18F-FDG-PET and MRI in autoimmune encephalitis: a systematic review of brain findings

In the diagnostic assessment of autoimmune encephalitis (AE) associated with antineural antibody (Ab) imaging with 18F-fluorodeoxyglucose (FDG), positron emission tomography (PET) was initially only used to screen for occult malignancies in paraneoplastic cases. Today accumulating evidence also supports the use of PET imaging for the objective assessment of metabolic changes in the brain of patients with AE. On the other hand, magnetic resonance imaging (MRI) of the brain reveals a variable picture depending on the specific syndrome and associated antibody, and may be normal in a sizable proportion of patients.From a systematic review of the literature, it seems that some specific metabolic patterns correlate with the presence of specific Ab, such as a cerebral posterior hypometabolism in anti-NMDAR encephalitis, and a mesiotemporal hypermetabolism (associated with hyperintensities and swollen structures on MRI T2) in encephalitis with LGI1 and onconeural Ab. To ascertain the prognostic value of FDG-PET and its role in driving therapy, larger (preferably longitudinal) studies are needed on age-matched, untreated patients with the same Ab status, who undergo imaging at a similar time after the onset of their symptoms. This would enable a systematic correlation between MRI and FDG-PET findings, and help to clarify a number of unsolved clinical and technical issues.