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Last October an OCT examination with a difference took place – approximately 370km above the earth.

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Last October an OCT examination with a difference took place – approximately 370km above the earth.

The “patient” in this particular case was an astronaut and the “clinic” was the microgravity environment of the International Space Station (ISS). The OCT examination was carried out as part of NASA’s ongoing Ocular Health Study, which seeks to understand ocular changes in astronauts during long-term space missions.

Since its arrival at the ISS in June 2013, the OCT device (Spectralis, Heidelberg Engineering) has been used for eye examinations of ISS crew members every four weeks. The astronauts have all had OCT baseline examinations prior to their space missions, with the follow-up examinations in orbit allowing observation of possible ocular changes developing during the mission.

Astronauts also perform an additional series of in-flight tests to include IOP measurements, Fundus photography, ocular ultrasound and visual acuity testing.

As well as OCT examinations, each astronaut undergoes pre-flight, and post-flight IOP measurements, dilated fundus examination with fundus photography, optical biometry, MRI, ocular ultrasound imaging and cycloplegic refraction.

Effect largely unknown

Although physiologic and pathologic changes associated with the microgravity environment have been studied extensively in the past, the effect of this environment on the eye and brain remains largely unknown.

A landmark study published in Ophthalmology in 2011 found that space flights lasting six months or more can cause a spectrum of changes in astronauts’ visual systems. Varying degrees of disc edema, globe flattening, choroidal folds and hyperopic shifts after long-duration space flight have all been documented by NASA in recent years.

Of seven astronauts in the Ophthalmology study who spent at least six continuous months in space, six reported that their vision became blurry, to varying degrees, while on the space station. Vision changes usually began around six weeks into the mission and persisted in some astronauts for years after their return to Earth.

Such visual impairment, even if only transitory, has wide-reaching implications for the future of space travel, and is being treated with appropriate seriousness by NASA, Thomas H Mader MD, lead author on the 2011 Ophthalmology study told EuroTimes.

“Prolonged or persistent optic disc edema is potentially sight-threatening and could theoretically limit interplanetary space travel for some astronauts. The good news is that once NASA researchers first identified these ocular changes they immediately began a series of tests to quantify the extent and possible progression of these anomalies. Thanks to the onboard equipment such as OCT and fundus camera we now at least have some limited objective data to base an opinion on,” he said.

Research studies

Dr Mader added that although the visual acuity problems appear, thus far, to be completely correctable with a new spectacle lens prescription, the research team is still very concerned about potential visual loss from chronic disc edema.

“At this point the focus of NASA ocular research is to carefully follow and critically evaluate data as it is acquired and to conduct prospective research studies to understand the mechanisms involved. Hopefully this information will help us to sort out who may be most at risk for visual impairment during long duration space flight,” he said.

While it is still too early to pinpoint the exact cause of the visual disturbances experienced by certain astronauts on longer space missions, a number of plausible theories exist, points out C Robert Gibson OD, an optometrist at NASA’s Flight Medicine Optometry Clinic.

“At this point we are simply not certain of the specific aetiology of our findings although a rise in intracranial pressure (ICP), an optic nerve (ON) compartment syndrome, and changes in intraocular pressure (IOP) have all been proposed as possible mechanisms,” he said.

While researchers hope that one day they might be able to identify risk factors for visual impairment during long-term missions, the current data is not sufficiently robust to allow anything more than tentative observations, said Dr Gibson.

“We simply have not examined enough astronauts pre, during and post mission to develop a list of proven risk factors. At this point the information we have suggests that men are more likely to have these ocular changes than women and the right eye is more likely to be involved than the left, but we do not have enough data to draw firm conclusions,” he said.

Intriguingly, Dr Gibson noted that a recent study on MRI imaging in 27 astronauts suggests that the ocular effects of space travel may have a dose-response.

“In other words anatomical changes that occur during an early mission may set the stage for recurrent or additional changes when the astronaut is again exposed to the physiologic stress of repeat space flight,” he said.

Thomas H Mader:

Charles Robert Gibson: Charles.Gibson-1@NASA.Gov

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