In NASA speak, it it called Risk of Microgravity-Induced Visual Impairment/Intracranial Pressure. The Washington Post simplifies it to - NASA studies why some astronauts suffer vision problems during spaceflights.
Apparently about 30 percent of the astronauts who have been in space for extended periods have reported gradual blurring of eyesight. (Those with more than six-months on the ISS have experienced the problem.) NASA explains that even those that have not reported symptoms have exhibited optic nerve sheath edema on MRI. The NASA reports speculates that all astronauts suffer idiopathic intracranial hypertension and those with unfavorable eye construction have a high probabilty of developing impairment. It could even be permanent. One astronaut could no longer read the gauges on the space station at the end of his mission.
Not a medical doc? Me neither. What this means is that because of the microgravity experienced in space, there is increased spinal-fluid pressure on the head and eyes. It is similar to papilledma, optic disc swelling, which is difficult to treat.
Mmm. This is pretty serious - a NASA panel has recommended pre- and postflight tests including: a) optic nerve sheath diameter with ultrasound, cornial topgraphy, pre- and post-dadolinium MRI/MRV of the brain and orbit. These are in addition to current testing protocols. They noted that a two week space shuttle mission was not a risk (that is certainly true now), but six-month missions on the space station could be a significant risk to some astronauts.
Sounds like a problem for a manned Mars mission, doesn't it?