To report a case of MALT lymphoma presenting as optic neuritis and highlight its inherent diagnostic challenges.

51-year-old healthy African American woman presented with one month of progressive left monocular blurry vision, color desaturation and pain . Exam showed left visual acuity of 20/60, afferent pupillary defect, and 0/13 Ishihara plates identified in left eye. Remainder of exam was intact.

MRI demonstrated left optic nerve sheath enhancement and enlargement. Another enhancing lesion was identified in the right inferior orbit. Despite visual impairment, corticosteroids were deferred prioritizing diagnostic tests. Workup was negative for autoimmunity, systemic neoplasm, and infection. CSF showed a bland profile without malignant cells. Serum and CSF IL2 receptor levels returned normal. Oligoclonal bands and IgG were elevated in serum and CSF; IgG4 levels were high at 156, raising concern for IgG4-related ocular disease. Right intra-orbital mass biopsy demonstrated atypical CD20+ lymphoid infiltrates with rare germinal centers, but <10% plasma cells expressed IgG4. Further molecular analysis was positive for monoclonal IgH gene rearrangement, diagnosing mucosa-associated lymphoid tissue (MALT) lymphoma in the right orbit and probable left optic nerve involvement. She was initiated on bendamustine and rituximab with stabilization of her ocular symptoms.

Atypical presentations of ON warrant thorough workup. IgG4-related ocular disease and ocular adnexal MALT lymphoma share similarities that require histologic and molecular analysis to differentiate. Early identification of MALT lymphoma is crucial to initiate proper therapies, maintain disease control, and preserve visual acuity.