Case Report: A case of anti-mGluR1 antibody-associated encephalitis treated with efgartigimod combined with rituximab.

Anti-metabotropic glutamate receptor 1 (mGluR1) encephalitis is a rare autoimmune disorder affecting both the central and peripheral nervous systems, typically characterized by a subacute cerebellar syndrome. Its effective treatment remains undefined. Here, we present the case of a 16-year-old male student who experienced dizziness upon walking and standing. Symptoms gradually worsened, leading to psychiatric and behavioral abnormalities. Neurological examination revealed somnolence, dysarthria, a broad-based unsteady gait, a prominent head tremor, and limb ataxia. The first magnetic resonance imaging scan showed no obvious abnormality. The background rhythm on the electroencephalogram was normal, with no epileptiform discharges observed. A whole-body PET/CT scan revealed a mild reduction in glucose metabolism in the left frontal and temporal lobes. Lumbar puncture showed normal cerebrospinal fluid pressure but elevated white blood cell count and protein level. The diagnosis of anti-mGluR1 encephalitis was confirmed by positive anti-mGluR antibody testing, specifically showing an anti-mGluR1-IgG titer of 1:1000 in the cerebrospinal fluid and an anti-mGluR1-IgG1 titer of 10,000 in the serum. The patient did not respond to first-line therapy with corticosteroids. During disease progression, innovative sequential therapy with efgartigimod, an FcRn antagonist, was followed by the anti-CD20 monoclonal antibody rituximab (RTX) to reduce the risk of profound B-cell depletion. The patient exhibited significant clinical improvement, evidenced by a reduction in the Modified Rankin Scale (mRS) score from 4 to 3 and a decrease in the Scale for Assessment of Ataxia (SARA) score from 40 to 13. Efgartigimod effectively cleared pathogenic IgG, whereas RTX induced rapid B-cell depletion to prevent disease recurrence, thereby demonstrating synergistic efficacy. At the 12-month follow-up, the patient's clinical symptoms continued to improve, with no observed relapse. This case report underscores the importance of early diagnosis and therapeutic intervention, highlighting the need to suspect this condition based on its characteristic clinical presentation.