(By Markian Hawryluk) Acute mountain sickness (AMS) and high altitude cerebral edema (HACE) have generally been considered part of the same process. But new research suggests those conditions might not be as closely related after all.
“It has long been hypothesized that general vessel leakage occurs at high altitude and previously been proposed to be the cause of the development of AMS, which in severe cases may progress to HACE,” explained Dr. Gabriel Willmann, a researcher at the University of Tubingen in Germany. “However, recent data suggests different pathophysiological mechanisms for the development of AMS.”
Doctors have been able to confirm from autopsies that HACE results from tiny blood vessels in the brain that leak, increasing the cranial pressure. Willmann and his colleagues recently tested to see if AMS was related to that same process. Because they couldn't look into the brains of test subjects at high altitude, they looked instead at the tiny vessels in the retina, the back layer of the eye.
“Retinal vessels are very similar to vessels in the brain,” Willmann said.
If test subjects with retinal leakage were also those with AMS, it would add credence to the theory that both AMS and HACE are two points on the same continuum. But the researchers found just the opposite.
Seven of the 14 test subjects they took from 341 meters to 4559 meters experienced retinal vessel leakage at the higher altitude. There was no correlation between those with retinal vessel leakage and those experiencing AMS. Three of the seven volunteers with AMS, but four of the seven without AMS showed leakage. Nor was there any correlation between oxygen saturation levels and retinal vessel leakage.
“Our study was—in our opinion—able to provide the medical community visual evidence that retinal vessel leakage, which in addition may occur in the brain analogously, appears at high altitude regardless of AMS,” Willmann said.
None of the test subjects experienced any noted visual symptoms, and the leakage stopped once they returned to lower altitude. The volunteers, age 25 to 54, were recruited from the university and did not necessarily have any experience in going to altitude. It's not clear whether better acclimatization might change the risk of retinal vessel leakage. But Willman said the four study doctors, who are also avid mountaineers, spent two weeks at altitude conducting their research and one still showed leakage after two weeks regardless of acclimatization status. The other three study doctors did not present with any leakage initially or after acclimatization.
The study was published June 5 in the Journal of the American Medical Association.
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