Military pilots who fly at high altitudes above 18,000 feet are typically exposed to a type of illness called "decompression sickness" that results from quick changes in environmental pressure. While relatively manageable, "the risk for decompression sickness among Air Force pilots has tripled from 2006, probably due to more frequent and longer periods of exposure for pilots,” Stephen McGuire, MD, with the University of Texas in San Antonio and the U.S. Air Force School of Aerospace Medicine, said in a press release. “To date however, we have been unable to demonstrate any permanent clinical neurocognitive or memory decline."

However, when researchers looked into this change, they found that pilots who fly at such high altitudes, whether they experience the symptoms of decompression sickness or not, may have nearly three times the number of brain lesions as non-pilots — a much more dangerous issue than decompression sickness. The study was published in the most recent issue of Neurology.

Decompression Effects on the Body

When the body rapidly changes from a high-pressure to a lower-pressure environment, such as may occur when a person flies in an airplane or dives deep into the ocean, decompression sickness can occur. With a quick ascent or descent, the small amount of gases in the air (mostly nitrogen) enters the bloodstream and forms bubbles in the tissues, including those of the brain.

If a person rises more slowly, though, the gases enter the bloodstream more gradually and then are expelled by the lungs. The body, in other words, can adjust at its own pace. Physical symptoms of decompression sickness include muscle cramps and joint pain; in divers this is commonly called “the bends.” Mental symptoms of decompression sickness include slowed thought processes, confusion, unresponsiveness, and permanent memory loss.

Although immediate treatment with 100 percent oxygen (often delivered in a high-pressure chamber) alleviates the symptoms and provides a “cure,” scientists, as noted by Wikipedia, have not been certain whether longer-lasting effects and damage may occur.

Brain Imaging Results: Uncovering Brain Lesions

For the recent study of pilots, a total of 102 U-2 United States Air Force pilots and 91 non-pilots between the ages of 26 and 50 underwent MRI brain scans. The U-2 is an aircraft developed by Lockheed Martin during the cold war, but used today by the United States Air Force as an aerial eavesdropping device, in the search for signs of IEDs and mines in Iraq and Afghanistan. Researchers matched the groups of U-2 pilots and non-pilots for age, education, and health factors and then studied their scans, which measured the amount of tiny brain lesions or white matter hyperintensities.

An unrelated study conducted in 2010 found that white matter hyperintensities were associated with an increased risk of stroke, dementia, and death.

In the current study, the researchers found that pilots had nearly four times the volume of brain lesions as non-pilots, plus they had three times the number of brain lesions as non-pilots. These results held up whether or not the pilots had a history of symptoms of decompression sickness. The research also found that while the lesions in non-pilots were mainly found in the frontal white matter, as occurs in normal aging, the lesions among the pilots were evenly spread throughout the brain — an abnormal distribution.

“This is consistent with our hypothesized pattern of damage produced by interaction between microemboli and cerebral tissue, leading to thrombosis, coagulation, inflammation, and/or activation of innate immune response,” wrote the authors in their study.

The study was supported by the U.S. Air Force Surgeon General. Further research will be necessary to clarify underlying causes. Still, the results are “valuable in assessing risk for occupations that include high-altitude mountain climbing, deep sea diving and high-altitude flying," McGuire stated in a press release.

Sources: McGuire S, Sherman P, Profenna L, Grogan P, Sladky J, Brown A, et al. White matter hyperintensities on MRI in high-altitude U-2 pilots. Neurology. 2013.

Debette S, Markus HS. The clinical importance of white matter hyperintensities on brain magnetic resonance imaging: systematic review and meta-analysis. BMJ. 2010.