Recent missions on board the International Space Station have revealed
previously unreported physiological consequences of long duration space
flight, particularly in eyesight, and in this Brief Dr. Seedhouse
reviews the existing theories on what causes this degeneration and how
long it will last. Notably, 60% of long-duration crews have reported
subjective degradation in vision, a clear indication that further study
is necessary before astronauts embark on even longer-term space
missions. Decreased near-visual acuity was reported in 46% of ISS/Mir
crewmembers, resulting in a change of up to 2 dioptres in their
refractive correction. It is possible that ophthalmic changes have been
present since the first days of spaceflight, but had been attributed to
other causes; this approach to the issue as well as other hypotheses are
all presented in full to give a broad foundation of the existing
knowledge on the topic. The changes have occurred at various times
during a mission with varying degrees of visual degradation. Some cases
resolved on return to Earth, but several crewmembers have not regained
pre-flight visual acuity, indicating the damage may be permanent. One
explanation of the syndrome has been attributed to hyperopic shift due
to aging, but onboard analysis techniques, including visual acuity
assessments, retinal imagery, and ultrasound examination of the eye, has
led to the acceptance of a wider syndrome. In addition to vision
changes, studies have reported flattening of the globe, swelling of the
optic disc (papilledema), choroidal folds in the retina, swelling of the
optic nerve sheath, and visual field defects. It is widely hypothesized
that this spectrum of symptoms may be explained by an elevation of
intracranial pressure (ICP). Establishing the provenance of this medical
problem, monitoring its occurrence and resolving the symptoms for future
long term space missions is a key challenge for space medicine. With
this book, readers have an entry point for understanding the full scope
of the problem and its possible origins.
