In the study, the scientists observed MRI scans of nine world-class mountain climbers, who had been climbing for at least 10 years, before and after expeditions to Mount Everest (8,848 meters) and K2 (8,611 meters) without an oxygen supply.
They compared their MRI brain scans with 19 age and sex matched healthy control subjects.
The researchers carefully checked both the climbers and controls to do away with the presence of any major systemic, psychiatric or neurological illnesses. None of the control group subjects had any history of high-altitude exposure over 3,000 meters.
When the baseline measurements of the climbers were compared with the control group, the climbers showed a reduction in both the density and volume of white matter in the left pyramidal tract, near the primary and supplementary motor cortex.
And when the researchers compared the before and after scans for the climbers, they also found a reduction in the density and volume of grey matter in the left angular gyrus.
"The aim of our study was to measure the quantitative loss of white and grey matter, using voxel-based morphometry, which takes spatial, unbiased MRI measurements independent of the operator" explained lead author Dr Margherita Di Paola from the Neuroimaging Laboratory at the IRCCS Fondazione Santa Lucia in Rome.
She added: "The scans were then assessed by two experienced observers who were unaware of whether the scans belonged to the climbers or control group."
All the climbers taking part in the study were male, in the age group of 31-52 years, with average age of just under 38, and were used to climbing to altitudes of at least 4,000 meters several times a year.
The first scans were taken eight weeks before the expedition began and the second set eight weeks after the climbers returned.
"Despite the loss of grey and white matter, the climbers in our study did not suffer any significant neuropsychological changes after the expedition" said Dr Di Paola.
She added: "Some of the subjects did show abnormal scores on the neuropsychological tests, but in these cases there was no significant difference between the baseline and follow up results. This suggests that there were no significant changes as a result of a single expedition.
"As they had been carefully checked for any pathological conditions that could cause these abnormal scores, we conclude that these test results are most likely to be due to progressive, subtle, brain insults caused by repeated high-altitude exposure."
It was found that the cognitive abilities most likely to be affected were the climbers executive function and memory. The researchers observed that six of the nine climbers had lower than average scores on the Digit Symbol test, which measures executive functions such as the ability to anticipate outcomes and adapt to changing situations.
Four scored lower than average on the Block Design test, which measures visuo-motor functions, and three out of nine scored lower than average on the Prose Memory test (immediate recall) and on the Rey's Figure test (delayed recall), which measure the verbal and visuo-spatial memory respectively.
The study is published in the recent issue of the European Journal of Neurology.