Japanese Society of Internal Medicine Acta Medica Okayama 0918-2918 64 12 2025 Subacute Upper Motor Neuron Dysfunction Possibly Associated with the Anti-GM1 Autoantibody 1900 1905 EN So Okubo Department of Neurology, Graduate School of Medicine, The University of Tokyo Meiko Maeda Department of Neurology, Graduate School of Medicine, The University of Tokyo Kazuto Katsuse Department of Neurology, Graduate School of Medicine, The University of Tokyo Hiroyuki Ishiura Department of Neurology, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences Yuichiro Shirota Department of Neurology, Graduate School of Medicine, The University of Tokyo Masashi Hamada Department of Neurology, Graduate School of Medicine, The University of Tokyo Wataru Satake Department of Neurology, Graduate School of Medicine, The University of Tokyo Tatsushi Toda Department of Neurology, Graduate School of Medicine, The University of Tokyo Anti-GM1 antibodies are associated with Guillain-Barré syndrome (GBS), primarily peripheral neuropathy. However, there are cases of anti-GM1 IgG antibody-positive GBS with upper motor neuron (UMN) signs. We herein report a case of gastrointestinal infection followed by subacute gait disturbance with predominant signs of UMN on a neurological examination. The serum and cerebrospinal fluid tests were positive for anti-GM1 and anti-asialo-GM1 IgG antibodies. An electrophysiological evaluation revealed normal nerve conduction and prolonged central motor conduction times. No magnetic resonance imaging abnormalities were observed. The symptoms improved with treatment, which was accompanied by decreased antibody titers. This case highlights the fact that anti-GM1 IgG-associated disorders may present with predominant UMN involvement. No potential conflict of interest relevant to this article was reported.