Half your face has gone numb, or the corner of your mouth will not lift when you smile in the mirror. The question every UK patient reaches for is the same one - is this a stroke, and do I need a scan today. The honest answer is that most facial numbness turns out to be benign, but a small subset is a genuine neurological emergency, and telling them apart in the first hour is the decision that matters.
This is the decision guide. When to call 999, when to push for an MRI inside a week, when to sit with it, and what a proper cranial-nerve MRI actually looks like in 2026 - NHS and private.
The one-line answer
Sudden facial weakness with a smooth forehead, or with any arm weakness, speech change or vision loss, is stroke until proven otherwise - call 999. Facial weakness affecting the forehead as well, coming on over hours, is usually Bell’s palsy, and warrants a same-week GP or A&E review and often an MRI within days. Persistent one-sided facial numbness lasting more than a week deserves a brain MRI with dedicated cranial-nerve sequences and contrast.
Central versus peripheral: the forehead is the tell
The most important clinical distinction in facial weakness is central versus peripheral, and the giveaway is the forehead. It sounds trivial. It is not.
- Forehead spared (central). The lower half of the face droops, the mouth pulls, but the patient can still wrinkle the forehead on the affected side. This pattern points to a lesion above the brainstem - most commonly a stroke in the middle cerebral artery territory. Treat as stroke.
- Whole face affected, forehead included (peripheral). The eyebrow does not lift, the eye will not fully close, the corner of the mouth droops, the whole side is flat. This points to the facial nerve itself - Bell’s palsy in around 70 per cent of cases, but also Ramsay Hunt syndrome, a parotid tumour, or an acoustic neuroma pressing on the seventh nerve.
Numbness, as opposed to weakness, follows a different logic. One-sided facial numbness sparing the forehead points centrally too. Numbness in a trigeminal distribution - a strip of cheek, or the chin - points to the trigeminal nerve, and the differential runs from a benign vascular loop to MS to a small cerebellopontine-angle tumour.
Emergency: A&E now
Any one of these is a 999 call, not a GP appointment:
- Sudden onset. The face was normal an hour ago and now it is not. Sudden is the word - not gradually noticed over a day.
- Forehead-sparing weakness. A crooked smile with a normal forehead lift is central until proven otherwise.
- Any limb weakness or clumsiness on the same side. Face plus arm is a stroke pattern.
- Speech change. Slurred, garbled, unable to find words.
- Vision loss or double vision that came on at the same time.
- Sudden severe headache or a change in consciousness alongside the facial change.
The FAST acronym - Face, Arms, Speech, Time - exists because time is brain tissue. Thrombolysis and thrombectomy windows close at 4.5 and 24 hours respectively. Do not drive to A&E. Call 999 so the stroke pathway starts on the ambulance.
MRI within days: the persistent, gradual and cranial-nerve patterns
Not everything is an emergency, but a real subset of facial-nerve problems needs imaging inside a week - not inside three months. The trigger patterns:
- Facial numbness persisting beyond seven days with no obvious dental or sinus cause.
- Gradual onset over days or weeks, especially with progression - each morning worse than the last. Tumours grow. Strokes do not.
- Cranial nerve pattern. Numbness in a clean trigeminal distribution, or facial weakness combined with hearing loss, tinnitus, or unsteadiness on that side - a classic cerebellopontine-angle presentation.
- Bell’s palsy that is not recovering by three to four weeks, or that recurs on the same side.
- Facial pain in a trigeminal distribution, especially with sensory change - not just classical trigeminal neuralgia.
- Any of the above under 40, where MS enters the differential far more strongly.
The MRI protocol: brain plus dedicated cranial-nerve sequences, with contrast
A generic brain MRI is not the right scan for facial neurology. The correct protocol has three layers, and knowing this before you book privately stops you paying for a scan that misses the diagnosis.
- Standard brain sequences. T1, T2, FLAIR and diffusion-weighted imaging (DWI) - to catch stroke, MS plaques, and gross intracranial pathology. DWI is non-negotiable if stroke is on the differential.
- Dedicated cranial-nerve sequences. Thin-slice, high-resolution T2 sequences (CISS or FIESTA) through the internal auditory canals and cerebellopontine angles - to see the facial and trigeminal nerves themselves.
- Gadolinium contrast. Post-contrast T1 sequences to pick up nerve enhancement in Bell’s palsy, small acoustic neuromas, active MS plaques and meningeal disease.
If a private clinic quotes you £350 for a plain brain MRI without contrast for persistent facial numbness, that is the wrong scan. A proper brain MRI with cranial-nerve sequences and gadolinium in 2026 sits between £550 and £950 all-in.
