The study’s lead author, Dr. Douglas H. Smith, director of the Center for Brain Injury and Repair at the University of Pennsylvania, emphasized that there is much more to learn.

“This is a preliminary report,” he said. “We thought it was important to get it out from a public health standpoint.”

But, he added, the patients’ symptoms were notable. “Uniformly, everyone who saw these patients was absolutely convinced,” he said. “It looked like concussion pathology. Processing speed, inability to remember — those are such classic symptoms we see in concussion.”

“We all believe this is a real syndrome,” Dr. Smith added. “This is concussion without blunt head trauma.”

Like concussion patients, some of the diplomats improved on their own, while others recovered only after therapy to regain balance or cognition. “Many went from being miserable to returning to work,” Dr. Smith said.

Dr. Smith and his colleagues do not think audible sound caused the injuries, as sound in an audible range does not damage the brain. Perhaps, they speculated, a device that produced another sort of harmful energy also produced an audible sound. Low-frequency infrasound, high-frequency ultrasound and microwaves have all been shown to damage the brain, the researchers noted.

One drawback to the report is that there was no comparison group, said Karen Postal, immediate past president of the American Academy of Clinical Neuropsychology.