Higher Risk of Brain Lesions For High-Altitude AF Pilots

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By Brenda L. Mooney

SAN ANTONIO — High-flying U-2 pilots may be paying a neurological price for their service.

The elite Air Force pilots flying at high altitudes increase their risks for developing brain lesions, according to a new study, although no permanent neurocognitive defects or memory decline have been documented at this point.

The study, led by researchers from the U.S. Air Force School of Aerospace Medicine and the University of Texas in San Antonio, was published recently in the journal Neurology.1

For the study, 102 U-2 U.S. Air Force pilots between the ages of 26 and 50 underwent magnetic resonance imaging (MRI) brain scans. The pilots, who don pressurized suits to fly on missions around 70,000 feet, then were matched by age, education and health status with 91 non-pilots whose brains also were MRI-scanned.

The scans measured the amount of white matter hyperintensities (WMH), or tiny brain lesions associated with memory decline in other neurological diseases. Researchers found that U-2 pilots demonstrated an increase in volume (394%; p = 0.004) and number (295%; p < 0.001) of WMH. The lesions also were more uniformly distributed throughout the brains of the U-2 pilots compared with mainly frontal distribution in the control participants, they added.

“Pilots who fly at altitudes above 18,000 feet are at risk for decompression sickness, a condition where gas or atmospheric pressure reaches lower levels than those within body tissues and forms bubbles,” explained lead author Stephen McGuire, MD, of the U.S. Air Force School of Aerospace Medicine and the University of Texas in San Antonio. “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. To date, however, we have been unable to demonstrate any permanent clinical neurocognitive or memory decline.”

Slowed thought processes, confusion, unresponsiveness and permanent memory loss have been documented with decompression sickness. The greater volume and number of brain lesions in the pilots compared with the nonpilots did not vary based on a history of decompression sickness symptoms, however, according to the report.

Staff Sgt. Vanessa Jordan (left) 9th Physiological Support Squadron launch and recovery technician, uses a mirror to confirm placement of a U-2 pilots’ specialized glasses within his helmet as Staff Sgt. Heather Doyle and Senior Airman Meghan Mattingly assist, earlier this year at Beale Air Force Base, CA. U.S. Air Force photo by Airman 1st Class Drew Buchanan

Increase in Lesions

“Pilots with occupational exposure to hypobaria showed a significant increase in WMH lesion volume and number,” the authors wrote. “Unlike the healthy controls with predominantly WMH in the frontal white matter, WMH in pilots were more uniformly distributed throughout the brain. 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, although further studies will be necessary to clarify the pathologic mechanisms responsible.”

A study published last year in the journal Aviation, Space, and Environmental Medicine documented the frequency of decompression sickness (DCS) in U-2 pilots from 1994 through 2010.2

Researchers from the 9th Medical Group, Beale Air Force Base, found 73 documented cases, showing an annual increase in recent years.

“Between 1994 and 2005, the number of annual cases ranged from 0-5; between 2006 and 2010, the number of annual cases increased to 6-10. Additionally, there was a trend toward more severe (neurologic and pulmonary) cases between 2006 and 2010 with 22 cases compared to 10 cases the preceding 12 years,” the authors wrote.

They also noted that, from 2006-2010, an increase in the average annual flight hours per pilot to meet wartime operational needs temporarily correlated with the increase in number and severity of decompression sickness cases.

The risk per flight rose from 0.076% from 1994-2005 to 0.23% from 2006-2010.

The report said the most common presentations of U-2 decompression sickness were joint pain (59%), mainly involving large joints, and generalized neurologic symptoms (44%).

“DCS remains prevalent among U-2 pilots. An increase in number and severity of cases correlated temporally with increased operational tempo of the U-2 squadron. Changes in cockpit pressurization and limiting the length and frequency of hypobaric exposure may reduce future incidence,” that study concluded.

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