A 75-year-old woman was admitted to the emergency department with symptoms of confusion, gait disturbances, and mutism, which occurred three weeks after she experienced acute carbon monoxide intoxication. Initially, the medical team suspected that she had infectious meningoencephalitis, a serious condition that involves inflammation of the brain and its surrounding tissues. As a result, she was treated with acyclovir and antibiotics in high doses, which are commonly used to treat meningoencephalitis. However, despite receiving this treatment in an intensive care setting, the patient's condition did not improve. This lack of improvement led the medical team to re-examine her case history and imaging studies, which ultimately revealed a different diagnosis. The patient was found to be suffering from post-interval syndrome, a rare and poorly understood complication that can occur after carbon monoxide intoxication. This condition is characterized by the development of neuropsychiatric disorders, such as confusion and mutism, after an initial period of improvement following the intoxication. In this case, the patient had experienced a three-week asymptomatic period before her symptoms appeared. Repeated magnetic resonance imaging (MRI) scans, combined with her clinical symptoms, helped to confirm the diagnosis of post-interval syndrome. Unfortunately, the patient's prognosis was poor, and she did not show any significant neurological improvement after six months of follow-up. The diagnosis of post-interval syndrome can be challenging, as it may be confused with other conditions, such as cytotoxic edema, in the early stages. However, repeated MRI scans performed more than a month after the onset of symptoms can help to distinguish post-interval syndrome from other conditions. Currently, there is no effective treatment available for post-interval syndrome, highlighting the need for greater awareness and research into this condition. Link: https://pubmed.ncbi.nlm.nih.gov/40051738/