This study investigated hyperreflexia in Guillain-Barré syndrome (GBS) and its relation to acute motor axonal neuropathy (AMAN) and anti-GM1 antibodies. The study found that some GBS patients with the AMAN pattern, anti-GM1 antibodies, and milder weakness developed hyperreflexia. Specifically, 33% of AMAN patients showed hyperreflexia compared to 12% of acute inflammatory demyelinating polyneuropathy patients. Hyperreflexia was associated with increased motor neuron excitability. The mechanism is unknown but may involve dysfunction of spinal inhibitory interneurons or upper motor neurons due to anti-GM1 antibodies. Hyperreflexia indicates less severe motor ax