Limbic Encephalitis: An Unusual Presentation on 18F-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography (18F-FDG PET/CT).

Limbic encephalitis is a relatively rare autoimmune neurological disorder, typically diagnosed based on clinical symptoms lasting less than three months, including seizures, memory deficits, psychiatric symptoms, bilateral mesial temporal lesions on MRI, inflammatory cerebrospinal fluid, and epileptiform activity on electroencephalogram (EEG). We report the case of a female patient who presented with an inaugural epileptic seizure, for which MRI, lumbar puncture, and cerebral positron emission tomography (PET) scan showed no pathological findings. The patient re-presented to the emergency department one month later with recurrent seizures. A subsequent PET scan revealed the emergence of a right mesial temporal lesion, and a hypermetabolic pulmonary lesion was identified, which was later diagnosed as small-cell lung carcinoma on histopathology. The patient showed favorable clinical improvement under Solu-Medrol treatment, and a follow-up imaging performed several months later showed complete resolution of the hypermetabolic cerebral lesion after chemotherapy. This case highlights an unprecedented early stage of limbic encephalitis, characterized by an initial absence of inflammation, suggesting that this might represent a nascent phase of the disease, which could be crucial for future management of similar cases.