There are many risks involved in spaceflight. Eye damage is one of stealthiest. NASA has documented at least seven cases where astronauts with healthy eyes returned to Earth with altered vision. For some, vision loss lasts only a few weeks. Others must live with the condition for much longer and in some cases it may not resolve. The cause remains unknown, but one possible culprit is elevated intracranial pressure caused by an extended stay in microgravity.