SAN FRANCISCO -- Studies show hanging or standing upside down, whether to exercise, practice yoga or relieve back pain, may be harmful to the eyes, a researcher reported Wednesday.
'An inverted position may be good for the mind and back, but it causes eye pressure to nearly double -- a concern that is especially serious for those who have glaucoma or certain other eye diseases,' said Dr. Thomas Friberg, associate professor of ophthalmology at the University of Pittsburgh.
He presented his findings at the annual meeting of the American Academy of Ophthalmology.
In the studies, all 60 subjects developed increased eye and retinal artery pressure following inverted exercises. Many also experienced temporary visual field losses and eyelid hemmorhages.
An increasing number of Americans are spending time with their feet above their heads -- handstanding, doing yoga exercises or using the so-called 'antigravity boots,' designed to ease back and joint pain, Friberg said in an interview.
'More than 1 million Americans have bought the inversion boots, which are hooked to bars so the person can hang upside down,' he said. 'Also, untold numbers of sports enthusiasts, youngsters conditioning for the Olympics and football players stay in the inverted position for exercise purposes.'
Friberg stressed the physical changes were not permanent.
'But we don't know what the outcome would be if a person continued such an activity over a prolonged period of time,' said Friberg, who conducted the studies through the University of Texas Health Science Center in Dallas.
As an extreme example, he cited the case of political prisoner in the Middle East who was hung upside down for 11 hours a day for six months.
'Even though the pressure in his eyes is now normal, he has suffered significant damage to his peripheral vision and is legally blind in the right eye,' Friberg said.
Another concern stems from the finding that in many of the test subjects, it took up to several minutes for the eye pressure to return to normal after the inverted exercise was completed.
'We are uncertain how much the eye can tolerate and for how long without permanent damage,' Friberg said.
'For most healthy people, there is no great danger, but we have the concern that some susceptible individuals may suffer damage similar to that caused by glaucoma. At this stage, we don't know exactly who could be susceptible.'
Glaucoma, the result of increased fluid pressure insid the eye that presses on and damages the optic nerve, can cause blind spots, a loss of peripheral vision and, eventually, blindness.
Although it is too early to offer firm guidelines, Friberg said, he recommends against staying in the upside-down position for longer than 10 minutes at a time and for taking two-minute right-side-up breaks between inverted exercises.
Those who suffer from glaucoma or retinal vascular diseases, which affect the retina -- a thin, transparent tissue of light-sensitive nerve fibers and cells -- should avoid inverted positions altogether, he cautioned.
Friberg said he and his colleagues began their research after noticing a recurrence of reddened eyes in users of the antigravity boots.
In addition to identifying the visual risks of upside-down positions, he said, the study also can serve as a useful model for further understanding glaucoma.