Carbon Monoxide Poisoning Presenting With Transient Visual Field Defects

Carbon monoxide (CO) toxicity typically presents with drowsiness, headache, and neurological disturbances. Visual field defects involving the optic chiasma are very rarely seen. A case of a 31-year-old female, with a history of heavy smoking and learning disability, presented to the acute medical ward with 3-4 days of frontal headache, neck pain, room spinning sensation, and an episode of loss of consciousness. Neurological examination revealed a bitemporal hemianopia that progressed to a temporal hemianopia over the next day and completely resolved within 48 hours. Blood gas analysis showed a high carboxyhaemoglobin (COHb) level of 9.7%, with normal pH and lactate. The CT brain showed no acute abnormality. Ophthalmology confirmed visual field defects with no diplopia. Neurology advised an MRI to rule out pituitary apoplexy, and the image showed no structural abnormality. The transient visual field changes were consistent with possible CO toxicity-induced hypoxia affecting the optic chiasma or its vascular supply. Symptoms quickly resolved after oxygen therapy and abstinence from smoking. This study highlights the diagnostic challenge of identifying CO toxicity in heavy smokers and their atypical clinical manifestations. It emphasises the need to consider toxic exposure when presenting with fluctuating visual field defects and particularly, when their neuro-imagining is normal. Environmental sources of CO exposure were assessed, and no clear source was identified, with significant tobacco use considered a potential contributing factor.