Different FDG‐PET metabolic patterns of anti‐AMPAR and anti‐NMDAR encephalitis: Case report and literature review

INTRODUCTION: (18)F‐fluorodeoxyglucose (FDG)‐PET metabolic patterns of brain differ among autoimmune encephalitis with different neuronal surface antigens. In this case report, we compared the topographical relationship of cerebral glucose metabolism and antigen distribution in the patients with anti‐NMDAR and anti‐AMPAR encephalitis. Literature review summarized the common features of brain metabolism of autoimmune encephalitis. METHODS: The cerebral glucose metabolism was evaluated by FDG‐PET/CT during acute‐to‐subacute stage of autoimmune encephalitis and after treatment. The stereo and quantitative analysis of cerebral metabolism used standardized z‐score and visualized on three‐dimensional stereotactic surface projection. To map NMDAR and AMPAR in human brain, we adopted genetic atlases from the Allen Institute and protein atlases from Zilles's receptor densities. RESULTS: The three‐dimensional stereotactic surface projection displayed frontal‐dominant hypometabolism in a 66‐year‐old female patient with anti‐AMPAR encephalitis and occipital‐dominant hypometabolism in a 29‐year‐old female patient with anti‐NMDAR encephalitis. Receptor density maps revealed opposite frontal–occipital gradients of AMPAR and NMDAR, which reflect reduced metabolism in the correspondent encephalitis. FDG‐PET hypometabolic areas possibly represent receptor hypofunction with spatial correspondence to receptor distributions of the autoimmune encephalitis. The reversibility of hypometabolism was in line with patients' cognitive improvement. The literature review summarized six features of metabolic anomalies of autoimmune encephalitis: (a) temporal hypermetabolism, (b) frontal hypermetabolism and (c) occipital hypometabolism in anti‐NMDAR encephalitis, (d) hypometabolism in association cortices, (e) sparing of unimodal primary motor cortex, and (e) reversibility in recovery. CONCLUSIONS: The distinct cerebral hypometabolic patterns of autoimmune encephalitis were representative for receptor hypofunction and topographical distribution of antigenic receptors. The reversibility of hypometabolism marked the clinical recovery of autoimmune encephalitis and made FDG‐PET of brain a valuable diagnostic tool.