Ocular structural and functional changes, including optic disk edema and reduction of near visual acuity, have been recently described in some astronauts returning from long-duration space travels. It is hypothesized that ocular changes related to spaceflight may occur, in predisposed individuals, as a result of cephalad shift of body fluids, possibly leading to elevated intracranial pressure (ICP). Results from head-down bed-rest studies (used to simulate the effects of microgravity) and from parabolic flight experiments (used to produce transient periods of microgravity) indicate that ocular blood flow and intraocular pressure (IOP) may undergo changes in a low-gravity environment. Recent studies suggest that changes in translaminar pressure (i.e., IOP minus ICP) may be implicated in the pathophysiology of optic disk neuropathies. Because postural changes exert an effect on both IOP and ICP, the head-down bed-rest analog may also be used as a platform to characterize the relationship between IOP and ICP, and their reciprocal influence in the pathophysiology of conditions such as optic disk edema or glaucoma.
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