Anti-LGI1 encephalitis with a pseudotumoral presentation: a case report

Abstract

Case Presentation: A 42-year-old man, previously healthy and fully independent, presented with a subacute 45-day history of behavioral change, memory impairment, progressive temporospatial disorientation, and loss of independence in activities of daily living. During the disease course, he developed tonic episodes involving the left hemibody, consistent with faciobrachial dystonic seizures (FBDS). Brain magnetic resonance imaging showed T2/FLAIR hyperintensity involving both hippocampi and left uncus, without diffusion restriction or significant contrast enhancement. Magnetic resonance spectroscopy demonstrated reduced N-acetylaspartate and altered choline-to-creatine ratio, initially raising suspicion for a low-grade glial neoplasm. Cerebrospinal fluid analysis revealed mild hyperproteinorrachia without pleocytosis, and electroencephalogram showed epileptiform paroxysms over the left anterior temporal region. Given the clinical picture, an autoimmune encephalitis panel was obtained and showed stark positivity for anti-LGI1 antibodies, confirming autoimmune limbic encephalitis. Initial treatment included high-dose intravenous methylprednisolone and intravenous immunoglobulin. Due to only a partial clinical response, with unsatisfactory cognitive improvement, therapy was escalated to rituximab. Clinical improvement was observed during hospitalization, documented by an increase in the MoCA score from 14 to 19, although residual cognitive deficits persisted at outpatient follow-up. Screening for associated malignancy, including CT and ultrasonography, remained negative. Discussion: This case highlights the diagnostic challenge of anti-LGI1 encephalitis with a pseudotumoral presentation, mimicking infiltrative lesions such as low-grade glioma and primary central nervous system lymphoma. Mesial temporal T2/FLAIR hyperintensity, hippocampal mass effect, and spectroscopy abnormalities suggestive of increased cellular turnover may closely resemble glial or lymphoproliferative neoplasms. In this context, identification of FBDS was the key diagnostic clue, prompting prioritization of the autoimmune workup. Final Comments: Anti-LGI1 encephalitis should be considered in the differential diagnosis of mesial temporal pseudotumoral lesions, particularly in patients with subacute cognitive decline, behavioural changes, and FBDS. Expansile imaging features and metabolic abnormalities on spectroscopy may lead to diagnostic error and unnecessary invasive procedures. Early recognition of suggestive clinical features and integrated interpretation of radiologic, electroencephalographic, and immunologic findings are essential for timely diagnosis, prompt immunotherapy, and reduction of long-term cognitive sequelae.