The significance of brain 18F-FDG PET/CT in the diagnosis of limbic encephalitis

Objectives Whole-body FDG-PET/CT has proven to be very effective in the diagnosis of primary tumors in patients developing associated paraneoplastic limbic encephalitis (LE). The aim of this study was to analyze Brain FDG PET/CT scans in patients suspected to have LE in order to evaluate whether FDG PET findings of the brain support the diagnosis of LE.

Methods 81 patients (47 men, median age: 54 y) with suspected LE were referred for whole body PET/CT to asses the paraneoplastic origin of LE. Additional FDG-PET scans of the brain was performed to evaluate the pathologic changes. Brain PET images were evaluated visually as well as with SPM 2. According to the diagnostic criteria used in our institution patients were classified as (definite LE), (possible LE), (suspected LE), and (no LE).

Results 13, 8 and 19 patients were classified as definite, possible, and suspected LE respectively. All patients with "definite LE" showed significant metabolic changes in limbic areas, with relative hypo- or hypermetabolism in 55 % and 45 % of cases, respectively. Furthermore, FDG-PET detected significant metabolic changes in limbic regions of 56 % and 63 % of cases with (suspected) LE and (possible LE), respectively. 17 patients were diagnosed with cryptogenic temporal lobe epilepsy (CTLE) and 78 % of these patients showed significant metabolic changes in the temporal lobe, 39 % in limbic regions.

Conclusions In "definite LE", interindividually consistent metabolic changes can be detected in limbic areas, which manifest either as relative hypo- or hypermetabolism. FDG PET is particularly promising in cases with (suspected) and (possible) LE, as the demonstration of limbic metabolic alterations can substantiate the diagnosis. It is also of clinical value in patients with CTLE with normal MRI. Thus, we recommend to include Brain FDG PET in the routine work up of LE patients