Inclusion Criteria:  Acute CO poisoning  Exclusion Criteria:  Age <19 years Patients with a history of ophthalmic surgery which might have affected the PLR Patients with baseline cognitive deficit Refusal to enroll in this study Discharge from the ED or transfer to another hospital within 24 hours Expired in the ED Co-ingestion of drugs, such as hypnotics, that may affect the PLR No follow-up for the neurocognitive outcome