Fig. 1: Magnetic resonance imaging (MRI) revealed only non-specific white matter lesions (see exemplary within red circle).

Veobrain analysis of the MRI showed no volume loss (https://www.veobrain.com/?page=veomorph). [¹⁸F]fluorodeoxyglucose positron emission tomography (FDG-PET) of the brain showed normal findings. Whole-body FDG-PET (not shown) revealed no evidence of tumor. Electroencephalography (EEG) showed intermittent slowing (see exemplary red marker in EEG). Tissue-based assays using cerebrospinal fluid (CSF) identified strong immunoglobulin (Ig) G antibody binding to the mossy fiber tract (mft) in the hippocampus. The well-characterized anti-neuronal and anti-glial autoantibodies were negative (not shown here). Serum was tested negative for paraneoplastic IgG antibodies against intracellular antigens (Yo, Hu, CV2/CRMP5, Ri, Ma1, Ma2, SOX1, Tr/DNER, Zic4, GAD65, amphiphysin) and glial antigens (MOG/AQP4). Neuronal IgG cell surface antibodies (NMDA-R, LGI1, CASPR2, GABA-B-R, AMPA1-R, AMPA2-R, DPPX) were negative in serum and CSF. The routine CSF analyses (not shown here) detected intrathecal synthesis for all three Ig isotypes (IgG, IgM, and IgA) and oligoclonal bands in CSF. Both the white blood cell count (4/µL; reference: <5/µL) in the CSF and the albumin quotient (2.9; reference: <6.3) were normal.