Effect of gravity and microgravity on intracranial pressure

Abstract

Key Pointsr Astronauts have recently been discovered to have impaired vision, with a presentation that resembles syndromes of elevated intracranial pressure on Earth.r Gravity has a profound effect on fluid distribution and pressure within the human circulation.In contrast to prevailing theory, we observed that microgravity reduces central venous and intracranial pressure.r This being said, intracranial pressure is not reduced to the levels observed in the 90 deg seated upright posture on Earth. Thus, over 24 h in zero gravity, pressure in the brain is slightly above that observed on Earth, which may explain remodelling of the eye in astronauts.Abstract Astronauts have recently been discovered to have impaired vision, with a presentation that resembles syndromes of elevated intracranial pressure (ICP). This syndrome is considered the most mission-critical medical problem identified in the past decade of manned spaceflight. We recruited five men and three women who had an Ommaya reservoir inserted for the delivery of prophylactic CNS chemotherapy, but were free of their malignant disease for at least 1 year. ICP was assessed by placing a fluid-filled 25 gauge butterfly needle into the Ommaya reservoir. Subjects were studied in the upright and supine position, during acute zero gravity (parabolic flight) and prolonged simulated microgravity (6 deg head-down tilt bedrest). ICP was lower when seated in the 90 deg upright posture compared to lying supine (seated, 4 ± 1 vs. supine, 15 ± 2 mmHg). Whilst lying in the supine posture, central venous pressure (supine, 7 ± 3 vs. microgravity, 4 ± 2 mmHg) and ICP (supine, 17 ± 2 vs. microgravity, 13 ± 2 mmHg) were reduced in acute zero gravity, although not to the levels observed in the 90 deg seated upright posture on Earth. Prolonged periods of simulated microgravity did not cause progressive elevations in ICP (supine, 15 ± 2 vs. 24 h head-down tilt, 15 ± 4 mmHg). Complete removal of gravity does not pathologically elevate ICP but does prevent the normal lowering of ICP when upright. These findings suggest the human brain is protected by the daily circadian cycles in regional ICPs, without which pathology may occur.

Adaptation of the cardiovascular system to weightlessness: Surprises, paradoxes and implications for deep space missions

“…It is therefore according to this hypothesis a possibility that venous accumulation of blood in the neck and head alone induces the ocular changes without a simultaneous increase in ICP. Another possibility for a mechanism for SANS, which has been suggested by Lawley et al, is that even though ICP decreases in 0 G to a level below that in the supine 1‐G position, it may still chronically be elevated in space compared to in the upright posture on the ground and thus on an average be somewhat higher and over time lead to SANS manifestations. These considerations, however, are purely speculative at present, because no invasive measurements of ICP have been conducted in space.…”

“…The weakness of these results is that the investigators did not detect a decrease in pressures on the ground before flight when changing posture from upright seated to supine . It has been thoroughly documented that such and similar posture changes induce significant decreases in brachial arterial pressures of some 10 mm Hg or more …”

Intracranial pressure in outer space: preparing for the mission to Mars

“…However, there is evidence that the slightly elevated resting ICP reported by Lawley et al . () is trivial since the authors’ raw data demonstrates that an individual with the lowest resting ICP had the greatest reduction in ICP during parabolic flight in the supine position. Additionally, the current study did not examine ocular specific measures (e.g.…”

“…In the microgravity environment, data from Lawley et al . () indicate that ICP is ∼13 mmHg, slightly lower than ICP when supine on Earth, meaning that 24 h ICP is likely to be elevated during long‐term microgravity compared to Earth since ICP is no longer reduced with upright posture. An approach to simulate microgravity (i.e.…”

“…However, VI becomes a very serious condition in regard to the future Mars missions, as the condition appears to be degenerative and corrective lenses may not be obtainable while astronauts are on a mission in outer space, or on Mars. The study by Lawley and colleagues () is a major step forward in research in this area, and they should be commended for attempting to address this current issue. Moving forward, this area of research should be viewed as a global issue, as it is one of the many obstacles that need to be overcome in order to reduce the risk of long‐term space travel, which will contribute to a successful mission to Mars, and potentially, in the foreseeable future, human habitation on Mars.…”