Aviators piloting aircraft at very high altitudes for the military have “significantly more” brain lesions known as white matter hyperintensities, US Air Force medical researchers have found through MRI scanning.

The study, published in Neurology, compared the MRI images from 102 USAF pilots of the U-2 reconnaissance aircraft with brain scans from 91 controls matched for age, health and education levels.

The U-2 Dragon Lady is an aircraft made by Lockheed Martin, originally designed to go high enough to evade destruction by Soviet anti-aircraft fire during surveillance missions in the Cold War.

It flies at altitudes above about 69,000 feet (above 21,000 meters) and maintains a cabin altitude – the altitude equivalent kept inside the cockpit or cabin of an aircraft – of between 28,000 and 30,000 feet.

The authors cite previous research from other scientists to show that the low air pressure (hypobaria) under which high-altitude pilots work leads to decompression sickness (DCS), including a type known as CNS neurologic decompression sickness (NDCS). They give the following range of example symptoms that can be seen in high-altitude pilots suffering the neurologic decompression sickness (and some “44% of episodes” are NDCS):

  • Slowed thought processes
  • Anomia (impaired ability to name things)
  • Confusion
  • Unresponsiveness
  • Permanent cognitive decline.

The study authors’ own previous research found a link between clinical NDCS and white-matter hyperintensity brain lesions, and they wondered if an increase in these “was directly or indirectly related to microbubbles of predominantly nitrogen gas formed during” the low pressure conditions of high altitude.

The researchers set out to test whether “the entire U-2 pilot population” would exhibit significantly more lesions.

The following is the extent to which they found more lesions among the military high-flyers, compared with the control subjects, higher in both volume and number:

  • Almost fourfold rise in volume of lesions (375% increase)
  • Threefold rise in the number of white matter hyperintensities (295%).

The authors say lesion volume and count are “important markers of cerebral integrity,” and that increases in these measures are seen in age-related cognitive decline – worsened executive functioning, processing speed and general cognitive status in particular.

The brain scans of the pilots, however, showed a different kind of lesioning – “white matter damage different from that occurring in normal aging.”

They discuss what mechanisms could lie behind the high-altitude lesions and distinguish the effects on U-2 pilots from other findings in high-altitude mountain-climbers due to low oxygen – not a problem within the aircraft.

Inflammatory effects to do with “microparticle-induced neutrophil activation and vascular damage” – seen in scuba divers – are proposed by the researchers among other suggested mechanisms.

The authors come to the conclusion that their radiological research adds to the idea that a “showering” of microemboli – tiny gas bubbles – in the brain tissue leads to cerebral injury at very high aviation altitudes.

Should the phenomenon found in the high-altitude military pilots be of concern to commercial airline pilots? Dr. Stephen McGuire from the US Air Force School of Aerospace Medicine, who designed the study and is lead author, told Medical News Today:

We believe these brain changes occur only in response to cabin altitudes above 18,000 feet.

Commercial airliners maintain a cabin altitude of 6,000-8,000 feet and therefore commercial airline passengers and crew are not at any increased risk.”

The authors call for animal research to gain a better understanding of how the white-matter brain damage happens in military pilots and to develop ways of mitigating it through “neuro-protection or neuro-treatment therapies.”

Lieutenant Colonel Edward Sholtis, the US Air Force’s deputy director of public affairs at the headquarters of Air Combat Command, explained how the USAF strives to minimize risks to its pilots.

Lt Col Sholtis told Medical News Today:

The Air Force’s first priority is our airmen, and we therefore seek to minimize known risks of any type.

In the realm of flight safety, for example, in U-2 operations overall since 1963, for every 100,000 hours flown there were 8 mishaps that resulted in the death of the pilot.”

The force has analyzed these mishaps, Lt Col Sholtis added, and implemented procedural, educational or technological changes. “We have reduced that fatality rate to 0.1 per 100,000 hours over the past 10 years.”

In light of the emerging information about white matter hyperintensity (WMH) brain lesions now being reported in the present Neurology study, the USAF is “taking steps to minimize the hypobaric exposure of our pilots, [and] continuing our efforts to better understand WMHs and identify other potential ways of avoiding them.”

Specifically, Lt Col Sholtis said:

The Air Force is in the process of reviewing flight operations guidance to potentially increase downtime between high flights longer than 9 hours, as well as limiting the length of deployments.”

“Additionally,” he added, “the Cockpit Altitude Reduction Effort (CARE) is a new program that will reduce the altitude U-2 pilots are exposed to during operational missions.”

More information on the CARE program has been published by the Beale Air Force Base (resource no longer available at www.beale.af.mil), California, where the the U-2 unit is stationed.