BACKGROUND AND AIMSBickerstaff brainstem encephalitis (BBE) is a rare disease included in the Clinical and immunological spectrum of the Miller-Fisher syndrome. It has an autoimmune etiology, usually related to anti-GQ1b antibodies.METHODWe report a case of BBE mimicking acute stroke as clinical debut that received early immunosuppressor treatment.RESULTS:24-year-old male patient with relevant medical history of sporadic ketamine consumption. He is attended at the emergency department with woke-up symptomps of lower facial paralysis and dysarthria. Neurological examination showed a complex ophthalmoplegia with multiple bilateral cranial nerve palsies, multidirectional nystagmus, dysarthria and right dysmetria. Myotatic reflexes were globally increased. A multiparametric-CT was performed without showing any alteration. CSF tests had normal values. Urgent MRI was performed, demonstrating an extensive medullary hyperintensity. The patient was admitted at the ICU after clinical worsening with poorly secretion management and compromised airway. Orotracheal intubation was performed. Aciclovir i.v and a 5-day immunoglobulin treatment was indicated due to the presumptive autoimmune etiology, which was eventually confirmed by positive CSF antiGM2-IgM antibodies. The patient progressively recovered during the next 29 days of hospitalization admission. Electroneurography/electromyography did not show any alteration. The patient was clinically asymptomatic at 3 months.CONCLUSION:This case report emphasises the importance of an early diagnosis of stroke mimics, which can lead to a rapid clinical worsening. Immunosuppressor treatment and compromised airway awareness is essential if BE is suspected. Our patient had a good clinical outcome in spite of showing extensive medullary involvement. The current evidence of BE mediated by antiGM2-antibody is scarce and dysimmune neuropathies have been more frequently reported.