Diagnostic Challenges in Seronegative Autoimmune Limbic Encephalitis Presenting as Faciobrachial Dystonic Seizures: A Case Report

Faciobrachial Dystonic Seizures (FBDS) are sudden brief seizure-like episodes, which are highly characteristic for Autoimmune Limbic Encephalitis (AIE), mostly associated with Voltage-gated Potassium Channel (VGKC), classically Leucine-Rich Glioma Inactivated-1 (LGI1). Timely diagnosis in seronegative patients poses a big challenge, often leading to delay in intervention, causing irreversible neuronal damage. A 66-year-old male with a history of hypertension for seven years on regular medications presented to neurology Outpatient Department (OPD) with one week history of recurrent brief involuntary dystonic movements involving left face and upper limb, along with neuropsychiatric manifestations. Initial assessment revealed mild hyponatraemia. Cerebrospinal Fluid (CSF) analysis, Magnetic Resonance Imaging (MRI) and Electroencephalogram (EEG) were unremarkable. Fluorodeoxyglucose-Positron Emission Tomography Computed Tomography (FDG PET-CT) revealed hypermetabolism in right mesial temporal cortex, bilateral caudate nuclei and putamen. Despite the fact that antibodies were absent in CSF, patient was still started on pulse intravenous corticosteroid therapy and Anti-Seizure Medication (ASM) along with immunotherapy resulting in significant improvement of the patient. The present case highlights the limitations in presently used diagnostic criteria, which rely heavily on MRI and CSF analysis. Advanced imaging techniques like FDG PET-CT are more sensitive, and hence support the diagnosis, when diagnostic criteria are not fulfilled. Timely initiation of immunotherapy based on strong clinical suspicion and advanced imaging is very important to prevent irreversible cognitive impairment.