Abstract
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Introduction: Autoimmune encephalitis (AE) is a relatively rare neurological disorder, which develops as a rapidly progressive encephalopathy caused by brain inflammation that, when it is diagnosed lately its prognosis is poor reaching up to 7-12% of deaths.
For patients with AE treatment should be implemented quickly to avoid irreversible sequelae and death, for this reason is important to find an early biomarker for no delaying therapy.
18F-FDG-PET has been recognized as a potentially useful biomarker in the initial evaluation and subsequent monitoring of patients with suspected autoimmune encephalitis.
Methods: This is a retrospective analysis that included 16 patients with memory or behavioral deficits or alteration in the level of consciousness treated in a third level attention hospital from 2018 to 2021. All patients were diagnosed like possible autoimmune encephalitis after that they were carefully examined for excluding other diseases that can cause rapidly progressive encephalopathy with routine blood, CSF analysis, EEG, complete neurological examination and MRI. Measurement of autoantibodies were done in 15 patients, however the results were recorded several weeks after sampling. A whole body and brain 18F-FDG PET/CT was done in all patients to rule out possible primary tumor and before steroid or immunotherapy began.
PET/CT were assessed qualitative and semiquantitative by one nuclear medicine and one radiologist. Intensity normalization was performed to the gray matter (primary sensorimotor cortex). For the semiquantitative analysis we use Scenium© software (Siemens), which was also normalized to the cortex.
Results: The average age was 62 years old. An MRI was performed in every patient, nevertheless, only 3 of them were diagnosed as suggestive of encephalitis, while brain PET/CT was pathologic in all the patients. Only in 2 patients antibodies were found to be positive (AC GAD65 and Ac CASPR2).
For qualitative analyses patients were divided into 2 groups according to the metabolic pattern found, group 1: hypermetabolic, and group 2 mixed pattern (hyper and hypometabolic). The most frequent pattern found was mixed (11 patients). The brain areas with the highest metabolism were the mesial temporal region, the lateral temporal region and lenticular nucleus. The area with the lower metabolism was the occipital lobe, reported in 7 patients, as well as global hypometabolism in 4 patients.
For the semiquantitative analyses the most frequent finding was hypermetabolism (Z score > 2) in the lenticular nucleus, found in.13 patients, followed by the mesial and lateral regions of the temporal lobe in 10 patients. Unlike the visual analyses there was found occipital hypometabolism in 9 patients (Z score < 2).
When whole body PET/CT were analyzed, we found a tumor in 25% of patients, malignant neoplasms found were breast, kidney and prostate cancer, in one patient an ovarian teratoma was documented, two of these patients showed improvement after surgery, one died and the other one was lost in the follow up.
According to the PET findings, treatment was stablished in the remaining 12 patients, which were treated with steroids, immunoglobulin, plasmapheresis or rituximab, according to the clinical evaluation by the neurologist, with total or partial clinical improvement in 100% of patients.
Conclusions: The clinical facts and evidence suggest that early therapy improves outcomes in AE so it should start quickly while other studies and comprehensive antibody tests are processed. In our study, 100% patients had abnormal FDG PET, while only 3 had abnormal MRI, suggesting the limited sensitivity of MRI in AE. We describe the main findings corresponding to AE, these metabolic patterns can add value to the diagnostic evaluation of AE. It is necessary more prospective studies to demonstrate the specificity and accuracy of FDG PET so that this study can be included in the future diagnostic recommendations for AE.
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