Chicago: Advanced brain scans found perplexing differences in American diplomats who say they developed concussion-like symptoms after working in Cuba, a finding that only heightens the mystery of what may have happened to them, a new study says.

Extensive imaging tests showed the workers had less white matter than a comparison group of healthy people and other structural differences, researchers said.

A classic convertible drives past the United States embassy in Havana, Cuba. Credit:AP

While they had expected the cerebellum, near the brain stem, to be affected given the workers' reported symptoms - balance problems, sleep and thinking difficulties, headaches and other complaints - they found unique patterns in tissue connecting brain regions.

Ragini Verma, a University of Pennsylvania brain imaging specialist and the lead author, said the patterns were unlike anything she's seen from brain diseases or injuries.