Intraocular/Intracranial pressure mismatch hypothesis for visual impairment syndrome in space.

Visual impairment intracranial pressure syndrome (VIIP) is considered a major risk for future human spaceflight. Loss of hydrostatic pressure gradients in vascular and cerebrospinal fluid systems due to the removal of gravity associated with subsequent intracranial and intraocular fluid shifts and the resulting intraocular/intracranial pressure mismatch might be important etiology factors causingVIIP syndrome. Acclimation changes in the ocular and cerebral circulation and the two fluid systems during chronic microgravity exposure and their underlying mechanisms need further elucidation. Relevant findings may help to validate the pressure differential hypothesis for VlIP syndrome and to evaluate whether a gravity based countermeasure is needed.