NMOSD and MOGAD

Atypical Optic Neuritis: NMOSD & MOGAD

Atypical features that deviate from typical MS-associated optic neuritis mandate an immediate search for non-MS autoimmune demyelinating conditions. Red flag warning signs include:

• Bilateral simultaneous or rapidly sequential vision loss
• Severe, catastrophic visual acuity impairment (e.g., light perception or no light perception)
• Absence of eye movement-induced pain
• Lack of visual improvement within 3 to 4 weeks of symptom onset
• Marked anterior disc swelling with extensive retinal haemorrhages or macular exudates
• Associated severe myelopathic or intractable systemic signs

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Neuromyelitis Optica Spectrum Disorder (NMOSD)

Pathophysiology & Biomarkers

NMOSD is a severe, relapsing, antibody-mediated autoimmune astrocytopathy with secondary demyelination and axonal injury. Aquaporin-4 antibodies (AQP4-IgG / NMO-IgG) identify the characteristic antibody-positive form; antibody-negative NMOSD requires stricter clinical and MRI criteria, with exclusion of alternative diagnoses. Aquaporin-4 is a bidirectional water channel highly concentrated on the foot processes of astrocytes that abut blood vessels at the blood-brain barrier and ependymal surfaces. Binding of AQP4-IgG triggers classical complement activation (membrane attack complex deposition), granulocyte and macrophage infiltration, astrocytic foot process necrosis, and secondary bystander demyelination with irreversible axonal transection.

Clinical Presentation & Distinctive Syndromes

NMOSD is characterised by a strong female preponderance (up to 9:1) and a slightly older age of onset (median 35-45 years), with increased incidence in individuals of African, Asian, and Latin American ancestry:

• Severe, Bilateral or Sequential Optic Neuritis: Visual loss is catastrophic; up to 50% of affected eyes deteriorate to counting fingers or worse. Spontaneous visual recovery is notoriously poor without aggressive acute rescue therapy.
• Posterior Optic Nerve & Chiasmal Involvement: MRI typically reveals longitudinally extensive gadolinium enhancement of the optic nerve, frequently involving the intracranial segment and extending across the optic chiasm, producing bitemporal hemianopic field defects.
• Longitudinally Extensive Transverse Myelitis (LETM): Contiguous spinal cord lesions spanning $\ge 3$ complete vertebral segments on sagittal T2-weighted MRI, presenting with severe paraplegia, sensory loss, and neurogenic bladder/bowel dysfunction.
• Area Postrema Syndrome: Intractable, unexplained nausea, vomiting, or persistent hiccups lasting $\ge 48$ hours, caused by inflammation of the medullary area postrema (a circumventricular organ rich in AQP4 lacking a tight blood-brain barrier).

Treatment Protocols in NMOSD

• Acute relapse: Treat urgently with specialist high-dose corticosteroids and consider early plasma exchange for severe disease or inadequate response; do not wait for an arbitrary deadline.
• Maintenance Immunotherapy: AQP4-positive NMOSD can cause severe recurrent attacks, and specialist preventive therapy is usually needed. The choice depends on antibody status, relapse course, comorbidity, infection precautions and local approval; do not infer that every attack is incapable of spontaneous improvement or apply one mandatory regimen to all seronegative optic neuritis:
  • B-cell depletion: Rituximab (anti-CD20) or Inebilizumab (anti-CD19)
  • Complement inhibition: Eculizumab or Ravulizumab (anti-C5 terminal complement inhibitor)
  • Interleukin-6 receptor blockade: Satralizumab or Tocilizumab

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Myelin Oligodendrocyte Glycoprotein Antibody-Associated Disease (MOGAD)

Pathophysiology & Biomarkers

MOGAD is a distinct inflammatory demyelinating disorder mediated by IgG autoantibodies targeting myelin oligodendrocyte glycoprotein (MOG-IgG). MOG is a transmembrane protein expressed on the outermost lamellae of the myelin sheath and oligodendrocyte plasma membranes. In contrast to NMOSD, MOGAD is a primary demyelinating condition rather than an astrocytopathy, resulting in less tissue necrosis and substantial potential for remyelination.

Clinical Presentation & Distinctive Syndromes

• Demographics: Occurs in both children and adults, with an equal male-to-female ratio (1:1 to 1.5:1).
• Prominent Papillitis (Optic Disc Oedema): In contrast to typical MS-ON (where 65% is retrobulbar), optic disc swelling is present in $>80\%$ of MOGAD-ON cases. The disc swelling is frequently severe, accompanied by hyperaemia, splinter haemorrhages, and peripapillary subretinal fluid.
• Bilateral Simultaneous Optic Neuritis: Seen in up to 50% of presentations, accompanied by severe retro-orbital pain.
• Optic Perineuritis on MRI: Orbital MRI characteristically reveals longitudinally extensive enhancement of the optic nerve sheath and adjacent intraorbital fat (optic perineuritis) with sparing of the optic chiasm and brain parenchyma.
• Visual Recovery vs. Steroid Dependence: Initial visual loss is often profound (nadir $<20/200$), but exhibits exquisite, rapid responsiveness to corticosteroid therapy. However, MOGAD demonstrates a notorious tendency for steroid-dependent relapse when oral prednisone is tapered rapidly or withdrawn within weeks.

Treatment Protocols in MOGAD

• Acute Therapy: High-dose IV methylprednisolone (1,000 mg/day for 3 to 5 days).
• Taper and prevention: Plan specialist taper and relapse prevention according to MOGAD course and toxicity; do not impose a mandatory 3–6-month minimum on every patient.

Diagnostic Comparison: MS-ON vs. NMOSD vs. MOGAD

The following table summarizes the differential diagnostic features essential for the EBOD examination:

Applying Historical ONTT Results Today

ONTT found increased recurrence with oral prednisone 1 mg/kg/day alone; this is a specific low-dose regimen, not a prohibition on all oral corticosteroid treatment. Bioequivalent high-dose oral regimens can be used in selected adults under specialist protocols. Typical optic neuritis often recovers, while corticosteroids mainly accelerate recovery. MRI findings and clinical features inform longer-term neurological assessment rather than a single white-matter lesion automatically establishing MS.

The 2024 McDonald revisions, published in 2025, allow the optic nerve as a fifth dissemination-in-space location when appropriately demonstrated and no better explanation exists. Diagnosis still requires the complete neurological criteria. See the revised criteria. Test AQP4/MOG antibodies when the phenotype supports these conditions and interpret assay quality and pretest probability; indiscriminate testing can yield misleading positives. Seronegative NMOSD requires its own stricter clinical/imaging criteria.

Severe atypical disease needs urgent specialist treatment. Plasma exchange may be started early for inadequate response rather than waiting a fixed five-to-seven-day deadline. MOGAD relapse risk and steroid taper are individualised; every patient does not require a mandatory three-to-six-month course. NMOSD preventive drugs depend on antibody status, jurisdiction and infection/vaccination precautions, including meningococcal protection for complement inhibitors.