This article requires an account. I posted it here as it is a rather interesting article concerning U-2 drivers.
High-Altitude Pilots at Increased Risk for Brain Lesions
Megan Brooks
Aug 22, 2013
High-flying U-2 pilots in the United States Air Force (USAF) have an abnormally high number and volume of white matter hyperintensity (WMH) brain lesions on magnetic resonance imaging (MRI), a study suggests.
"These are novel observations in the U-2 population," neurologist Stephen McGuire, MD, from the University of Texas in San Antonio and the USAF School of Aerospace Medicine, told Medscape Medical News.
"We have been unable to identify any clinical deficits in any of the U-2 pilots studied, and all of the U-2 pilots that were part of this study are still performing the extremely complex tasks associated with flying the U-2 plane," he noted. "What we do not know is the long-term implication of these findings, nor what the threshold might be for inciting clinical symptoms," Dr. McGuire added.
The study was published in the August 20 issue of Neurology.
More Missions Since 9-11
The U-2 is a high-altitude reconnaissance aircraft that maintains a cabin altitude of approximately 9000 m while operating above 21,000 m, the investigators note in their report. Decompression sickness (DCS), including central nervous system neurologic DCS (NDCS), is a known occupational risk from exposure to low ambient pressure (hypobaria) in high-altitude pilots.
The risk for DCS among U-2 pilots has tripled since 2006, probably as a result of more frequent and longer periods of exposure for pilots. Dr. McGuire said the current study was "precipitated by the increased incidence of clinical NDCS" observed in U-2 pilots in association with the increase in intelligence, surveillance, and reconnaissance operations conducted since 2001.
He and his colleagues obtained MRI brain scans of 102 USAF U-2 pilots and 91 non-pilots matched for education, health factors, and age. All were between 26 and 50 years old.
They found that the U-2 pilots had nearly 4 times the volume (394%; P = .004) and 3 times the number (295%; P < .001) of WMH as non-pilots. The results were the same whether or not the pilots had a history of symptoms of DCS.
"Currently, the consensus opinion of the scientific community is these lesions are markers of an underlying disease process," Sean Jersey, MD, from the David Grant USAF Medical Center, Travis AFB, California, who was not involved in the study, told Medscape Medical News.
"A normal, healthy, young adult should not have very many, if any, detectable brain lesions. In the case of our U-2 pilots or divers, they are probably the result of an occupational exposure which was previously unrecognized," he said. Dr. Jersey emphasized that more research is needed "before we can say for certain that these lesions are dangerous."
Dr. McGuire and colleagues also found that the brain lesions in the nonpilots were mainly located in the frontal white matter, whereas in the U-2 pilots, they were more uniformly distributed throughout the brain and did not increase with age. This suggests, the authors say, that hypobaric exposure during high-altitude flights produces white matter damage different from that occurring in normal aging. "Both findings suggest injury produced by microemboli entering cerebral circulation at random," they note.
The researchers hypothesize WMH in U-2 pilots stems from "interaction between microemboli and cerebral tissue, leading to thrombosis, coagulation, inflammation and/or activation of innate immune response." More study is needed to clarify the pathologic mechanisms.
"We've suspected for some time that these pilots are having subclinical microbubble or microthrombi disease in the brain," Claude Piantadosi, MD, director of the Duke Center for Hyperbaric Medicine and Environmental Physiology, Durham, North Carolina, told Medscape Medical News. Dr. Piantadosi was not involved in the study.
An important DCS countermeasure for high-altitude pilots is prebreathing 100% oxygen before flying to denitrogenate body fluids and tissues. Dr. Piantadosi explained that when the U-2 program got started back in the 1950s and 1960s, pilots had a "very long oxygen prebreathe of 4 hours, and there wasn't as much [DCS] as seen recently."
"Two things have changed since 9-11," he said. The prebreathe has been shortened to 2 hours, "so the pilot has more time at altitude and more mission time, and the pilots fly more frequently: Instead of every couple of weeks[, they] may be flying twice a week."
In terms of the clinical significance of these lesions for long-term neurodegeneration and, in particular, the risk for Alzheimer's disease, Dr. Piantadosi said, "we don't have those answers yet, and this study doesn't address that."
USAF Being Proactive
Dr. McGuire said the USAF has been "very proactive in (1) addressing factors that will lessen the risk and (2) instituting a long-term monitoring program to better understand the long-term implications."
Edward T. Sholtis, deputy director of public affairs, Headquarters Air Combat Command at Langley AFB, Virginia, told Medscape Medical News the USAF has completed the evaluation of U-2 pilots, and ongoing occupational MRI monitoring by the Aeromedical Consult Service is now being implemented.
"Each pilot will receive a baseline MRI, a follow-on MRI every 3 years, and a final MRI when they leave the U-2 program by cross-training, separating, retiring, etc. These evaluations will provide medical documentation of any significant changes and allow long-term monitoring," he said.
"Current AF policy is no pilot will be disqualified based on WMH identified by MRIs," he added.
Sholtis also said the USAF is in the process of restricting exposure for the U-2 pilots via several measures: "flight operations guidance is under review to potentially increase down time between high flights longer than 9 hours, as well as limiting the length of deployments. Additionally, the Cockpit Altitude Reduction Effort (CARE) is a new program that will reduce the altitude U-2 pilots are exposed to during operational missions."
The study was supported by the USAF Surgeon General. The authors, Dr. Jersey, and Dr. Piantadosi have disclosed no relevant financial relationships.