Thiamine Deficiency After Bariatric Surgery: Early Neurological Complications and Nutritional Monitoring.

Thiamine (vitamin B1) deficiency is a recognized complication after bariatric surgery (BS) that can lead to severe neurological manifestations. Early identification is critical to prevent irreversible complications. We report a 23‑year‑old woman who presented two months after BS-Roux-en-Y gastric bypass (RYGB) with diplopia and limitation of ocular abduction in the right eye, consistent with isolated sixth cranial nerve palsy. She had no history of head trauma, infection, or prior neurological disease. Cranial CT angiography and brain MRI excluded ischemic, hemorrhagic, vascular, or demyelinating lesions. Laboratory tests revealed thiamine deficiency (20 ng/mL) and folic acid deficiency (1.7 ng/mL). The patient was treated with thiamine 600 mg/day, folic acid 5 mg/day, and cyanocobalamin 1 mg/day, resulting in complete resolution of symptoms.