We aim to emphasize the importance of FDG-PET testing for the prompt diagnosis of limbic encephalitis (LE), especially in MRI-negative cases.  

Faciobrachial dystonic seizure are the classic feature for autoimmune LE. This diagnosis has become increasingly more recognized as a cause of intractable focal epilepsy, in no short part, due to advances in imaging modalities and immunohistochemically detection. 

We report the case of a 61 year old man presenting with several months of progressive memory loss and frequent faciobrachial seizures. Continuous EEG monitoring demonstrated myoclonic seizures, with no clear epileptogenic zone. The patient’s MRI brain was initially unremarkable and therefore FDG-PET scan was obtained, which showed an increased area of metabolic activity exclusively in the region of the left amygdala. This finding correlated with subsequent MRI brain imaging, demonstrating increased size and FLAIR signal of the left amygdala and left hippocampal formation. Autoimmune panel detected the presence of both LGI1 and VGKC antibodies. Despite continuous treatment with multiple antiepileptic drugs and steroids, the patient continued to seize. He was given both Plasmapheresis and Intravenous immunoglobulin, and subsequently became a candidate for Rituximab therapy.

This case illustrates a clear unilateral and focal area of involvement in LE, specifically the left amygdala and hippocampal formation. It also highlights the usefulness of FDG-PET imaging to both localize the affected area, and aid in arriving to a prompt diagnosis when other imaging modalities and immunohistochemical tests are pending.