JOIN NOW

Vision Science in Space

by | Oct 1, 2022 | Manuscripts | 0 comments

Saptarshi Mukherjee, M. Optom

Director of Advance Studies Vision Science Academy, India

 

Vision Science Academy Exclusive

Now-a-days, human has dream to fly to the space. This type of deep space exploration needs healthy body as well as healthy eyesight. Already we have seen the walking capacity of astronauts temporarily disrupted after returning from space exploration.

Vestibular dysfunction, loss of mechanical weight bearing, subjective sensation of shifting of fluid towards head, reduction in muscle volume and bone loss were intensely studied in the past five decades.

However, since 2009, ocular health problems have taken in more focus. Scientists have found that long duration of space exploration in astronauts develops in Space-Associated Neuro-Ocular Syndrome (SANS). Symptoms which include swelling in the optic disc and flattening of the eyeball. Human Exploration and Operations (HEO), Vision Impairment and Intracranial Pressure (VIIP) by NASA used MRI and other tools to examine changes in the eye structure of crew members.(1)

Hypothesis is that increased flow towards the brain can increase intracranial pressure which in-turn causes optic head swelling. It has been noticed that after deep space exploration Hypermetropic shifting, globe flattening, optic disc Oedema, choroidal fold in the focal areas of ischemic retina, refractive changes in cornea and temporary loss of peripheral vision.(2)

In 2005 astronaut John Phillips had experienced blurry vision after 6 months space exploration, his visual acuity dropped from 6/6 to 6/30.(3)  Follow up tests showed posterior flattening of eyeball, choroidal fold caused hypermetropic shift. According to NASA, 70-80 percent of astronauts are affected by SANS. While they experience chronic weightlessness. Only 15% show more concerning signs of this. When they return to Earth’s gravity, these changes can take up to years to resolve, with some changes to the eye never fully returning to how they were before space-exploration.

Microgravity ocular changes in Vision like metamorphopsia caused by choroidal fold, restriction of Visual field due to kink of optic nerve, axial movement of crystalline lens.

Elongation of the eyeball creates pressure on the posterior cranial cavity of the globe which leads to kink of optic nerve. Hypothesis is that increased blood volume in the anterior choroid or ciliary body area may displace the lens or iris which could cause refractive changes. Optical coherence tomography (OCT) imaging technique uses to identify and for better understanding of manifest lymphedema.(4,5)

It has been noticed that there is a lack of a day-night cycle in intracranial pressure. Researchers found that in zero-gravity conditions, intracranial pressure is higher than when people are in a head up position as standing or sitting on Earth, but lesser than sleeping people on Earth. (6) Dr John Berdahl, proposed a hypothesis of eye glass that would create a slight vacuum in front of the eye to reduce some of the atmospheric pressure.

Few evidence has also shown that thin cornea or post refractive surgery cornea changes refractive status. Hypothesis is that hypoxic corneal expansion occur in the unstable area of cornea like Refractive Keratotomy (RK) incision results in the circumferential elevation of peripheral cornea as resultant hyperopic shift.(7) However, in cases of LASIK, the incision affected by mild oedema in central part results in myopic shift.(8)

In zero gravity, dynamic behaviour of choroid obstructs the trabecular meshwork outflow due to peripheral iris elevation which leads to elevated IOP. Astronauts from east Asian region have shorter Axial length, flatter cornea, shallow AC are more prone to terrestrial narrow angle glaucoma.(9)

Although the possible mechanisms of SANS are not completely understood, developing preventive measures before or during deep manned space missions will certainly be useful.

 

References:

    1. Lee AG, Mader TH, Gibson CR, Tarver W, Rabiei P, Riascos RF, Galdamez LA, Brunstetter T. Spaceflight associated neuro-ocular syndrome (SANS) and the neuro-ophthalmologic effects of microgravity: a review and an update. npj Microgravity. 2020 Feb 7;6(1):1-0.
    2. Mader TH, Gibson CR, Pass AF, Kramer LA, Lee AG, Fogarty J, Tarver WJ, Dervay JP, Hamilton DR, Sargsyan A, Phillips JL. Optic disc edema, globe flattening, choroidal folds, and hyperopic shifts observed in astronauts after long-duration space flight. Ophthalmology. 2011 Oct 1;118(10):2058-69.
    3. washingtonpost.com/national/health-science/the-mysterious-syndrome-impairing-astronauts-eyesight/2016/07/09 [Last accessed: 12Sep2022, 6:28am]
    4. Mathieu E, Gupta N, Ahari A, Zhou X, Hanna J, Yücel YH. Evidence for cerebrospinal fluid entry into the optic nerve via a glymphatic pathway. Investigative ophthalmology & visual science. 2017 Sep 1;58(11):4784-91.
    5. Mader TH, Gibson CR, Otto CA, Sargsyan AE, Miller NR, Subramanian PS, Hart SF, Lipsky W, Patel NB, Lee AG. Persistent asymmetric optic disc swelling after long-duration space flight: implications for pathogenesis. Journal of Neuro-Ophthalmology. 2017 Jun 1;37(2):133-9.
    6. Lawley JS, Petersen LG, Howden EJ, Sarma S, Cornwell WK, Zhang R, Whitworth LA, Williams MA, Levine BD. Effect of gravity and microgravity on intracranial pressure. The Journal of physiology. 2017 Mar 15;595(6):2115-27.
    7. Koe MT, Goodman RL, Waller SG, Johnson DA. Case report: myopic shift in a stable corneal graft following high altitude exposure. Aviation, space, and environmental medicine. 2001 Dec 1;72(12):1145-7.
    8. Nelson ML, Brady S, Mader TH, White LJ, Parmley VC, Winkle RK. Refractive changes caused by hypoxia after laser in situ keratomileusis surgery. Ophthalmology. 2001 Mar 1;108(3):542-4.
    9. Quigley HA, Friedman DS, Congdon NG. Possible mechanisms of primary angle-closure and malignant glaucoma. Journal of glaucoma. 2003 Apr 1;12(2):167-80.

Submit a Comment