How do you differentiate and treat compressive optic neuropathy versus Myelin Oligodendrocyte Glycoprotein (MOG) optic neuritis?

Differentiating Compressive Optic Neuropathy from MOG Optic Neuritis

MRI of the orbits and head with contrast is the definitive imaging study to differentiate these conditions: compressive optic neuropathy shows a mass lesion compressing the optic nerve, while MOG optic neuritis demonstrates longitudinally extensive optic nerve enhancement (typically >50% of pre-chiasmal length) with perioptic enhancement, bilateral involvement, and prominent optic disc swelling without a compressive lesion. 1, 2

Clinical Differentiation

Compressive Optic Neuropathy Features

• Progressive, painless visual loss is typical, though pain can occur with rapid expansion 3
• Unilateral presentation is most common unless bilateral compression (e.g., pituitary adenoma) 4
• Visual field defects correspond to anatomic compression site 3
• Optic disc appearance varies: normal (retrobulbar compression), pale (chronic), or edematous (anterior compression) 3
• No response to corticosteroids distinguishes from inflammatory causes 4

MOG Optic Neuritis Features

• Acute onset with severe visual loss (often to no light perception or counting fingers) occurring over days 5, 6, 7
• Retrobulbar pain with eye movement in most cases 5, 6
• Prominent optic disc edema/papillitis is characteristic (seen in 39-60% of cases) 2, 6
• Bilateral simultaneous or sequential involvement is common 2, 8
• Steroid-responsive but often steroid-dependent with relapses after tapering 2, 5
• Red color desaturation is frequently reported 7

Imaging Characteristics

MRI Findings for MOG Optic Neuritis

• Longitudinally extensive optic nerve lesions: median length 23.1 mm (>50% of pre-chiasmal nerve length) 2
• Involvement of >6/12 optic nerve segments or all three pre-chiasmal segments (intraorbital, canalicular, intracranial) 2
• Perioptic gadolinium enhancement during acute phase (not just T2 hyperintensity) 2
• Chiasmal and optic tract involvement may occur 2
• Brain MRI typically lacks MS-typical lesions: no Dawson's fingers, no juxtacortical U-fiber lesions, no ovoid periventricular lesions 2

MRI Findings for Compressive Optic Neuropathy

• Identifiable mass lesion (tumor, aneurysm, thyroid eye disease with apical crowding) compressing the optic nerve 3
• Tendon-sparing extraocular muscle enlargement in thyroid eye disease 3
• Orbital apex crowding may be evident 3
• No abnormal optic nerve enhancement unless secondary inflammation present 1

Diagnostic Algorithm

Step 1: Clinical Assessment

• Age matters: MOG optic neuritis peaks in children and young adults; compressive lesions increase with age 2, 5
• Tempo: Acute onset over hours-to-days suggests MOG; gradual progression over weeks-to-months suggests compression 5, 6
• Pain: Retrobulbar pain with eye movement strongly favors MOG optic neuritis 5, 6
• Bilaterality: Simultaneous bilateral involvement strongly suggests MOG (rare in compression unless midline lesion) 2, 8

Step 2: Fundoscopic Examination

• Prominent papillitis/disc swelling: Highly suggestive of MOG optic neuritis 2, 6
• Normal or pale disc: More consistent with retrobulbar compression 3
• Disc edema with normal or elevated intracranial pressure: Consider both entities, but MOG can mimic IIH 4

Step 3: Imaging Protocol

• Order MRI orbits AND head with and without contrast as the primary study 1
• Look for compressive lesions first (tumor, aneurysm, thyroid-related muscle enlargement) 1, 3
• If no compression identified, assess optic nerve for:
  • Length of T2 hyperintensity/enhancement (>50% pre-chiasmal length suggests MOG) 2
  • Perioptic enhancement (specific for MOG) 2
  • Brain lesion pattern (absence of MS-typical lesions supports MOG) 2

Step 4: Serologic Testing (if no compression found)

• Order MOG-IgG by cell-based assay (NOT ELISA) if imaging shows inflammatory pattern 2
• Test AQP4-IgG simultaneously to exclude NMOSD 2, 9
• CSF analysis if diagnosis unclear: neutrophilic pleocytosis or WCC >50/μL suggests MOG; absence of oligoclonal bands (in 87-88% of MOG cases) distinguishes from MS 2, 9

Step 5: Ancillary Testing

• OCT: Increased peripapillary RNFL thickness acutely in MOG; normal or decreased in chronic compression 7
• Visual evoked potentials: Delayed/reduced in both, but pattern may help if imaging equivocal 1, 2
• Thyroid function studies if thyroid eye disease suspected 3

Treatment Implications

For Compressive Optic Neuropathy

• Surgical decompression is definitive treatment when vision-threatening 3, 4
• Corticosteroids do not improve outcomes and should not delay surgical intervention 4
• Teprotumumab for thyroid eye disease may reduce muscle volume and improve diplopia 3

For MOG Optic Neuritis

• High-dose intravenous methylprednisolone (typically 1g daily × 3-5 days) is first-line acute treatment 5, 6, 7
• Plasma exchange if inadequate response to steroids 2, 5, 7
• Oral prednisone taper over weeks-to-months (rapid taper causes relapses) 2, 7
• Long-term immunosuppression (rituximab, mycophenolate mofetil, azathioprine) for relapsing disease 2, 5, 7, 8
• Delaying treatment causes irreversible optic nerve damage 7

Critical Pitfalls

• MOG optic neuritis can mimic IIH with disc edema and elevated opening pressure; repeat orbital imaging if IIH treatment fails 4
• Compressive lesions may coexist with MOG disease; always image to exclude mass before attributing symptoms solely to MOG 4
• ELISA testing for MOG-IgG has poor specificity; only cell-based assays are acceptable 2, 6
• Normal brain MRI does not exclude MOG optic neuritis; in fact, it increases pre-test probability 2
• Steroid responsiveness does not exclude compression; some compressive lesions show transient improvement with steroids 4
• Bilateral optic neuropathy in obese patients: Do not assume IIH; consider MOG if atypical features present 4