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The Impact of Hypobaric Exposure on Aviators and High-Altitude Special Operations Personnel

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Human Factors and Medicine

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Aircrew, Decompression Sickness, Hypobaric


The 300% increased incidence of neurological decompression sickness (NDCS) in USAF U-2 pilots, attributed to increased U-2 employment during the recent SWA engagements, led to USAF/SG sponsored research that demonstrated subcortical brain injury believed related to non-hypoxic hypobaric exposure in both U-2 pilots and inside safety monitors for altitude chamber training. Furthermore, clinical symptoms were found to be a poor indicator of the brain injury occurring. On computer-based neurocognitive testing U-2 pilots demonstrated a slowing of executive processing when compared to USAF pilot controls; specifically, this reduced performance was in the realms of memory, reasoning and calculation, and information processing speed and accuracy on the computer-based testing although no clinical symptoms were detected. As similar findings were present in both U-2 pilots (a USAF-unique asset) and in hypobaric operators (a NATO-wide asset), this raised international concerns regarding the current and long-term impact of standard military operational procedures across multiple platforms on cognitive abilities. As a consequence the USAF has (1) modified operational procedures and U-2 cockpits; (2) funded a follow-on study examining routine aircrew occupational training exposures in altitude chambers; and (3) pursued a relevant animal model to further understand the pathophysiology underlying this brain injury. Additionally the United Kingdom and Norway are pursuing their own research while simultaneously expanded Department of Defense (DoD) and NASA participation is under development. The hypobaric aerospace environment will remain an important domain for NATO operations for the foreseeable future. Lacking a clear understanding of the underlying pathophysiology, any operational adaptations such as those already implemented by the USAF represent “expert consensus” rather than accurate scientific knowledge. Synergizing research efforts will facilitate a more rapid and fiscally responsive understanding of this human factors challenge and a more rational and economical adaptation of equipment and operating procedures, optimizing operational capabilities while appropriately protecting NATO personnel. After the first meeting of the NATO ET-138 group, there was consensus that our 8-country member team was already in the process of, or about to implement, hypobaric related studies in both humans and animals. There was unanimous concern for understanding the pathophysiology of hypobaric exposure related brain injury in order to prevent or mitigate its effects in our aircrew and special operations personnel. There was unanimous support for continuing with a research program after completion of our ET-138 year. All agree that determining the scope of the problem, defining pathophysiology of white matter injury, and developing relevant exposure guidelines in aircrew and aerospace altitude chamber physiology personnel as well as special operations forces operating at high altitudes could not be completed in one year on an exploratory team.


The Objective of this research group is to identify and define an operational structure that will address S&T issues such as: (1) Understanding the scope of the problem in various career fields (2) Defining the underlying pathophysiology through: a. Demographic and genomic studies b. Basic and applied research (3) Rapidly share and disseminate findings from all member nations and make recommendations for additional research or operational changes (4) Coordinate research efforts to avoid duplication and to enhance synergy (5) Contribute to risk assessment and recommend potential operational limitations for injury mitigation (6) Recommend optimally effective prevention and/or treatment protocols


(1) Identify the prevalence of WMHs and other evidence of white matter injury in various career fields exposed to hypobaria (2) Identify the fundamental underlying pathophysiology (3) Develop an animal model(s) and protocols for hypobaric exposure (4) Recommend relevant exposure guidelines.

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