Were U.S. Diplomats Attacked in Cuba? Brain Study Deepens Mystery.

“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,” that report concluded.

In the new study, the research team focused on those 21 patients and 19 subsequent ones, examining brain areas known to support hearing, balance and motor control. It also measured each individual’s volume of gray matter, the overall population of neurons, and of white matter, the connective tissue between the neurons. Gray matter makes up the bulk of the brain’s distinct organs, specialized to manage functions like vision, hearing and movement. White matter comprises the wiring that connects cells and organs into wider circuits. Sharp deficits in either can compromise brain function.

Those measures were then compared to an identical battery of brain images from 48 healthy adults representing the same mix of ages, genders and educational attainment.

On average, the diplomats had a lower volume of white matter than individuals in the control group. They also showed clear differences in the volume, connectivity and tissue properties of the cerebellum, which is involved in maintaining balance, and lower connectivity among neurons in the auditory and visual-spatial areas of the brain. The analysis found no difference between the groups in so-called executive-control networks, which are involved in thinking and planning.

The researchers could not say for certain what these differences mean, except that they are consistent with the symptoms the diplomats have reported. The overall pattern was unlike anything found in studies of people with traumatic brain injuries, multiple sclerosis, stroke damage or other neural disorders.

“If you look at the totality of these findings — an increase in this, a decrease in that — and combine them all, you have a presentation that is very unique to this cohort,” Dr. Verma said.

Whether that presentation reflects a physical injury from an enigmatic weapon, or something else, is still far from clear, the authors acknowledged. Reaching a firm conclusion would likely require many more new cases, a situation that nobody is eager to see.