In 2016, 24 US diplomats reported hearing loss, fogginess, vertigo, and problems with their vision after hearing chirping sounds.
The first public reports of the problems, released in August, sparked fears of a sonic weapon attack in Havana.
This theory, however, was rebuked under the knowledge that inaudible sound waves are not able to physically cause concussion symptoms.
Nonetheless, the US withdrew more than half of its staff from Cuba and warned American tourists not to travel to the island as a health precaution in September.
The study’s authors wrote: “The unique circumstances of these patients and the consistency of the clinical manifestations raised concern for a novel mechanism of a possible acquired brain injury from a directional exposure of undetermined aetiology.”
If a weapon was involved experts have not been able to say what would have been used.
Seth Horowitz, a neuroscientist, said: “There isn’t an acoustic phenomenon in the world that would cause those type of symptoms.”
Although unwarranted sonic fears have once again been lambasted.
With help from a team of specialists at the University of Pennsylvania, the new medical report has confirmed a cluster of concussion symptoms among 21 of the 24 embassy personnel affected.
Their research has found scant evidence of the brain damage described in early reports of the outbreak, but has yet to identify the trigger for the mysterious health issues.
The study’s lead author, Randel Swanson, said: “If you didn’t know their history, they would look to you like other concussion patients.
“It’s premature to call this a syndrome, but what we have is a constellation of symptoms.”
Unlike typical concussions, many of the diplomats reported problems that began or worsened weeks after their initial injuries.
Swanson added: “By and large, concussions get better or stay the same, they don’t do new things.”
With 18 out of the 21 patients experiencing symptoms so severe they required rehabilitative treatment, all but two reported their symptoms started with hearing a high-pitched sound in Havana.
MRI images from the report show only three of the diplomats to have suffered from white matter changes unusual for their ages and none of the changes appeared to be tied to symptoms.
Without images of the diplomats’ brains before their time in Havana, the doctors are unable to confirm whether the changes are relevant.
In a commentary accompanying the study neurologists Christopher Muth and Steven Lewis wrote: “Nonetheless, the similarities among the 21 cases merit consideration of a common medical, environmental or psychological event as the potential cause.”
The authors found no sign of poisoning, tropical viruses (a populist explanation linked to neurological illness), or head trauma.
They also ruled out a mass psychology event – or mass psychogenic illness (MPI) – as recently suggested by the Cuban government.