There is a particular kind of quiet that follows the sentence "you have a small acoustic neuroma, and we are going to watch it." It is not the language people expect after a brain scan. It sounds like being left alone with something. In practice, watch and wait is one of the most carefully choreographed pathways in UK ENT and neurosurgery, built around a schedule of surveillance MRIs designed to catch change before it costs you your hearing or your facial nerve.
This piece walks through what an acoustic neuroma actually is, why observation is the default, what the surveillance schedule looks like in the UK in 2026, and the specific findings on MRI that shift the plan from watching to treating.
One-line answer
A small acoustic neuroma is almost always observed with a structured MRI schedule (baseline, 6 months, 12 months, then annually if stable, then 2-yearly), and only escalated to Gamma Knife or surgery if it grows more than around 2mm per year, or if symptoms progress meaningfully.
What an acoustic neuroma actually is
An acoustic neuroma, more properly called a vestibular schwannoma, is a benign tumour that grows from the Schwann cells that wrap the vestibular (balance) portion of the eighth cranial nerve. It sits in the internal auditory canal, the narrow bony tunnel between the inner ear and the brainstem, and sometimes extends outwards into the cerebellopontine angle.
Two things about it are worth holding onto. First, it is almost never malignant. Fewer than one in a thousand behave aggressively, and even those remain locally invasive rather than metastatic. Second, it is slow. The average acoustic neuroma grows around 1 to 2mm per year, and roughly 60 to 70 per cent of small tumours do not grow meaningfully at all over a five-year window. This is the biology that underwrites watch and wait.
The typical presentation
The classic triad is one-sided hearing loss, one-sided tinnitus, and imbalance. In real UK ENT clinics, most patients present with only one or two of these, and the hearing loss is often subtle - a phone that feels less clear on one side, difficulty picking a voice out of background noise at dinner. Unilateral tinnitus is the symptom that most reliably triggers imaging, because asymmetry is what raises the index of suspicion.
Larger tumours can produce facial numbness (fifth nerve), facial weakness (seventh nerve), headache and, in a small minority, hydrocephalus from brainstem compression. By the time these appear the tumour is usually well beyond the internal auditory canal.
The Koos classification and what it means for management
The Koos grading system is the shorthand your ENT consultant or neurosurgeon will use to describe your tumour, and it is worth understanding because it maps directly onto management.
| Koos grade | Where the tumour sits | Typical management (2026 UK) |
|---|---|---|
| I | Confined to the internal auditory canal | Watch and wait with serial MRI |
| II | Extends into the cerebellopontine angle, no brainstem contact | Watch and wait; Gamma Knife if growing |
| III | Fills the cerebellopontine angle, touches brainstem, no compression | Gamma Knife or microsurgery |
| IV | Compresses the brainstem, may cause hydrocephalus | Microsurgery, sometimes with adjuvant radiosurgery |
The great majority of tumours found in 2026 are Koos I or II. MRI is more sensitive than it was even a decade ago, and many neuromas are now caught early enough that the first conversation is about scanning, not treating.
The standard surveillance schedule
UK ENT and skull base units broadly follow the same pattern, adapted from the British Skull Base Society consensus. The intervals below assume a small, stable Koos I or II tumour with no red-flag symptoms.
- Baseline scan at the point of diagnosis, with contrast. This becomes the reference every future scan is measured against.
- Repeat MRI at 6 months. This early scan is deliberately close to baseline, because the fastest-growing tumours will declare themselves in the first year.
- Annual MRI for years 2 and 3 if the tumour has not grown.
- Every 2 years from year 4 to year 10 if still stable.
- Every 2 to 5 years indefinitely thereafter, depending on age, tumour size and consultant preference.
If growth is detected at any interval, the schedule resets to 6 monthly until a new pattern is established. The single most common reason a patient falls off this schedule in the NHS is a rescheduled scan that quietly slips from 12 months to 15 to 18, at which point growth can be missed by an interval rather than by a reading. This is the case where going private for a single scan, then handing the report back to the NHS team, is often quietly sensible.
The IAC MRI protocol for surveillance
Not every MRI of the head is the right MRI for an acoustic neuroma. The correct study is a dedicated internal auditory canal (IAC) MRI, and the protocol matters.
The baseline scan is done with gadolinium contrast, using thin-section post-contrast T1 sequences through the IAC and cerebellopontine angle, alongside a high-resolution heavily T2-weighted sequence called CISS on Siemens scanners, or FIESTA on GE. Together these show the tumour, its margins, and its relationship to the seventh (facial) and eighth (cochleovestibular) nerves inside the canal.
For follow-up surveillance of a known, stable tumour, many UK centres now use a non-contrast protocol built around T2 CISS or FIESTA alone. Modern non-contrast sequences are sensitive enough to measure a small schwannoma to within about 1mm, and they spare the patient a decade or more of repeated gadolinium exposure. Contrast is added back if growth is suspected, if the tumour becomes cystic, or if there is any diagnostic doubt.
The point of a surveillance MRI is not to prove the tumour is still there. It is to prove, in millimetres, that it has not changed. That takes a specific protocol, a subspecialist reader, and the same measurement method every time.
