Antibody-negative limbic encephalitis

Case Discussion

In this case of antibody-negative limbic encephalitis, the patient had been 5 years in remission after resection and chemotherapy for a right lower lobe non-small cell lung cancer (NSCLC). He presented to Neurology with 4 months of spouse-reported progressive encephalopathy, including personality changes, memory loss, and hypersomnia. His initial MOCA at the time of the presented imaging was 20/30. Initial CT was unremarkable, but MRI revealed the diffuse T2/FLAIR hyperintensities described previously and 18F-FDG PET/CT revealed the hypometabolism with a focus of hypermetabolism described previously. In this patient's case, numerous infectious, neoplastic, and paraneoplastic evaluations were negative for any causative etiology, but they did reveal CSF inflammation. The patient was started on high dose steroids for antibody-negative limbic encephalitis, and the patient's symptoms improved, resulting in a MOCA score of 30/30. After steroid therapy, his encephalopathy returned (MOCA 23/30), and he was started on IVIG therapy, which lead to improvements in his MOCA score (27/30). 

18F-FDG PET/CT or PET/MRI can be a helpful adjunct diagnostic imaging tool in diagnosing and potentially monitoring limbic encephalitis. Studies have demonstrated the ability of PET imaging to identify limbic encephalitis in the case of negative MRI studies. Regarding the classic patterns for 18F-FDG uptake, the pictures are mixed, with some encephalitis etiologies demonstrating only hypermetabolism, some demonstrating only hypometabolism, and some demonstrating a mixture of hyper- and hypometabolism. Studies have shown that the pattern can be diagnostic of the type of antibody leading to encephalitis. The mixed hypo-/hypermetabolism pattern shown in this case has been shown to be more strongly found in antibody-negative and NMDAR antibody limbic encephalitis, compared with other antibody-positive cases.