In Massachusetts, unique occurrences of sudden, onset amnesia and acute, complete, and bilateral ischemia of the hippocampus were identified in a number of individuals. Over the last 4 years, 14 cases of this unusual amnestic syndrome came to light, and many of the patients affected were linked to previous or current opioid use.
According to a report in the CDC’s Morbidity and Mortality Weekly Report (MMWR), 13 of the 14 affected patients tested positive for either opioids, cocaine, or benzodiazepines after undergoing a routine clinical toxicology screening.
CDC researchers noted that no readily apparent evidence for the etiology of the syndrome was available, despite the patients’ exhibiting abnormal MRI brain scan results. An investigation prompted researchers to consider the role of substance use as a possible trigger.  
In November 2015, 4 patients with the syndrome were evaluated. Three out of the 4 patients tested positive for opiates on initial toxicology screening, and the fourth had a reported history of heroin use. The Massachusetts Department of Public Health requested health professionals to report similar cases for a medical review, which included patients identified with new onset amnesia without evidence of cause. An analysis of records from patients who met the case definition included a review of demographic, behavioral, and clinical data, including information related to substance use.
Thirteen of the 14 patients who demonstrated sudden onset amnesia had a history of substance use disorder, and the remaining patients tested positive for opiates and cocaine at evaluation. The 1 patient who tested positive for cocaine, also tested positive for opiates, amphetamines, and benzodiazepines. Marijuana, LSD, MDMA, mushrooms, and PCP were among other substances reported, as well as gabapentin. Six patients had a history of at least 1 vascular disease risk, such as hypertension dyslipidemia, diabetes, and sleep apnea.
The researchers concluded that the cluster of amnestic syndrome with bilateral complete hippocampal ischemia with no evidenced etiology is uncommon, especially due to the temporospatial clustering, young patient age, and history of extensive substance use. The report offers cardiopulmonary, cerebrovascular, or other mechanisms as possible underlying causes for some of the findings, although, they noted, further investigation would be needed to determine a correlation to substance use.
Barash JA, Somerville N, DeMaria A. Cluster of an unusual amnestic syndrome – Massachusetts, 2012-2016. MMWR. 2017; 66(3); 76-79. DOI: .