According to the study, published in the medical journal JAMA, 21 workers sought medical attention beginning in late 2016 after suspected exposure to "auditory and sensory phenomena in their homes or hotel rooms."
These findings have left doctors wondering whether this pattern of symptoms can occur in a previously unseen way, without head trauma.
"If you took any one of these patients and put them into a brain injury clinic, and you didn't know their background, you would think that they had a traumatic brain injury from being in a car accident or a blast in the military," Dr. Randel Swanson
, one of the paper's authors and a specialist in brain injury rehabilitation at the University of Pennsylvania, said in a separate report
published with the study. "It's like a concussion without a concussion."
Of the 10 men and 11 women in the study, many reported hearing "intensely loud" sounds coming from a specific direction, which they described as "buzzing," "grinding metal," "piercing squeals" and "humming."
"The sounds were often associated with pressurelike or vibratory sensory stimuli," according to the report. "The sensory stimuli were likened to air 'baffling' inside a moving car with the windows partially rolled down."
One patient said they heard two short 10-second pulses, while other patients said they could hear the sound for more than 30 minutes, the report said.
A majority of the patients reported problems with memory, concentration, balance, eyesight, hearing, sleeping or headaches that lasted more than three months. Three people eventually needed hearing aids for moderate to severe hearing loss, and others had ringing or pressure in their ears.
Many reported feeling "mentally foggy" or "slowed" for months, the authors said. Some reported irritability and nervousness, with two meeting criteria for post-traumatic stress disorder. Poorer job performance was also observed.
More than half needed to be prescribed medication in order to sleep or to deal with headaches. Many were, at least for a period of time, unable to return to work.
Certain symptoms that some patients had -- such as pain and ringing only in one ear -- are not typically seen in a concussion, the study noted. The report also pointed out that while concussion patients often make a quick and full recovery, these patients experienced symptoms for months.
It's possible those were the only severe cases, the authors said, noting that other individuals may have been affected and simply didn't know it, either because they recovered fully or had only minor symptoms.
Conventional MRIs didn't reveal anything conclusive, although the doctors said they are continuing to conduct advanced scans to look for clues.
"In many cases of severe or moderate or mild brain injury, you won't see anything on a conventional MRI," said Dr. S. Andrew Josephson
, professor and chairman of the neurology department at the University of California, San Francisco. Josephson is the editor of a related journal, JAMA Neurology, but was not involved in this study or evaluating the patients.
"Part of the reason why this story is interesting is because you can't point to a spot on the MRI and say, hey, there's the injury," Josephson said.
The noise itself is unlikely to have caused the symptoms directly, the authors said, noting that audible sound "is not known to cause persistent injury to the central nervous system."
"We actually don't think it was the audible sound that was the problem," said Dr. Douglas Smith
, another of the study's authors.
"We think the audible sound was a consequence of the exposure," added Smith, who is the director of the University of Pennsylvania's Center for Brain Injury and Repair
The doctors note that, because the patients were initially seen elsewhere, at the University of Miami, not all of them had a complete examination.
The report found "audible or sensory phenomena" were from an unknown source but came from a specified direction. In conclusion, the authors say their study raises "concern about a new mechanism for possible acquired brain injury from an exposure of unknown origin."
This is in line with what the State Department and federal investigators told a Senate Foreign Relations subcommittee last month. They testified
that they were unable to determine the source or cause of the ailments, stating only that they "were most likely related to trauma from a non-natural source."
In the past, the Cuban government has claimed ignorance when it comes to the cause of the incidents, and more recently it has aimed to discredit the accounts of affected government workers. These efforts have been met with doubt from Senators Marco Rubio and Bob Menendez, both on the Senate Foreign Relations subcommittee.
The idea that such an attack would go unnoticed by Cuban security officials in heavily surveilled Havana, said Rubio, was "outside the realm of reasonable -- it's ridiculous."