General view of the US Embassy in Cuba after the US government pulled more than half of its diplomatic personnel out of Cuba who suffered health problems due to an alleged sonic attack on September 29, 2017 in Havana, Cuba.

Doctors investigating "sonic attacks" against U.S. embassy staff in August have concluded that government personnel serving in Cuba appear to have "widespread" neurological damage.

Doctors from the University of Pennsylvania's Perelman School of Medicine found in a preliminary study that "these individuals appeared to have sustained injury to widespread brain networks without an associated history of head trauma."

Eighteen out of 21 individuals evaluated by the doctors reported hearing a "novel, localized sound" in their homes and hotel rooms in Havana. The sound was likened to "buzzing, grinding metal, piercing squeals, and humming" and lasted anywhere from 10-second pulses to more than 30 minutes long. Affected individuals perceived this sound as "directional, intensely loud, and with pure and sustained tonality."

Patients also associated the sound with a "pressure-like sensory stimuli" described as "air 'baffling' inside a moving car with the windows partially rolled down."

Both the sound and sensory stimuli seemed to stem from a distinct direction. In fact, after changing locations, patients said the sensation disappeared and associated symptoms went away.

All but one of the patients "reported an immediate onset of neurological symptoms" when exposed to the sound, according to the report.

The cause of the symptoms has not yet been identified, and it's still unclear how the noises are related to the patients' reported symptoms.

"It appears that we have identified a new syndrome that may have important public health implications," said Dr. Douglas Smith, the study's senior author and director of Penn's Center for Brain Injury and Repair, in a statement.

The patients' symptoms lasted for days and sometimes weeks after exposure and included, but weren't limited to, memory problems, mental fogginess, impaired concentration, irritability, sadness and nervousness. These symptoms were made worse by physical exertion and cardiovascular exercise.

Doctors said they didn't uncover any evidence of patients feigning their illnesses.

The State Department, which provided background information and patient referrals to the study's authors, claims its staff was specifically targeted while in Cuba. "The investigation into who or what is causing these attacks is ongoing," a State Department rep told CNBC.

Last September, the department withdrew nonemergency personnel and all family members from the U.S. embassy in Havana. In October, the State Department ordered the removal of 15 Cuban officials from its embassy in Washington, D.C., citing "Cuba's failure to take appropriate steps to protect our diplomats in accordance with its obligations under the Vienna Convention."



The State Department also issued a travel warning advising U.S. citizens to avoid travel to Cuba. Although there were no reports of private U.S. citizens being affected, Secretary of State Rex Tillerson said the attacks are known to have occurred in diplomatic residences and hotels frequented by American citizens.

The Cuban government has repeatedly denied responsibility for the attacks in the past year, claiming that U.S. authorities did not provide enough information in order to investigate the allegations.

The Cuban government also said the nation "rigorously" complies with all obligations under the Vienna Convention and that Cuba is "universally considered as a safe destination for both visitors and foreign diplomats, including Americans."

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