What is the appropriate investigation and management for unilateral optic nerve swelling?

Unilateral Optic Nerve Swelling: Investigation and Causes

Immediate Diagnostic Approach

The investigation of unilateral optic nerve swelling must be guided by visual function status: if vision is normal, optic nerve head drusen is most likely; if vision is impaired, obtain MRI orbits and brain with contrast immediately to evaluate for optic neuritis or non-arteritic anterior ischemic optic neuropathy (NAION). 1

Initial Clinical Assessment

The critical first step is determining whether visual function is preserved or impaired 1:

• Visual acuity testing - Document baseline acuity in both eyes 2
• Color vision assessment - Red-green desaturation (dyschromatopsia) strongly suggests optic neuritis 2
• Visual field testing - Central scotomas are characteristic of optic neuritis 2
• Pupillary examination - Relative afferent pupillary defect indicates optic nerve dysfunction 1
• Pain assessment - Periocular pain worsening with eye movement suggests optic neuritis 2
• Temporal profile - Subacute onset over hours to days indicates optic neuritis, while sudden onset suggests NAION 2, 3

Primary Imaging Investigation

MRI of orbits and brain with contrast is the primary diagnostic study for unilateral optic nerve swelling with visual impairment. 4, 5

The dual imaging approach serves two critical purposes 4:

1. Orbital MRI evaluates for:

   • Abnormal enhancement and T2 signal changes within the optic nerve 4, 2
   • Optic nerve swelling 2
   • Coronal fat-suppressed T2-weighted sequences optimally visualize optic nerve lesions 2

2. Brain MRI assesses for:

   • Intracranial demyelinating lesions predicting multiple sclerosis risk 4, 2
   • Even one clinically silent T2 hyperintense brain lesion is highly associated with eventual MS diagnosis 2
   • Absence of brain lesions strongly predicts monophasic illness 2

Differential Diagnosis by Visual Function Status

Normal Visual Function (Unilateral Swelling)

Optic nerve head drusen is the most likely diagnosis 1, 6:

• Particularly common in pediatric patients where drusen are typically buried 6
• Diagnostic imaging options include ultrasound, OCT, enhanced depth imaging-OCT, fundus autofluorescence 6
• Critical caveat: Unilateral optic disc edema does not exclude intracranial abnormalities, including benign intracranial hypertension or posterior fossa masses 7
• If drusen cannot be confirmed, neuroimaging is mandatory to exclude papilledema from increased intracranial pressure 7

Impaired Visual Function (Unilateral Swelling)

The two most likely diagnoses are 1:

1. Demyelinating Optic Neuritis

• Subacute visual loss over hours to days 2
• Periocular pain worsening with eye movement 2
• Red-green color desaturation 2
• MRI shows T2 hyperintensity, optic nerve swelling, and gadolinium enhancement 2
• Visual evoked potentials demonstrate delayed conduction 2

2. Non-Arteritic Anterior Ischemic Optic Neuropathy (NAION)

• Age ≥50 years without pain: NAION accounts for 87.5% of cases 8
• Sudden onset vision loss 3
• Important finding: Disc swelling may precede vision loss by 2-10 weeks 3
• Fellow eye typically shows "disc at risk" configuration (small cup-to-disc ratio) 3

Red Flags Requiring Expanded Investigation

Certain features suggest atypical optic neuritis requiring additional workup 2, 9:

• Bilateral simultaneous involvement - Suggests neuromyelitis optica spectrum disorders (NMOSD) 2, 9
• Posterior optic nerve involvement including chiasm - Associated with anti-AQP4-IgG-seropositive NMOSD 2, 9
• Long optic nerve lesions - Suggest NMOSD or anti-MOG-IgG disease 2, 9
• Soft tissue enhancement extrinsic to nerve - Indicates granulomatous disease, tumor, or infection rather than typical MS 9

Additional Diagnostic Testing

When atypical features are present 9:

• Serum testing for AQP4-IgG and MOG-IgG antibodies 9
• Cerebrospinal fluid analysis with oligoclonal bands 2
• CSF oligoclonal bands combined with brain MRI lesions dramatically reduce likelihood of monophasic illness 2

Complementary Imaging

CT has limited role 4:

• Not routinely indicated for optic neuritis evaluation 4
• May be used before lumbar puncture if optic disc edema present to exclude space-occupying mass 4

Optical coherence tomography (OCT) 2:

• Documents both acute changes and chronic sequelae 2
• Retinal nerve fiber layer (RNFL) thinning provides evidence of optic nerve damage 2
• Can provide objective evidence when MRI is equivocal 2

Critical Clinical Pitfalls

1. Do not assume unilateral disc swelling excludes increased intracranial pressure - Papilledema can be unilateral due to optic nerve sheath anomalies 7

2. Do not delay imaging in patients >50 years with painless vision loss - While NAION is most likely, other serious conditions must be excluded 8

3. Do not perform only brain MRI without orbital sequences - Optic nerve pathology requires dedicated orbital imaging with fat suppression 2

4. Recognize presymptomatic NAION - Disc swelling may precede vision loss, representing a potential treatment window 3