Case Report: Rare brainstem involvement in anti-metabotropic glutamate receptor 1 encephalitis.

Anti-metabotropic glutamate receptor 1 (mGluR1) encephalitis is a rare form of autoimmune encephalitis that typically presents with cerebellar dysfunction. Predominant brainstem involvement has seldom been reported. We report an 87-year-old man who presented with a subacute onset of fever, dysarthria, limb weakness, and altered mental status. Comprehensive diagnostic evaluation revealed anti-mGluR1 antibodies in both serum and cerebrospinal fluid using a cell-based assay, while cerebrospinal fluid next-generation sequencing (NGS) was negative for infectious pathogens. Brain MRI demonstrated extensive T2-weighted and fluid-attenuated inversion recovery (T2/FLAIR) hyperintensity involving the midbrain, pons, and medulla, without diffusion restriction. These findings support the possibility that anti-mGluR1 encephalitis may, in rare instances, present with predominant brainstem involvement. Testing for anti-mGluR1 antibodies may be considered in patients with otherwise unexplained subacute inflammatory brainstem syndromes.