Complete bilateral hippocampal ischemia is highly suggestive of toxic exposure. Acute anterograde amnesia with the use of opioids is an emerging clinical entity. We present such a case of acute bilateral hippocampal injury in a patient unable to form any new memories or provide any relevant history.

Case Report: A 49-year-old female with past medical history of tobacco, polypharmacy, and substance use disorder presented to the hospital after she was found down for an unknown period of time on her birthday. On neurological examination, she is found to have severe anterograde amnesia with an inability to form new memories. Computed tomography (CT) head showed bilateral globus pallidus hypoattenuation. Diffusion-weighted magnetic resonance imaging (MRI) brain was consistent with acute infarction of the bilateral globus pallidus along with complete bilateral hippocampus. Initial toxicology screen was negative. Expanded toxicology screen revealed elevated serum levels of Fentanyl (2.0 ng/ml) and Norfentanyl (0.6 ng/ml). Patient required cognitive therapy for a few weeks followed by gradual symptom improvement.

Patients with acute amnesia should be evaluated with brain MRI as well as expanded urine and serum drug screen. In our case, brain MRI showed bilateral hippocampal ischemic injury consistent with the clinical finding of anterograde amnesia. The mechanism of brain injury is likely related to opioid-induced hypoxia following respiratory depression. Some illicit drugs, including specific opioids like fentanyl, are not easily detected on standard urine or drug screen.  The involvement of bilateral hippocampal ischemia should warrant a thorough investigation into recreational drug use or exposure to other toxins.