The great mimicker: myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD) masquerading as a hematologic malignancy

Abstract

Case Presentation: A 15-year-old previously healthy male presented with acute paraparesis (grade 4/5), an L5-S1 sensory level, and urinary retention, associated with febrile peaks, emerging three weeks after infectious mononucleosis. Initial cerebrospinal fluid (CSF) analysis revealed marked pleocytosis (68 cells/µL) with a predominance of monomorphic lymphoid cells morphologically suggestive of immaturity or blasts. Under suspicion of lymphoproliferative disease with CNS infiltration, the patient was managed by Hematology and underwent triple intrathecal chemotherapy (methotrexate, cytarabine, and dexamethasone) without clinical response. Diagnostic refinement via CSF flow cytometry evidenced a mature lymphoid pattern, while bone marrow aspirate and biopsy confirmed a reactive process, negative for malignancy. Following the exclusion of infectious etiologies, neuroaxis MRI demonstrated T2/FLAIR hyperintensity characterizing Longitudinally Extensive Transverse Myelitis (LETM) involving the cervical segment and conus medullaris (L5-S2), notably featuring the "medullary H-sign" (central gray matter involvement). The antibody panel was reactive for Anti-MOG (CBA) and negative for Anti-AQP4 and oligoclonal bands, confirming MOGAD. The patient achieved complete remission after 1g methylprednisolone pulse therapy for 5 days. Due to clinical severity, maintenance oral immunosuppression was initiated. Discussion: MOGAD is an antibody-mediated demyelinating disease with heterogeneous phenotypes; LETM and conus medullaris syndrome represent its most severe manifestations. This case underscores a significant diagnostic pitfall: prominent CSF pleocytosis. In MOGAD, high cell counts (>100 cells/µL in 12% of cases) with mononuclear predominance can mimic lymphoproliferative syndromes or neuroinfections. Immature cell morphology often acts as a confounder, as seen here, leading to unnecessary oncological intervention. Flow cytometry immunophenotyping remains the gold standard for differentiating reactive lymphoid activation from neoplastic infiltration. Radiologically, the "H-sign" is highly suggestive of MOGAD, reflecting antibody tropism for areas of high myelin density or metabolic vulnerability in the gray matter. Furthermore, the temporal link with Epstein-Barr virus reinforces the role of infectious triggers in the immune dysregulation that drives anti-MOG antibody production. Final Comments: This case highlights MOGAD as critical differential diagnosis in acute spinal cord syndromes, specifically alerting to the risk of hematologic "masking" due to intense intrathecal inflammatory responses. Distinguishing reactive pleocytosis from malignancy is vital to prevent delays in appropriate therapy. The patient's excellent response to treatment and radiological remission underscore the importance of early diagnosis. Maintenance immunosuppression is important to stabilize the blood-brain barrier and prevent relapses, especially in severe LETM cases where untreated cases carry a high risk of permanent sequelae.