Solving the space puzzle
Space flight-associated neuro-ocular syndrome must be solved to make way for Mars mission
During and after prolonged space flight, about half of flyers experience hyperopic shift, and a majority of those studied show optic disc oedema, globe flattening, choroidal folds and sometimes cotton wool spots, and these effects often persist long after return to normal gravity. In some flyers, the neuro-ocular findings appear to correlate with mildly elevated cerebrospinal fluid (CSF) pressure. The findings can affect the choroid, the retina and the optic nerve, Andrew G Lee MD told the American Academy of Ophthalmology 2017 annual meeting in New Orleans, USA.
Understanding the mechanisms involved is crucial to enable years-long flights to Mars, and could shed light on terrestrial vision problems such as idiopathic intracranial hypertension and postoperative vision loss in spinal surgery.
However, determining the relationship between CSF pressure and space flight-associated neuro-ocular syndrome (SANS) has been challenging, added Dr Lee, who holds ophthalmology, neurology and neurosurgery appointments at Houston Methodist Hospital, UTMB Galveston, Baylor College of Medicine and the Center for Space Medicine, Texas A & M College of Medicine, The University of Iowa, the UT MD Anderson Cancer Center in Texas and Weill Cornell Medical College, New York City, USA.
Part of the difficulty in assessing SANS is reflected in the length of time it took to characterise it. Choroidal folds were first observed in an astronaut returning from the International Space Station in 2003, followed by observations of optic disc oedema and cotton wool spots in flyers through 2008. By that time flyers were equipped with higher-magnification reading glasses in anticipation of the hyperopic shift often seen on long flights.
Several months into a 200-day mission in 2009, astronaut Michael Barratt MD noticed his near visual acuity declining, especially in his right eye. He and another physician on board conducted ophthalmoscopic exams on each other and thought they saw disc oedema. Within six weeks NASA sent up additional diagnostic equipment, and high-resolution fundus imaging and ultrasound showed flattening of the globe and marked distension of the optic nerve, as well as slight disc oedema in Dr Barratt’s right eye.
Michael Barratt MD
Spinal taps also have been performed to assess cerebrospinal opening pressure after flight in some cases, and these have been slightly elevated. Initial results of this research were published in 2011, and recurrent SANS on a second flight was documented in 2013, Dr Lee noted.
Astronauts now routinely undergo and perform visual acuity, fundus, ultrasound, IOP and OCT exams on one another in flight, and undergo orbital 3T MRI scans and VA exams before and after flight, Dr Lee said. The goal is to understand better how zero gravity might affect longer space flights, including whether the condition continues to progress, or if it plateaus.
Another question is why not all flyers experience SANS. In particular, it does not seem to affect women as much according to Dr Barratt, though they may be more susceptible to radiation effects of interplanetary flight.
ANSWERS NEEDED FOR MARS FLIGHT
Closer to home, research into the interaction of CSF pressure and IOP may produce insights for treating idiopathic intracranial hypertension and vision loss after spinal surgery – and understanding these earthbound conditions could help solve the zero-G riddle. “We see some of the same things in SANS as with cephalic fluid shift in the head-down prone spine surgery, which is ischaemic optic neuropathy and related vision loss, so there are some potential terrestrial analogues that could help answer the question of what is going on in space,” Dr Lee said.
It may be that weightlessness increases CSF pressure while IOP remains normal, resulting in a flattening of the globe and disc oedema, Dr Lee said. This hypothesis is consistent with the slow resolution in disc oedema seen when flyers return to earth, which could be due to the compartmentalised anatomy of the optic nerve channel.
However, the estimate flight duration time to Mars will be about 15 months, and the problems currently seen begin within a few days of zero-G, and can become significant problems after three-to-six months, Dr Lee pointed out. “We’ve got to learn more about this problem to prepare for any potential manned mission to Mars in 2030.”
Andrew G Lee: firstname.lastname@example.org