Painless Sudden Visual Loss in a Young Adult With a 'Normal' Fundus
A 26-year-old woman presents with painless loss of vision in the left eye noticed on waking 2 days ago. She has no eye pain, no flashes or floaters, and no prior ocular history. She takes the combined oral contraceptive and has had no recent illness. Visual acuity is 6/6 right and 6/36 left. There is a left relative afferent pupillary defect. Colour vision is reduced on the left (Ishihara 3/17 vs 17/17). Fundoscopy shows a slightly swollen left disc with blurred margins; the right disc looks normal. She has mild pain on eye movement that she had not volunteered. Blood pressure and neurological examination are otherwise normal.
1. What is your differential, and which single investigation most changes management?
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Differential of a unilateral optic neuropathy in a young woman:
- Optic neuritis (demyelinating, including NMOSD/MOGAD) - pain on eye movement, RAPD, reduced colour vision, disc swelling in a third.
- Anterior ischaemic optic neuropathy - unusual at 26, but consider if vasculopathic risk factors or disc-at-risk anatomy.
- Infectious/inflammatory optic neuropathy - syphilis, cat-scratch, Bartonella, sarcoid, TB.
- Compressive or infiltrative lesion - tumour, thyroid eye disease (usually bilateral), lymphoma.
- Leber hereditary optic neuropathy - usually young men, painless, bilateral sequential.
- Toxic/nutritional - less likely given acuity and disc swelling.
The single most management-changing investigation is an MRI of the brain and orbits with gadolinium and fat suppression, looking for optic nerve enhancement and, critically, white matter lesions meeting dissemination-in-space criteria. It determines whether she meets criteria for a clinically isolated syndrome and whether disease-modifying therapy is indicated. A normal MRI substantially lowers the risk of progression to MS. Also send FBC, ESR/CRP, ANA, syphilis serology, and consider NMO/MOG antibodies if atypical features (bilateral, severe, poor recovery, or longitudinally extensive).
2. How do you treat the acute episode, and what is the evidence and the counselling around steroids?
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Acute treatment:
- IV methylprednisolone 1 g daily for 3 days (or high-dose oral equivalent) speeds visual recovery but does not change final visual outcome. The Optic Neuritis Treatment Trial showed faster recovery with IV steroids and a lower early recurrence rate with oral prednisone alone, but no difference in final acuity.
- Oral prednisone alone at low dose is avoided because it was associated with a higher recurrence rate in ONTT.
- If MRI shows lesions typical of MS, discuss disease-modifying therapy with neurology; steroids do not prevent conversion to MS.
Counselling:
- Most patients recover to 6/9 or better, but subtle deficits (contrast, colour, stereopsis, fatigue-related worsening, Uhthoff's phenomenon) often persist.
- Warn about steroid side effects (hyperglycaemia, mood change, insomnia, infection) and the need for a tapering plan if oral steroids are used.
- Explain that treatment is for speed of recovery and comfort, not for 'saving' the nerve.
- Arrange neurology follow-up and repeat MRI timing per their protocol.
3. Two weeks later she returns with pain and reduced vision in the right eye. How does this change your thinking?
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Sequential bilateral optic neuritis in a young woman shifts the differential toward NMOSD (AQP4-IgG) or MOGAD, and away from typical MS:
- NMOSD - severe bilateral or sequential optic neuritis, poor recovery, longitudinally extensive optic nerve lesions on MRI, often with area postrema or spinal cord involvement. Test AQP4-IgG and MOG-IgG by cell-based assay.
- MOGAD - often bilateral, disc swelling prominent, good recovery, may be steroid-dependent with relapses on tapering.
- MS - sequential optic neuritis can still occur, but bilateral simultaneous or severe poor-recovery episodes are atypical.
Management changes:
- Do not treat as routine MS - some MS disease-modifying therapies can worsen NMOSD.
- Acute attacks: high-dose IV steroids; consider plasma exchange or IVIG if steroid-refractory.
- Urgent neurology involvement for long-term immunosuppression (e.g. rituximab, azathioprine, mycophenolate) depending on antibody status.
- Repeat MRI brain, orbits, and spine with contrast; check for other neurological symptoms.
- Counsel about the risk of further attacks and the importance of early treatment of relapses.