A 35-year-old woman with MS presents with painful loss of vision in the right eye over 3 days. Visual acuity is 6/60 right eye, 6/6 left. There is no disc swelling on fundoscopy. Which sign, if present, strongly supports the diagnosis of retrobulbar optic neuritis rather than papilledema?

• A Relative afferent pupillary defect (RAPD) in the right eye
• B Normal optic disc appearance (retrobulbar sign: 'patient sees nothing, doctor sees nothing') ✓
• C Impaired color vision in the right eye (red desaturation)
• D Pain on eye movement in the right eye

Explanation

The classic axiom for retrobulbar optic neuritis is 'the patient sees nothing, the doctor sees nothing' — meaning the patient has significant visual loss but the optic disc appears completely normal on fundoscopy because the inflammation is posterior to the globe. RAPD, red desaturation, and pain on eye movement are all features of optic neuritis but can occur in papilledema too. The normal disc appearance in the setting of significant visual loss is the specific distinguishing feature of retrobulbar versus anterior optic neuritis (papillitis) where disc is swollen.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.