If you’ve noticed hearing loss that’s clearly worse in one ear than the other, or tinnitus that only affects one side, it’s worth knowing that asymmetric hearing loss is one of the clearest red flags in audiology. Most cases turn out to have a benign explanation (wax, middle ear fluid, noise exposure or natural variation), but the investigation is there to screen for acoustic neuroma, a benign but potentially serious tumour that grows on the hearing and balance nerve.
Plenty of people arrive worried they have a brain tumour after searching “hearing loss one ear”. Very few do. The difference between appropriate concern and catastrophising comes down to understanding what acoustic neuromas actually are, how we detect them, and why the investigation pathway exists the way it does.
What Actually Is an Acoustic Neuroma?
An acoustic neuroma (technically a vestibular schwannoma, because it grows from the Schwann cells of the vestibular, or balance, nerve rather than the hearing nerve) is a benign, usually slow-growing tumour. It’s not cancer. It doesn’t metastasise. But left undetected, it can grow large enough to press on surrounding structures including the facial nerve, the brainstem, and the cerebellum.
These tumours are rare. Reported incidence is in the order of a few cases per 100,000 people per year, and it has risen as wider use of MRI picks up small tumours that might once have gone unnoticed. Most are diagnosed between ages 40 and 60, and while the majority are sporadic with no clear cause, bilateral acoustic neuromas can occur in people with neurofibromatosis type 2, a genetic condition.
The tumour grows in the internal auditory canal, the bony channel that carries the hearing and balance nerves from the inner ear to the brainstem. As it enlarges, it compresses these nerves. That’s why the classic presentation is gradual, one-sided hearing loss, often accompanied by tinnitus on the same side. Balance issues can develop, though because the tumour grows slowly, many people’s brains compensate and they don’t notice significant dizziness until the tumour is quite large.
Symptoms That Should Prompt Investigation
The hallmark symptom is asymmetric sensorineural hearing loss. On pure tone audiometry, we’ll see a difference between the ears, often a high-frequency loss on the affected side that doesn’t match the pattern in the other ear. Sometimes it’s subtle: a 15 to 20 dB difference at 4000 Hz and above. Other times, one ear is profoundly deaf and the other is completely normal.
Unilateral tinnitus is the other key flag. If your tinnitus is only in one ear, or substantially worse in one ear, that’s reason enough to investigate. The tinnitus is typically high-pitched and constant, though patients describe it in every way imaginable: ringing, hissing, buzzing, roaring.
Balance problems are less specific because they occur in many vestibular conditions, but the pattern with acoustic neuromas tends to be vague unsteadiness rather than the room-spinning vertigo you’d get with something like Meniere’s disease or BPPV. Some patients report feeling “off” when they turn their head quickly or walk in the dark.
Larger tumours can cause facial numbness or weakness if they compress the facial or trigeminal nerves, and very large tumours can cause headaches or symptoms of raised intracranial pressure. These are late signs. We’re trying to catch it well before this happens.
Why Early Detection Changes Everything
Small acoustic neuromas can often be monitored with serial MRI scans. Many grow extremely slowly or not at all, particularly in older patients. Observation is a legitimate management strategy for small, asymptomatic tumours, especially if the patient has other health conditions that make surgery or radiation risky.
But once a tumour reaches a certain size, or if it’s growing rapidly, treatment becomes necessary. The options are microsurgery or stereotactic radiosurgery (delivered with systems such as Gamma Knife or CyberKnife). Both have their place depending on tumour size, location, patient age, and hearing status.
The larger the tumour, the more difficult the surgery and the higher the risk of complications including facial nerve damage and complete hearing loss in that ear. Smaller tumours caught early offer better odds of hearing preservation and lower surgical risk. Radiotherapy can control growth in many cases but doesn’t remove the tumour, and there’s still a significant chance of losing useful hearing in the treated ear over time.
This is why audiologists take asymmetric loss seriously. Finding a tumour while it’s small generally leaves more options open.
The Investigation Pathway: What Happens Next
At The Audiology Place, an unexplained asymmetric sensorineural hearing loss on a diagnostic audiogram prompts a referral discussion. The difference doesn’t need to be large. An asymmetry of 15 dB or so at neighbouring frequencies, or a clear difference in word recognition scores between ears, can be enough to warrant investigation, depending on the overall picture.
The gold standard investigation is MRI with gadolinium contrast. CT scans can miss small acoustic neuromas entirely. The MRI protocol needs to include thin cuts through the internal auditory canals: a standard brain MRI won’t necessarily pick up a small tumour in this location. This is why the referral should make the question clear, so the right imaging protocol is used.
The usual pathway is for us to send our results to your GP, who can arrange the MRI and, where needed, a referral to an ear, nose and throat (ENT) surgeon or neurotologist. If the MRI does show an acoustic neuroma, you’ll need specialist assessment anyway. Ask the radiology practice in advance about any out-of-pocket cost.
In our clinic, the diagnostic hearing test includes pure tone audiometry, speech audiometry, and tympanometry to rule out middle ear causes of the asymmetry. If the tympanogram is normal (Type A) and the hearing loss is sensorineural, not conductive, that points to an inner ear or nerve issue. Acoustic reflex testing can provide additional information: reflexes are often absent or abnormal on the affected side with acoustic neuromas, even when hearing loss is mild.
We also conduct speech-in-noise testing because some patients with acoustic neuromas have disproportionately poor speech understanding relative to their pure tone thresholds. The tumour affects the quality of the signal reaching the brain, not just the volume.
What Outcomes Actually Look Like
Most people we refer for MRI don’t have an acoustic neuroma. The asymmetry turns out to be idiopathic sudden sensorineural hearing loss (which has its own investigation and treatment pathway), noise-induced hearing loss affecting one ear more than the other, or simply normal variation that sits on the edge of clinical significance.
For those who do have an acoustic neuroma, outcomes depend on size and treatment chosen. Small tumours under observation may never require intervention. Hearing preservation after surgery for small to medium tumours varies widely between studies and centres, and it falls with larger tumours. Facial nerve function is preserved in the large majority of small-tumour cases, but the risk increases with size.
Radiosurgery controls tumour growth in around nine out of ten cases, but hearing often declines gradually in the treated ear over the following years. For someone with good hearing in the other ear, this may be an acceptable trade-off to avoid surgery.
Single-sided deafness, whether from the tumour itself or from treatment, is manageable with CROS (Contralateral Routing of Signal) hearing aids or bone conduction devices. Life goes on. But it’s obviously preferable to catch things early enough that hearing preservation is still possible.
The Bottom Line: Don’t Ignore Asymmetry
If you’ve noticed hearing loss or tinnitus that’s clearly one-sided, book a diagnostic hearing test. It’s probably nothing sinister, but asymmetric hearing loss is one scenario where “wait and see” is not appropriate. The investigation is straightforward, and early detection genuinely improves outcomes if an acoustic neuroma is found.
One exception to the “book a test” advice: if the hearing loss in one ear came on suddenly, over hours or a few days, don’t wait. See a GP or go to an emergency department the same day or the next day, because sudden sensorineural hearing loss needs prompt medical treatment.
At The Audiology Place, every unexplained asymmetric sensorineural hearing loss is followed up with a referral discussion. That’s standard good practice, not overcaution.
If you’re concerned about your hearing, see an audiologist or GP. Don’t talk yourself out of getting checked because you don’t want to seem like you’re overreacting. When it comes to asymmetric hearing loss, appropriate concern is exactly what the situation calls for.
