Tinnitus is one of the most common symptoms in UK primary care. Most of it is bilateral, benign, and something a GP will reassure you about without a single scan. But when the ringing, buzzing or hissing is in only one ear, and it has stuck around for more than a month, the calculation changes. UK guidance is unusually consistent on this: unilateral tinnitus deserves an internal auditory canal MRI to rule out something structural, and it deserves it early rather than late.
This piece is written for the patient who has typed ‘tinnitus one ear’ into a search bar at 11pm and now wants to know what a sensible next step actually looks like. What the scan is, what it finds, what happens if it is positive, and what happens if it is normal.
The one-line answer
If you have tinnitus in one ear that has lasted more than four weeks, ask your GP for a referral for a dedicated internal auditory canal (IAC) MRI, or self-refer to a private imaging centre. This is standard, evidence-based UK practice. The scan is quick, contrast is usually not needed, and in the overwhelming majority of cases it will be normal. When it is not, catching the problem early materially changes what can be done about it.
Why unilateral is different
Bilateral tinnitus, ringing in both ears at once, is almost always a symptom of the auditory system itself. Age-related hair-cell loss, noise damage, ear-wax, middle-ear congestion, certain medications. It is common, it correlates with hearing loss, and it does not usually indicate anything sinister sitting behind the ear.
Unilateral tinnitus is asymmetric, and asymmetry is what neuro-otology cares about. When one side of the auditory pathway is behaving differently from the other, there is a small but genuinely elevated probability of a retrocochlear cause, meaning something further up the pathway than the cochlea itself: the vestibulocochlear nerve, the internal auditory canal, or the cerebellopontine angle. NICE guidance for tinnitus assessment and management (NG155) and the British Society of Hearing Sciences both flag persistent unilateral tinnitus as an indication for MRI. In practice, most UK ENT departments and audiology-led services investigate any unilateral tinnitus lasting more than four to six weeks.
The base rate of vestibular schwannoma, the classic retrocochlear cause, is low, around one in every 100,000 people per year. But in a population presenting with unilateral audio-vestibular symptoms, the prevalence rises meaningfully. The scan is quick and non-invasive, and the consequence of missing a schwannoma for two years is significantly worse than the consequence of scanning a hundred patients whose imaging comes back reassuring.
The five red flags for an IAC MRI
UK ENT guidance groups the audio-vestibular red flags into a short, memorable list. Any one of these, present for more than four weeks, is enough to justify an IAC MRI:
- Unilateral tinnitus. Ringing, buzzing, hissing, roaring or pulsatile noise confined to one ear.
- Unilateral or asymmetric hearing loss. A drop in hearing in one ear, especially at higher frequencies, or a measured asymmetry of more than 15 dB on audiometry at two adjacent frequencies.
- Unilateral vertigo or unsteadiness. Balance disturbance that consistently favours one side, or drop attacks associated with one ear.
- Aural fullness in one ear. A persistent sensation of pressure or blockage that is not explained by wax, effusion or Eustachian dysfunction.
- Facial numbness or weakness on the same side. Any facial nerve or trigeminal nerve symptom that maps to the same side as the ear symptom is a stronger red flag and should prompt urgent imaging.
Two or more of these together move the probability further, and the scan should not be delayed.
The IAC MRI protocol: what a good scan looks like
An IAC MRI is not the same thing as a general brain MRI. It is a focused, high-resolution protocol dedicated to the internal auditory canals and the cerebellopontine angles. If you book a ‘brain MRI’ expecting to answer this question you may not get the right sequences.
The core sequence in 2026 UK practice is a heavily T2-weighted 3D gradient-echo sequence with sub-millimetre isotropic voxels. On Siemens scanners this is called CISS (Constructive Interference in Steady State). On GE scanners it is called FIESTA (Fast Imaging Employing Steady-state Acquisition). On Philips it is called DRIVE or bFFE. They all do the same job: they produce exquisitely thin slices in which the cerebrospinal fluid inside the IAC is bright white and any soft-tissue lesion, even one just two or three millimetres across, stands out as a dark filling defect against it.
Most UK centres now use this high-resolution T2 sequence as a stand-alone screen. If it is completely clean, the scan is over. If it shows something suspicious, or if the referral specifies it, the radiographer will add intravenous gadolinium contrast and repeat T1-weighted sequences to characterise the lesion, its enhancement pattern, and its exact anatomical relationships. The Royal College of Radiologists endorses this two-tier approach, and it means the majority of patients avoid a contrast injection.
Total scan time is around 15 to 20 minutes for the non-contrast screen and 25 to 30 minutes if contrast is added on the same visit. You will need to lie still, ideally with your head immobilised in a small cushioned coil, and the machine will be loud in the way MRI scanners always are. There is no injection unless contrast is indicated, no radiation, and no recovery time.
What an IAC MRI actually finds
The overwhelming majority of IAC MRIs performed for unilateral tinnitus in the UK come back normal. That is a good result and does not mean the scan was unnecessary. It means the scan did what it was meant to do, which is to exclude the small handful of conditions where the diagnostic label materially changes the treatment plan.
When something is found, the differential is short. In order of frequency in a UK audiology-referred population:
- Vestibular schwannoma (acoustic neuroma). A slow-growing benign tumour of the Schwann cells wrapping the vestibulocochlear nerve. Accounts for roughly 90 per cent of positive IAC findings. Typically presents in adults aged 40 to 70 with unilateral tinnitus, gradual asymmetric hearing loss, and sometimes mild unsteadiness.
- Meningioma of the cerebellopontine angle. Benign tumour arising from the meninges, can compress the IAC contents and present identically to a schwannoma.
- Epidermoid cyst. Benign, slow-growing inclusion cyst of the CPA, usually incidental but occasionally causing cranial nerve symptoms.
- Vascular loop. An anterior inferior cerebellar artery loop pressing on the vestibulocochlear nerve. A recognised but debated cause of unilateral tinnitus, often reported and rarely treated surgically.
- Small multiple sclerosis plaque. An MS plaque in the pons or middle cerebellar peduncle can occasionally present with unilateral tinnitus and asymmetric hearing loss.
Each of these has a different management pathway, and each is the reason the scan is worth doing.
