Normal Early Investigations Do Not Exclude Bickerstaff Brainstem Encephalitis: Highlighting Diagnostic and Treatment Pitfalls

Bickerstaff brainstem encephalitis (BBE) is a rare immune‐mediated disorder characterized by ophthalmoplegia, cerebellar ataxia, and impaired consciousness. It is now recognized as a variant of Guillain–Barré syndrome (GBS), sharing common pathogenic mechanisms with Miller Fisher syndrome. However, early diagnosis remains challenging, particularly when initial investigations are normal. We report a 20‐year‐old previously healthy woman presenting with rapidly progressive ophthalmoplegia, bulbar dysfunction, flaccid quadriparesis, areflexia, and impaired consciousness following a febrile upper respiratory illness. Early cerebrospinal fluid analysis and brain MRI were normal. Anti‐GQ1b antibodies were positive, and nerve conduction studies showed isolated proximal conduction abnormalities with absent F‐waves. The patient developed respiratory failure requiring mechanical ventilation and was treated with an extended course of plasma exchange, resulting in near‐complete recovery. This case highlights a critical diagnostic pitfall: BBE, a variant of GBS, can present with a life‐threatening brainstem syndrome despite entirely normal early investigations. We emphasize the value of recognizing the paradoxical combination of areflexia and extensor plantar responses and the use of F‐wave studies as an early electrophysiological foothold when standard studies are unrevealing. Furthermore, it demonstrates that even in catastrophic, ventilator‐dependent presentations, an extended course of plasma exchange can lead to a favorable outcome, suggesting a potential therapeutic strategy for severe presentations of BBE.