Role of FDG PET/CT in definitive and presumed autoimmune encephalitis

F-18 Fluorodeoxyglucose PET/CT (FDG-PET) is emerging as a useful imaging adjunct to MRI in the initial diagnostic evaluation of autoimmune encephalitis (AIE)-though presently it is not included in the diagnostic criteria.

In this prospective study we enrolled a total of 52 patients with clinically diagnosed and treated AIE. MRI evaluation was done in each case along with CSF and EEG where feasible. FDG-PET was done for all and images were interpreted visually and using SPM.

The mean age group of patients included was 38.5 ± 22.6 years with 31 females and 21 males. 23 antibody-positive cases underwent PET, the most common antibody detected was anti-NMDAR type followed by anti-LGI 1. Most common metabolic pattern in NMDARE was hypermetabolism in basal ganglia and hypometabolism in parieto-occipital cortices and ovarian teratoma was detected in two of these patients on whole-body PET. A metabolic pattern consistent with AIE was demonstrated in 22/29 (75.8%) antibody-negative patients with hypermetabolism in basal ganglia and mesial temporal cortices. The overall sensitivity of FDG PET was 86% (45/52). MRI abnormalities were detected in 22/52 (42%) cases, 10/23 antibody positive and 12/29 antibody negative cases. PET was positive in 23/30 (76%) MRI negative cases.

Sensitivity of FDG PET for supporting a diagnosis of AIE was higher compared to MRI in both antibody-positive (definitive) and antibody-negative (presumed) AIE. Specific metabolic patterns can be demonstrated on FDG PET in AIE, prompting an early diagnosis so that timely treatment can be instituted.