Case Report: Pediatric MOG/NMDAR overlap syndrome with delayed cortical MRI evolution and incidental venous sinus hypoplasia.
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BACKGROUND: Pediatric overlap of myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD) and anti-N-methyl-D-aspartate receptor encephalitis (MNOS) is uncommon. At onset, such cases may resemble infectious or vascular disorders, leading to premature diagnostic closure and delayed immunotherapy. Most published reports emphasize antibody coexistence itself, whereas fewer describe how misleading early clinical or imaging clues can redirect the diagnostic pathway and how longitudinal multimodal reassessment can correct the initial working diagnosis. CASE PRESENTATION: A 16-year-old girl presented with a 20-day history of progressive headache, transient focal deficits, meningeal irritation, and inflammatory CSF abnormalities. Relative-timeline reassessment showed inflammatory CSF on Day 4, a left parieto-occipital cortical-sulcal FLAIR abnormality by Week 2, MRV findings favoring unilateral transverse-sigmoid sinus hypoplasia/low-flow change rather than active thrombosis at Month 1, and relapse with seizure and a new right occipital cortical-sulcal lesion at Month 3. Paired CSF/serum antibody testing revealed CSF anti-NMDAR-IgG positivity together with CSF/serum MOG-IgG positivity, supporting a final diagnosis of MNOS-associated MOGAD cortical encephalitis within the FLAMES/UCCE spectrum. The patient improved after high-dose corticosteroids and intravenous immunoglobulin, with no further documented seizures during follow-up. CONCLUSIONS: This case illustrates that MNOS-related FLAMES/UCCE-spectrum cortical encephalitis may initially raise concern for CVST or viral meningoencephalitis when early parenchymal MRI is nondiagnostic and venous imaging abnormalities coexist. Diagnostic adjudication should not rely on a single imaging clue or isolated antibody result, but on integrated reassessment of the clinical phenotype, serial MRI evolution, CSF inflammatory features, and standardized antibody testing. Persistent serum MOG-IgG may support concern for a relapsing-prone disease course, but a low-positive titer alone should not be used as a stand-alone marker of active relapse without clinicoradiological support.