To report a case of subacute optic neuropathy caused by thiamine (vitamin B₁) deficiency.

A 44-year-old woman was referred to the ophthalmology department due to decreased vision which began 10 days prior to presentation. The patient history indicated that she had undergone chemotherapy for ovarian cancer and she had been dependent on total parenteral nutrition for 3 weeks due to nausea and vomiting. Her best corrected vision of the right and the left eyes were 0.15 and 0.2, respectively. Color vision was severely impaired in both eyes without retrobulbar pain. There was marginal blurring on the temporal side of the optic discs of both eyes. The optic nerves were unremarkable on orbital and brain magnetic resonance imaging (MRI). There was high signal intensities in both the mammillary body and periaqueductal gray matter on T2-weighted imaging. In addition, the patient exhibited ataxia along with short-term memory loss. She was diagnosed with Wernicke's encephalopathy. Thiamine was administrated based on the diagnosis, and after 2 days of administration, the patient's vision and neurologic symptoms began to improve. Two weeks later, the patient recovered her vision.

Thiamine deficiency may cause optic neuropathy and can be recovered with early thiamine supplementation. This condition could occur due to deficient oral thiamine administration. We should keep this point in mind and try to prevent or diagnose early.