More than a dozen people in Massachusetts suddenly developed severe amnesia, but there was no clear cause of their memory loss, leaving researchers puzzled about what exactly could have been behind these cases, according to a new report.

In the report, researchers describe a cluster of 14 cases involving people who experienced sudden amnesia, and who were treated in Massachusetts from 2012 to 2016. All of the patients were relatively young (19 to 52 years old), and all had either tested positive for drugs or had a history of substance abuse, according to the report.

The patients showed "striking anterograde amnesia," according to the report, from researchers at the Lahey Hospital & Medical Center in Burlington, Massachusetts, and the Massachusetts Department of Public Health (MDPH). People with this type of amnesia have trouble forming new memories, and they often cannot recall events from the immediate past, such as something that has just happened to them. [27 Oddest Medical Cases]

In nine of the cases, the patients were unconscious at the time they were brought to the hospital, and they experienced amnesia when they regained consciousness. In the five other cases, family members or friends noticed that the individuals were experiencing severe memory loss, and they brought them to the emergency room.

Twelve of the patients had a history of using opioid drugs, including prescription painkillers or heroin. Many of the patients had also used other drugs, including marijuana, cocaine and amphetamines. These drugs have many negative effects on people's health, but they typically have not been linked with the development of anterograde amnesia.

Notably, the brain scans of all of the patients revealed an unusual finding: MRI tests showed significantly reduced blood flow to a part of the brain called the hippocampus, which is important for memory formation. Humans have two hippocampi, one on either side of their brain, and the patients in the report had reduced blood flow to both hippocampi. But the researchers could not find a clear cause of this problem.

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Sudden amnesia that's tied to reduced blood flow to both hippocampi is rare, the researchers said. A few similar cases have been reported in the past, but these were stand-alone cases rather than clusters of cases, the researchers noted. In some of those earlier cases, the sudden amnesia was tied to exposure to a toxic substance, such as carbon monoxide, the report said.

Investigation of the current cases is ongoing, and health authorities need to continue to watch for more cases, to determine whether this new cluster "represents an emerging syndrome related to substance use or other causes," such as exposure to a toxic substance, the researchers wrote in the Jan. 27 issue of the journal Morbidity and Mortality Weekly Report, which is published by the Centers for Disease Control and Prevention. [10 Things You Didn't Know About the Brain]

The cluster was first discovered in November 2015, when a neurologist in the Boston area reported four cases of unusual amnesia he had seen during the last three years. The MDPH conducted a subsequent search for similar cases and discovered the 10 additional ones.

Information on the long-term outcome of the cases was available for just four patients. Of these, one person's memory problems were resolved after five months, but two others continued to experience cognitive problems more than a year later. One of the patients still had severe short-term memory problems after eight weeks, and later died from cardiac arrest, the report said.

Doctors should consider performing an MRI and screening for drugs in all adult patients who have sudden-onset amnesia, the researchers said. Advanced laboratory testing, including testing for substances that were not assessed in the current report, might clarify why these cases were linked with drug use, they said.

Original article on Live Science.

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