Positron Emission Tomography with 18F-Fluorodeoxyglucose Imaging Patterns in Autoimmune Encephalitis

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Autoimmune encephalitis (AE) is an underrecognized and potentially curable disease, which has been the focus of intense neurologic research. In the present manuscript, we review recent updates and the current role of brain positron emission tomography imaging with 18F-fluorodeoxyglucose (FDG-PET) in the detection of AE. We appraise the many metabolic imaging manifestations described in this disease, the role of PET-FDG in its diagnosis and follow-up, and the possible relationship between some patterns and specific autoantibodies. We also briefly discuss recently recognized imaging patterns and the potential impact of new technologies in recognition of such metabolic imaging appearances. AE findings on FDG-PET may have various patterns, but three are dominant and can be summarized as follows: (1) hypermetabolism in cortical areas, mainly in mesial temporal regions and less frequently in basal ganglia and higher cortical regions, is a common pattern in early stages of the disease. Such pattern is highly suggestive of limbic AE, since it has not been described in many other entities, except for brain tumors and active epileptic foci. (2) Also common is a reduced metabolism in the regions described above, which could happen both in the detection of the disease or in previous hypermetabolic areas which changed their pattern during the course of illness. (3) Other areas with hypometabolism can also occur, especially the “diffuse whole-brain cortical hypometabolism” manifestation, which is unspecific and can have degenerative diseases and other conditions as differential diagnoses. Some antibodies are more related to specific metabolic imaging patterns, but others do not correlate closely with imaging appearances. We consider that FDG-PET imaging can aid in the early diagnosis of AE and may also be helpful while accessing the disease longitudinally while showing functional changes that occur after therapy. In both situations it can provide valuable information that is not provided by anatomic imaging alone.