A 28-year-old woman presents with acute painful vision loss in the left eye over 3 days. She reports colour desaturation and pain on eye movement. Examination reveals decreased visual acuity (6/36), a relative afferent pupillary defect on the left, and a central scotoma on visual field testing. The fundus appears normal. MRI orbits with fat-suppressed post-contrast T1 sequences shows the structure marked **A** with enhancement and T2 hyperintensity. What is the most likely diagnosis?
A. Acute angle-closure glaucoma
B. Optic neuritis (retrobulbar demyelinating disease)
C. Optic nerve compression from orbital mass
D. Posterior uveitis with vitritis
Explanation
Why Optic neuritis (retrobulbar demyelinating disease) is right
The clinical presentation—acute painful unilateral vision loss in a young woman with dyschromatopsia, pain on eye movement, RAPD, and a central scotoma with a normal fundus—is pathognomonic for retrobulbar optic neuritis. The MRI finding of enhancement and T2 hyperintensity of the optic nerve (structure A) confirms the diagnosis. Optic neuritis is an inflammatory demyelinating disorder of the optic nerve, classically affecting young adults (15–45 years, F:M ≈ 3:1). The normal fundus appearance in ~two-thirds of cases (as in this patient) is characteristic of retrobulbar neuritis, where "the patient sees nothing, doctor sees nothing." MRI with fat-suppressed post-contrast T1 sequences typically shows enhancement and T2 hyperintensity of the affected optic nerve, as demonstrated by structure A in this case (Kanski's Clinical Ophthalmology, 9th ed; Optic Neuritis Treatment Trial).
Why each distractor is wrong
Optic nerve compression from orbital mass: While compression can cause vision loss and RAPD, it does not typically present with acute painful vision loss, dyschromatopsia, or pain on eye movement. MRI would show a mass (structure D is explicitly labeled as "no retro-orbital mass"), not diffuse enhancement and hyperintensity of the nerve itself.
Acute angle-closure glaucoma: This presents with severe eye pain, halos, corneal edema, and elevated intraocular pressure. Vision loss is typically more rapid and accompanied by nausea and vomiting. There is no RAPD or central scotoma, and the fundus findings differ markedly.
Posterior uveitis with vitritis: While this can cause vision loss and floaters, it does not typically present with pain on eye movement, dyschromatopsia, or RAPD. Fundoscopic examination would reveal vitritis and retinal inflammation, not a normal fundus as seen here.
High-YieldNEET PG
Optic neuritis = acute painful unilateral vision loss + RAPD + central scotoma + normal fundus (retrobulbar) + MRI enhancement of optic nerve = demyelinating disease, often MS.
