The natural and artificial intraocular lens in spaceflight

Introduction

With increasing numbers of space travelers (professional astronauts and civilians alike), there is a need to better understand the impact of spaceflight on human health. Short and long duration spaceflight is associated with a myriad of spaceflight-associated hazards including microgravity and radiation exposure; psychological isolation and confinement; and changes to diet, sleep, circadian rhythm, and exercise [1, 2]. Systemic repercussions of spaceflight include loss of muscle mass and bone density, upward fluid shift towards the head, orthostatic intolerance, and radiation sickness [1, 3–5].

Spaceflight Associated Neuro-Ocular Syndrome (SANS) is a condition unique to spaceflight and is characterized by a constellation of neuro-ocular findings (e.g., optic disc edema, chorioretinal folds, flattening of the posterior globe and hyperopic refractive shift, and thickening of the retinal nerve fiber layer) [6]. Although the precise mechanism for SANS remains ill-defined, several hypotheses have been proposed including microgravity-associated cephalad and orbital fluid shifts; elevated intracranial pressure; and compartmentalization of cerebrospinal fluid within the optic nerve sheath [6]. In contrast to SANS, we describe the effect of spaceflight on the natural and artificial intraocular lens.

Spaceflight and the natural lens

On Earth and in space, the major risk factor for a lens subluxation or dislocation is trauma [7]. Meer et al. [8] evaluated the types of ocular diagnoses and symptoms that astronauts experienced during International Space Station (ISS) and space shuttle missions [8]. Although one astronaut in 135 missions suffered ocular trauma sufficient to have produced lens damage, there have been no reported cases of natural lens subluxation or dislocation during or after spaceflight [8].

Anterior lens subluxation or dislocation of the native lens can produce loss of vision or angle closure glaucoma. Anterior lens subluxation may block the pathway of aqueous humor drainage via the trabecular meshwork, leading to acute angle-closure glaucoma. On Earth, ocular ultrasound biomicroscopy (UBM) can be used to study lens pathology and secondary glaucoma in vivo and the UBM has the potential for use on astronauts [9]. With frequencies ranging from 40 to 100 MHz, UBM is a relatively novel ultrasound-based modality that is used for imaging the anterior segment with higher resolution than anterior segment optical coherence tomography [10]. The refractive shift seen in astronauts in SANS has been presumed to be related to posterior globe flattening but structural changes in the anterior segment and lens have not been assessed to the same extent [6, 11]. The cephalad fluid shifts in the microgravity environment could impact the anterior choroid and ciliary body, leading to a reduction in the volume of the anterior chamber [11]. Increased blood and fluid in the anterior choroid have the potential to affect the position of the iris and lens and could be contributing to the refractive shift seen in SANS [6, 11].

Spaceflight and radiation-induced cataract

Terrestrially, research has well-documented the association between radiation exposure and the risk of cataract development, especially due to the radiosensitive nature of the lens [1, 12]. There are many hypotheses for the pathophysiology behind the link between ionizing radiation and cataract development, including damage of the cell membrane and DNA of the lens and reduction in the level of enzymes that typically protect the lens [13]. There is risk of lens opacification (i.e., cataract) due to exposure to radiation during spaceflight [14]. On Earth, exposure to at least 3 x-rays (equivalent to at least 1 mSv) is linked with a hazard ratio of 1.25 for cataract development [5, 14]. However, astronauts may be exposed to anywhere from 50 to 2000 mSv of ionizing radiation depending on the duration of spaceflight, thus magnifying astronauts’ risk of cataract development at a younger age in comparison to their general population counterparts [5, 15].

In the Longitudinal Study of Astronaut Health (LSAH), 48 individuals out of 295 astronauts experienced lens opacity, of which 25 experienced cataract assessed as more than “trace” [16]. Three of 48 individuals required cataract surgery for posterior-subcapsular and nuclear cataracts at a relatively younger age than typical terrestrial cataract (below 60 years of age). It may be that these early cataracts were the results of space-associated radiation exposure [16]. Space missions with high-inclinations and lunar missions, which are associated with higher flux of heavy ions, has been linked to a higher incidence of cataract development among astronauts [16].

Spaceflight with artificial intraocular lenses

One 60-year-old physician astronaut with intraocular lens placement after cataract extraction experienced no lens related complications after short-duration space travel [17]. This astronaut had cataract surgery only a few months prior to space travel. He was in a microgravity environment for 18 days but experienced no visual symptoms or lens related problems during spaceflight [17]. Neither liftoff-associated vibration nor the microgravity environment affected his vision or the intraocular lenses during or following spaceflight [17]. Another 58-year-old astronaut with an intraocular lens also had no lens related problems during spaceflight [18]. In this case, the astronaut developed a unilateral cataract, underwent cataract surgery and intraocular lens implantation, and subsequently boarded the spacecraft 15 months later [18]. The astronaut’s vision remained unaffected and stable without any movement in the intraocular lens position from liftoff, through the 6-month experience on the ISS, and return to Earth [18]. It is believed that intraocular lenses are safe and well tolerated in the microgravity environment [18].

Conclusion

Spaceflight-associated microgravity and radiation exposure have the potential to affect the natural and artificial intraocular lens. Although the hyperopic shift seen in SANS has been previously attributed to choroidal thickening and folds and posterior globe flattening, there is a theoretic risk that these refractive shifts could be related to anterior movement of the lens position during spaceflight [6, 11]. The LSAH study showed that more than half of astronauts developed lens opacities which were “non-trace” or significant cataracts [16]. Further work is needed on the potential risks of radiation exposure during spaceflight on the natural lens; the possible lens related hazards associated with ocular trauma; and the precise mechanism of the refractive hyperopic shift in SANS. These risks of spaceflight related to the natural and artificial intraocular lens may be important as we prepare for longer duration crewed missions to the Moon, the asteroid belt, and Mars.

Author contributions

RS and JO were responsible for the conceptualization of this manuscript. RS conducted the literature review and wrote the initial manuscript. JO, EW, and AGL reviewed, edited, and provided intellectual support for the development of this manuscript. All authors participated substantially in the preparation of this paper and approved the final version.

Competing interests

AGL is a consultant for the National Aeronautics and Space Administration (NASA) and is a member of the Eye editorial board.