What investigations should be done for a 40-year-old man with suspected acute left retrobulbar optic neuritis and a history of similar episodes?

Investigation of Suspected Acute Left Retrobulbar Optic Neuritis with Recurrent Episodes

Primary Diagnostic Investigation

MRI of the orbits and brain with contrast is the essential first-line investigation for this patient with suspected acute retrobulbar optic neuritis and a history of recurrent episodes. 1, 2 This dual imaging approach serves multiple critical purposes: evaluating for abnormal enhancement and T2 signal changes within the optic nerve, and assessing for intracranial demyelinating lesions that predict multiple sclerosis risk. 1

Key MRI Features to Identify

The MRI should specifically evaluate for:

• T2 hyperintensity with associated optic nerve swelling and contrast enhancement, which characterizes typical acute optic nerve lesions 1
• Coronal fat-suppressed T2-weighted sequences, which are optimal for visualizing optic nerve lesions 1
• Brain lesions: Even a single clinically silent T2 hyperintense brain lesion is highly associated with eventual MS diagnosis 1, 3
• Long optic nerve lesions or posterior involvement extending to the chiasm, which suggest neuromyelitis optica spectrum disorders (NMOSD) or anti-MOG-IgG disease 1

Critical Clinical Context: Red Flags in This Case

This patient's history raises important concerns that must guide the investigation:

• Recurrent episodes (similar event years ago) significantly increase the likelihood of an underlying demyelinating disorder rather than isolated optic neuritis 4, 5
• Visual symptoms worsened by Valsalva maneuver is an unusual feature that warrants careful evaluation for alternative diagnoses
• Spontaneous resolution after one month is consistent with typical optic neuritis recovery patterns 4

Essential Complementary Investigations

Visual Function Testing

• Visual evoked potentials (VEPs) provide objective evidence of optic nerve dysfunction with slowed conduction, even when imaging is equivocal 1
• Optical coherence tomography (OCT) to document retinal nerve fiber layer (RNFL) changes, which can show both acute changes and chronic sequelae 1
• Formal visual field testing to document central scotomas or other characteristic defects 1

Laboratory Evaluation for MS Risk Stratification

• Cerebrospinal fluid analysis for oligoclonal bands: When combined with brain MRI lesions, CSF oligoclonal bands dramatically reduce the likelihood of monophasic illness and indicate high MS risk 1, 4
• Routine blood work to exclude other inflammatory or infectious etiologies 4

Serology for Atypical Optic Neuritis

Given the recurrent nature, testing for:

• Anti-aquaporin-4 (AQP4) antibodies to evaluate for NMOSD, particularly important as this carries a poor visual prognosis with only 30% maintaining visual acuity >20/25 3
• Anti-MOG antibodies to assess for MOG-antibody disease, which is associated with long optic nerve lesions 1

Clinical Examination Priorities

Before imaging, document:

• Relative afferent pupillary defect (RAPD) assessment 1
• Red-green color desaturation (dyschromatopsia), a characteristic feature 1
• Periocular pain with eye movement, typical of optic neuritis 1
• Fundoscopic examination to determine if this is truly retrobulbar (normal disc appearance) versus papillitis 4

Critical Pitfalls to Avoid

Do not delay MRI imaging while waiting for other test results, as early diagnosis impacts treatment decisions and MS risk stratification. 1, 5

CT has limited utility in optic neuritis evaluation and should not substitute for MRI; it may only be considered before lumbar puncture if optic disc edema is present to exclude space-occupying lesions. 1

Do not assume idiopathic optic neuritis without excluding NMOSD and MOG-antibody disease, as these require different treatment approaches and have worse prognoses. 3

Prognostic Implications of Investigation Results

The investigation results directly impact prognosis:

• Absence of brain lesions strongly predicts monophasic illness with low MS risk 1, 3
• Presence of brain lesions with CSF oligoclonal bands indicates high MS risk requiring different monitoring and potential early disease-modifying therapy 1, 3
• Bilateral involvement or long lesions suggest NMOSD/MOGAD with higher recurrence risk (50-60% during steroid taper) and worse visual outcomes 3