It usually starts over a day or two. Colours look washed out in one eye, then a smudge appears in the centre of vision, and moving the eye hurts in a way that feels wrong. In a healthy person in their twenties or thirties, and particularly in a woman, that picture almost always means one thing to a neurologist. Optic neuritis. It is a medical urgency, it is often the first sign of multiple sclerosis, and the scan that answers what happens next is a neurological MRI of the brain and orbits with contrast.
This piece walks through what optic neuritis actually looks like, why the emergency route through A&E and ophthalmology comes first, what the MRI protocol is, what it can show, and what the results mean for the years ahead. It is written for patients and families who have just been handed this diagnosis, or who are trying to work out whether the eye symptoms they are having warrant a hospital trip today.
The one-line answer
Sudden loss of vision in one eye with pain on eye movement, in an adult under 50, needs same-day ophthalmology assessment via A&E and a neurological MRI of the brain and orbits with gadolinium contrast within 72 hours. That MRI is what tells you whether this is an isolated event or the opening scene of multiple sclerosis.
The classic optic neuritis presentation
Optic neuritis has a recognisable pattern, and the fact that it is recognisable is why every UK neurology and ophthalmology trainee is drilled on it. The typical patient is a woman aged between 20 and 45. Vision in one eye deteriorates over hours to a few days, not seconds. There is dull, aching pain behind the eye that gets worse with eye movement, particularly on looking up or to the side. Colours, especially reds, look faded or washed out - patients often say a red bus looks brown, or that traffic lights look muddy.
On examination, an ophthalmologist will look for a relative afferent pupillary defect, or RAPD - the affected eye's pupil constricts less when a torch is swung into it than the healthy eye. This is the single most reliable clinical sign of optic nerve dysfunction, and its presence in the right clinical picture makes optic neuritis the leading diagnosis before any scan is done.
Vision loss can range from a slightly smudged central patch to complete loss of light perception. Most patients notice it is one eye only. If both eyes are affected simultaneously in an adult, the differential shifts sharply towards NMOSD or MOG antibody disease rather than typical MS-associated optic neuritis, and the urgency is even higher.
A&E first: the ophthalmology workup
Optic neuritis is not something to book a private MRI for directly from home. The correct first step in the UK is A&E or an urgent same-day ophthalmology clinic, because the diagnosis has to be established clinically and other causes of sudden vision loss have to be excluded before imaging is planned.
The ophthalmology assessment covers visual acuity in each eye, colour vision testing (usually with Ishihara plates), formal visual field testing to map exactly where vision is lost, a slit-lamp examination, a dilated fundoscopy to look at the optic disc, and increasingly an OCT scan - a high-resolution optical scan of the retinal nerve fibre layer that can show swelling of the optic disc in the acute phase and thinning weeks later.
Blood tests taken at the same visit typically include aquaporin-4 antibodies (for NMOSD), MOG antibodies (for MOG antibody disease), inflammatory markers, and often a syphilis and vitamin B12 panel. These results shape the interpretation of the MRI that follows.
The neuro MRI protocol: brain and orbits, with gadolinium
The scan requested for suspected optic neuritis is not a standard brain MRI. It is a dedicated brain and orbits protocol with intravenous gadolinium contrast, and the difference matters. A neurology department requesting the wrong protocol is one of the more common reasons for a repeat scan.
A typical protocol at a UK MRI centre in 2026 includes:
- Brain sequences - axial and sagittal T2/FLAIR to hunt for white matter lesions in the classic MS locations (periventricular, juxtacortical, infratentorial), plus diffusion-weighted imaging to exclude stroke.
- Dedicated orbit sequences - coronal and axial fat-suppressed T2 and STIR through both optic nerves, to show swelling and high signal in the affected nerve.
- Post-contrast sequences - fat-suppressed T1 with gadolinium through the orbits, which lights up any actively inflamed segment of the optic nerve as a bright line. Post-contrast T1 through the brain shows any actively enhancing white matter lesions - the marker of ongoing demyelination.
The scan itself takes 40 to 55 minutes. Patients lie still with a head coil around the head, the radiographer injects the contrast through a small cannula partway through, and there is no recovery time afterwards.
What the MRI shows
Three findings dominate the neuroradiologist's report in a suspected optic neuritis case.
An enhancing segment of the optic nerve. On post-contrast fat-suppressed T1, the inflamed portion of the affected optic nerve enhances brightly, often over a short segment. The length and location of the enhancement matter - a short, unilateral, retrobulbar enhancement is classic for MS-associated optic neuritis. A long segment (more than half the length of the nerve) or chiasmal involvement raises the suspicion of NMOSD.
Brain white matter lesions consistent with MS. The radiologist looks specifically for oval, T2-hyperintense lesions in the periventricular white matter (often oriented perpendicular to the ventricles - the "Dawson's fingers" appearance), in the juxtacortical white matter, in the infratentorial regions (brainstem and cerebellum), and in the spinal cord if imaged. The 2017 McDonald criteria, still the reference standard in 2026, allow a diagnosis of MS at first presentation if lesions are present in two or more of those locations, with at least one enhancing.
Look-alikes that change the diagnosis. Long-segment optic nerve enhancement, area postrema lesions, or longitudinally extensive spinal cord lesions shift the picture to NMOSD. Fluffy, poorly-marginated cortical or juxtacortical lesions in a paediatric or young adult patient raise MOG antibody disease. Granulomatous disease, sarcoid and lymphoma have their own patterns. The radiologist's job is to report not just what is there, but what pattern it fits.
An MRI in suspected optic neuritis is doing two jobs at once. It confirms the optic nerve is the site of the inflammation, and it screens the entire central nervous system for the evidence that this is the first attack of a lifelong disease.
