Optic Neuritis

Optic neuritis (ON) is inflammatory demyelination of the optic nerve, classically presenting as subacute, painful monocular vision loss. Its clinical identity was shaped by the Optic Neuritis Treatment Trial (ONTT), which established natural history, MS association, and corticosteroid response — but modern practice recognizes ON as biologically heterogeneous rather than one uniform syndrome.¹ Four subtypes are now distinguished: MS-associated ON (typically unilateral, retrobulbar or mild disc edema, MRI demyelinating lesions, oligoclonal bands on CSF), MOG-ON (frequently bilateral or with severe disc edema, longitudinally extensive optic nerve enhancement, generally more favorable visual recovery than AQP4-NMOSD despite dramatic acute presentation), AQP4-NMOSD-ON (severe vision loss, high bilateral/chiasmal involvement, high recurrence and permanent-deficit risk), and double-negative ON (antibody-negative, non-MS pattern, incompletely understood).¹

ON classically affects young adults, with a strong female predominance in MS-associated disease. Risk of eventual MS diagnosis after a first ON episode correlates strongly with baseline brain MRI demyelinating lesion burden.¹ MOGAD and AQP4-NMOSD each have distinct demographic and epidemiologic profiles from MS-ON and from each other.²³ MS-associated ON results from T-cell-mediated autoimmune demyelination as part of the broader CNS demyelinating process of MS. AQP4-NMOSD-ON is mechanistically distinct, driven by pathogenic IgG against the aquaporin-4 water channel on astrocytes, producing complement-mediated astrocytopathy with more severe axonal injury than typical MS-ON.³ MOG-ON is driven by antibodies against myelin oligodendrocyte glycoprotein, associated with a demyelinating rather than primarily astrocyte-destructive injury pattern.¹²

Patients report subacute (hours to days) unilateral vision loss, often with pain worsening on eye movement. Some report Uhthoff's phenomenon (transient worsening with increased body temperature) and photopsias. MOG-ON and AQP4-NMOSD-ON more frequently present with bilateral involvement and more severe disc edema at onset than MS-ON.¹

Key differentials: anterior ischemic optic neuropathy (typically painless, older patients, altitudinal defect), compressive optic neuropathy, toxic/nutritional optic neuropathy (bilateral, gradual, painless), hereditary optic neuropathy (e.g., Leber, in atypical/painless/bilateral young male presentations), and infectious/inflammatory optic neuropathy (syphilis, sarcoidosis).

Exam shows reduced visual acuity (ranging to no light perception in severe AQP4-NMOSD-ON), relative afferent pupillary defect, dyschromatopsia often disproportionate to acuity loss, an optic disc that can appear normal (retrobulbar) or edematous (papillitis, more severe/bilateral edema suggesting antibody-mediated disease), and a visual field defect, classically central or cecocentral.¹ Workup after a first episode should include contrast-enhanced brain/orbital MRI and AQP4/MOG antibody testing via live cell-based assays, which have significantly improved diagnostic accuracy.¹ CSF analysis, including oligoclonal bands, further refines diagnosis and is most supportive of an MS-associated process.¹ OCT structural biomarkers help differentiate subtypes: severe early ganglion cell loss is typical of AQP4-NMOSD-ON, while MOG-ON shows relative structural preservation despite dramatic acute edema.¹

High-dose corticosteroids remain the acute mainstay across ON subtypes for accelerating visual recovery, though long-term management diverges sharply by etiology.¹ AQP4-NMOSD-ON requires aggressive short- and long-term immunosuppression given its high recurrence rate and risk of permanent visual and neurological deficits.³ For MOGAD, high-dose corticosteroids remain the mainstay of acute management, while long-term immunotherapy (IVIg, rituximab, other steroid-sparing agents) has shown efficacy in reducing relapse risk, though optimal maintenance regimens are still under investigation and treatment should be individualized.²

• Optic neuritis is no longer a single diagnosis; antibody testing and MRI enhancement pattern should be pursued after essentially every first presentation¹
• Bilateral involvement or severe disc edema at presentation should raise suspicion for MOG-ON or AQP4-NMOSD-ON rather than typical MS-ON¹
• Despite often dramatic acute presentation, MOG-ON generally has a more favorable visual recovery prognosis than AQP4-NMOSD-ON¹
• Common pitfall: applying ONTT-era natural history and prognosis assumptions uniformly across all ON, when these are most directly applicable to MS-associated (and double-negative) ON
• Common pitfall: failing to test for AQP4 and MOG antibodies after a first ON episode, delaying recognition of a high-recurrence-risk subtype
• Common pitfall: assuming painless vision loss excludes optic neuritis, particularly in some antibody-mediated subtypes