The NIH conducted two studies over nearly five years, which were published Monday in the Journal of the American Medical Association.
Researchers used advanced imaging techniques, including MRIs, to examine federal employees stationed in Havana and other parts of the world, who had complained of anomalous health incidents -- or AHIs -- involving odd sounds and head pressure followed by dizziness, headache and cognitive dysfunction.
"We didn't see differences in the structure of the brain or even in the functional connectivity of the brain, said Dr. Leighton Chan, leading author and acting chief scientific officer at NIH Clinical Center. "Our goal was to conduct thorough, objective and reproducible evaluations to see if we could identify structural brain or biological differences in people who reported AHIs."
"While we did not identify significant differences in participants with AHIs, it's important to acknowledge that these symptoms are very real, cause significant disruption in the lives of those affected and can be quite prolonged, disabling and difficult to treat," Chan added.
In one study, NIH researchers examined the brains of those believed to have Havana syndrome and those of a healthy comparison group. Participants underwent a battery of clinical, auditory, balance, visual, neuropsychological and blood biomarkers testing. They also received several MRI scans to look at the volume, structure and function of the brain. The test results show no significant differences and no evidence of brain injury, according to researchers.
In a second study, researchers tested 86 federal workers, and adult family members who had reported Havana syndrome, and 30 people with similar jobs who had experienced no symptoms. Again, the two groups were the same.
While it is still undetermined what caused the symptoms, the Office of the Director of National Intelligence said last year that the health incidents were "very unlikely" to have been caused by a foreign adversary. Instead, Director of National Intelligence Avril Haines blamed "pre-existing conditions, conventional illnesses and environmental factors."
People for the Ethical Treatment of Animals also weighed in last year, blasting the Pentagon for weapon-wounding tests on dogs, cats, marine animals, ferrets and monkeys to determine the health of effects of the mysterious "Havana syndrome" on humans.
The first symptoms, which were consistent with head trauma, were reported in 2016 by a number of diplomats who were stationed in Havana. Since then, cases have been reported from around the world, including in Bogota, Colombia and Vienna, Austria.
While researchers noted there were limitations to the studies, "a lack of evidence for an MRI-detectable difference between individuals with AHIs and controls does not exclude that an adverse event impacting the brain occurred at the time of the AHI," said Dr. Carlo Pierpaoli, lead author and chief of the Laboratory on Quantitative Medical Imaging at the National Institute of Biomedical Imaging and Bioengineering.
"It is possible that individuals with an AHI may be experiencing the results of an event that led to their symptoms, but the injury did not produce the long-term neuroimaging changes that are typically observed after severe trauma or stroke," Pierpaoli said.
"We hope these results will alleviate concerns about AHI being associated with severe neurodegenerative changes in the brain."
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