A theory for why the spaceflight-associated neuro-ocular syndrome develops

The spaceflight-associated neuro-ocular syndrome (SANS) occurs in some long-duration astronauts and includes ocular changes such as globe flattening and optic disk edema (1-3). This syndrome has defied an easy explanation. Because of the globe flattening, some suggested SANS resembled idiopathic intracranial hypertension. This, combined with the distended appearance of jugular veins in space, led to the hypothesis that intracranial pressure (ICP) was elevated in space possibly due to increased venous pressures in the head. This hypothesis had problems. Elevated head venous pressures should increase not only ICP but also intraocular pressure (IOP). Consistently increased IOP in space compared with supine measurements has not been seen (4). Also, starting with the pioneering measurements of peripheral venous pressure by Kirsch et al. (5) on the Spacelab 1 flight and followed by the measurements of central venous pressure (CVP) on the Spacelab Life Sciences-1, Spacelab Life Sciences-2, and Deutsche-2 missions, venous pressures have been at or below supine levels in weightlessness (5-9). No plausible theory exists to explain how internal jugular venous vein (IJV) pressure could be elevated above supine levels, when peripheral venous pressure, CVP, and IOP are not.