You wake up and one ear sounds like it is stuffed with cotton wool. Voices around you are flatter on that side. You shake your head, you swallow, you poke a finger at it. Nothing changes. Most people wait a day, a weekend, sometimes a week, hoping it settles. In sudden sensorineural hearing loss, that wait is the part that costs you the most, because the drug that could have given the hearing back has a window of roughly 72 hours.
This piece is written for the person Googling with one ear muffled at 8am on a Tuesday. It is not a substitute for seeing a clinician. It is a plain-English map of the pathway: what sudden sensorineural hearing loss actually is, why the first 72 hours matter, why the MRI comes afterwards but must not be skipped, and how to get both the assessment and the scan done in time in the UK in 2026.
The one-line answer
If you have lost hearing suddenly in one ear over minutes to days and it is not obviously due to wax or a cold, you need to be assessed by an ENT clinician the same day, started on steroids inside 72 hours if sensorineural loss is confirmed on audiometry, and booked for an MRI of the internal auditory canals within the following weeks to rule out an acoustic neuroma. Any part of that pathway that slips is a part you may have to push privately to keep on schedule.
What sudden sensorineural hearing loss actually is
Sudden sensorineural hearing loss, usually shortened to SSNHL, has a specific clinical definition: a drop of 30 decibels or more across three consecutive audiometric frequencies, occurring within 72 hours. That is the audiogram result, not the sensation. The sensation the patient describes is almost always the same - a muffled ear, sometimes with tinnitus, sometimes with a sense of fullness or mild dizziness, almost always on one side.
It is uncommon but not rare. Incidence in the UK sits at roughly 5 to 27 cases per 100,000 per year, with most people between 40 and 70 at onset. The overwhelming majority - somewhere between 85 and 90 per cent of cases - have no clear cause identified even after full investigation. That is why it is often called idiopathic sudden sensorineural hearing loss. What we do know is that early treatment matters and that a small but important subset have a structural cause on MRI.
The 72-hour treatment window
The single most important reason to move fast is the steroid window. Oral corticosteroids, typically a course of prednisolone over one to two weeks, are the standard first-line treatment for SSNHL. Multiple randomised trials and international guidelines converge on the same message: treatment started inside 72 hours of onset gives meaningfully better hearing recovery than treatment started after seven days. After two weeks the benefit is much smaller. After a month it is close to zero.
Where oral steroids are not tolerated or the loss is severe, intratympanic steroid injection - a small injection through the eardrum delivering steroid directly to the inner ear - is used, either alone or as a rescue if oral treatment fails. That is an ENT clinic procedure and needs a consultant appointment, not a GP visit.
None of this needs the MRI first. The MRI is the second step, not the first. Anyone telling you to wait for a scan before starting steroids is losing you time you cannot get back.
Why IAC MRI is standard care afterwards
Once steroids are started, the second job is to work out why this happened. In roughly 2 to 10 per cent of patients presenting with sudden sensorineural hearing loss, an MRI of the internal auditory canals finds a structural cause - most commonly a vestibular schwannoma, also known as an acoustic neuroma. This is a benign tumour of the eighth cranial nerve, usually small at diagnosis, and it changes everything about the follow-up plan.
Other findings the IAC MRI is looking for include a meningioma at the cerebellopontine angle, an aberrant vascular loop compressing the nerve, a demyelinating plaque of multiple sclerosis affecting the brainstem, and rare inflammatory or ischaemic causes. Missing any of these does not just deny the patient a diagnosis, it removes the option to treat the underlying condition.
UK and international ENT guidelines are clear: every adult with confirmed sudden sensorineural hearing loss should have an MRI of the internal auditory canals, usually within four to twelve weeks of presentation. In practice, the sooner the better - once treatment is started, patients want to know what caused it, and the follow-up plan depends on the answer.
The IAC MRI protocol in plain English
The scan is a focused study of a small area at the base of the brain where the seventh and eighth cranial nerves run from the brainstem through the internal auditory canal to the inner ear. It is not a whole-brain MRI, though most centres include an overview brain sequence in case something is picked up incidentally.
A modern IAC MRI protocol includes:
- High-resolution T2 3D sequences (typically CISS, FIESTA or DRIVE) - millimetre-slice imaging of the internal auditory canals that shows the individual nerves against the bright cerebrospinal fluid. This is the workhorse sequence and picks up most acoustic neuromas even without contrast.
- Gadolinium contrast T1 sequences - added when the T2 shows something suspicious, when symptoms are atypical, or as part of a full protocol. Contrast highlights small enhancing lesions and inflammatory changes.
- Overview brain imaging - a quick set of sequences covering the whole brain to catch multiple sclerosis plaques, stroke or incidental findings.
- 3T field strength where available - modern 3 Tesla scanners give substantially better resolution of the internal auditory canal than older 1.5T machines, and are the standard of care for this indication in private UK imaging in 2026.
A well-run private MRI for this indication uses a subspecialist neuroradiologist to read the images, not a general radiologist. Small acoustic neuromas can be subtle, and reader experience matters.
What the MRI actually finds
The reporting radiologist is looking at four main categories:
- Vestibular schwannoma (acoustic neuroma) - most common structural finding, typically a small enhancing mass at the internal auditory canal or cerebellopontine angle. Management depends on size, symptoms and patient preference: watch and rescan, stereotactic radiosurgery, or microsurgical removal.
- Meningioma - less common at this location, usually managed similarly to schwannoma.
- Vascular loop - a normal artery in an abnormal position pressing on the nerve. Often incidental, but can be relevant in the right symptom context.
- Demyelinating plaque - a small lesion of multiple sclerosis on the brainstem or nerve pathway. Rare as a cause of isolated hearing loss but not to be missed.
