable growth when monitored over several years. When growth does occur, the average rate is 1 to 3 millimetres per year in diameter. Some tumours remain stable for a decade or longer, while others grow more rapidly. Serial MRI scans, typically performed at six to twelve month intervals, are used to track growth. The pattern of growth, rather than the size alone, often guides treatment decisions. Tumours that are stable may be managed with observation, while those showing consistent growth usually warrant active treatment.
Can acoustic neuroma cause headaches?
Headaches are reported by some patients with acoustic neuroma, particularly when the tumour is large enough to compress surrounding structures or cause a buildup of cerebrospinal fluid (hydrocephalus). Small tumours confined to the internal auditory canal are less likely to cause headaches. Larger tumours that extend into the cerebellopontine angle can increase intracranial pressure, leading to diffuse headaches, nausea, and visual disturbances. Headaches can also occur as a side effect of treatment, particularly in the weeks following surgical removal. Any new or worsening headaches in a patient with known acoustic neuroma should be reported to the treating specialist promptly.
What happens if acoustic neuroma is left untreated?
The outcome depends on the tumour's size and growth rate. Many small acoustic neuromas remain stable for years and cause minimal symptoms, requiring only regular monitoring. However, untreated tumours that continue to grow can progressively compress the cochlear nerve (causing worsening hearing loss), the facial nerve (causing facial weakness or paralysis), the brainstem (causing balance problems and coordination issues), and the trigeminal nerve (causing facial numbness). In rare cases, very large untreated tumours can obstruct the flow of cerebrospinal fluid, leading to hydrocephalus, which is a medical emergency. Early detection through audiometry and imaging allows for treatment before these complications develop.
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