Purpose
To study changes in ocular biometry and refraction in the largest cohort to date of astronauts who have experienced long-duration spaceflight on the International Space Station (ISS).
Design
A prospective cohort study.
Participants
29 astronauts.
Methods
Preflight and postflight cycloplegic refraction and ocular biometry measurements were obtained from 56 eyes among 29 subjects. For each eye, the preflight-to-postflight changes in spherical equivalent (SE), axial length (AL), average corneal curvature (K), and anterior chamber depth (ACD) were calculated. The Fyodorov and Olsen-C formulas were used to estimate the relative contribution of each biometric parameter individually to the total change in SE. A linear mixed-model approach was used to assess the relationships between refraction measurements, biometric parameters, optic disc edema, and duration on the ISS.
Main Outcome Measures
Preflight-to-postflight changes in spherical equivalent, axial length, average corneal curvature, and anterior chamber depth.
Results
27/56 (48.2%) eyes underwent a hyperopic shift, 8/56 (14.3%) underwent a myopic shif , and 21/56 (37.5%) eyes had no measurable change in SE. On average, this equated to a mild hyperopic shift of +0.12 D (95% CI, +0.02 to +0.22 D) that arose from a decrease in AL of –0.09 mm (95% CI, –0.14 to –0.04 mm), mitigated by a shortening in ACD of -0.09 mm (95% CI, –0.12 to –0.06 mm). Changes in K were variable and had little contribution to SE changes at the group level but often showed substantial change at the individual level. Statistical modeling revealed the greatest predictor for refractive change was baseline preflight refraction ( P = .034), with myopic individuals experiencing the largest hyperopic shifts (and never a myopic shift) and baseline hyperopic individuals experiencing variable myopic to mildly hyperopic shifts.
Conclusions
Spaceflight is associated with decreases in AL and ACD and variable changes in K. On average, these changes result in a mild hyperopic shift in SE, although myopic shifts can be observed at the individual level. Prior reports of greater hyperopic shift may be a result of subjects being more myopic at baseline.
INTRODUCTION
S paceflight-associated Neuro-ocular Syndrome (SANS) is a condition that astronauts on the International Space Station (ISS) frequently develop during missions spanning multiple months or longer. SANS is characterized by a collection of signs and symptoms including optic nerve head edema, globe flattening, choroidal folds, and a hyperopic shift in refraction. The optic disc edema is the most concerning long-term change and under considerable investigation. However, it is important to emphasize that the optic nerve structural alterations observed on the ISS are thus far mild, at least in comparison to the optic disc edema associated with clinically significant terrestrial eye diseases such as idiopathic intracranial hypertension. Typically, SANS changes eventually resolve upon return to Earth except for some persistent choroidal folds, shifts in refractive error, or changes to globe shape in mild-moderate cases. Thus, given the current lengths of spaceflight missions (up to ∼12 months), no astronaut to date has experienced permanent vision loss. As a result, from an operations standpoint, in-flight change in refraction is the most immediately relevant functional ocular change in astronauts.
Changes in refraction have long been known to impact astronauts on the ISS, with astronauts frequently using “space anticipation glasses” to combat typical hyperopic shifts. The magnitude of hyperopic shift is generally considered to be ≥0.75 diopters, but variation exists. To investigate this, comparisons of pre- and postflight examinations have been reported. In the Ocular Health (OH) study, astronauts showed decreases in both average axial length (AL) and average anterior chamber depth (ACD).
However, a comprehensive study of astronaut refraction and biometry (including keratometric measurements) in larger numbers has not been previously conducted. Such a study is important due to the anticipated extended duration of future spaceflight missions. As a mission to Mars could span multiple years, it needs to be determined whether spaceflight-associated changes in refraction continue over longer durations and at what trajectory. This is particularly important because change in refraction is easily mitigated with proper planning and access to spectacle correction. Additionally, understanding the magnitude of all biometric changes in the eye can be used to assess the risk for other conditions. For example, the previously reported reduction in ACD, could hypothetically put astronauts at risk for acute angle closure glaucoma.
In this study, we evaluate refraction and biometric data from individuals who were involved in consented research studies (NASA <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='’>
Fluid Shifts” [FS], and <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='’>
Ocular Health” [OH], studies). We then supplement these data with outcomes acquired from standard medical operations on astronauts who were not a part of the above studies but who had consented to allow their results to be used for research (the <SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='’>
Lifetime Surveillance of Astronaut Health” [LSAH] program). We evaluate biometric endpoints, including keratometric (K) results, which have not been previously reported. Additionally, we calculate the relative effect of each biometric change on total refraction. Lastly, we evaluate these changes on not just a group level, but also on an individual level. Ultimately, an improved understanding of the changes in refraction and ocular biometry that occur during spaceflight will allow NASA to better plan the refractive needs of astronauts on future missions.
METHODS
Research Participants
Preflight and postflight measurements of cycloplegic refraction and ocular biometry were obtained from NASA’s ‘Ocular Health’ (OH) (IRB: 285), ‘Fluid Shifts’ (FS) (IRB: 998), and <SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='’>
Lifetime Surveillance of Astronaut Health” (LSAH) (IRB: 277) studies. The OH and FS studies were reviewed and approved by the NASA-JSC IRB, and all subjects provided written informed consent. LSAH data came from standard medical operations in which astronauts consented for the use of their data for research. All aspects of this study followed the tenets of the Declaration of Helsinki.
Acquisition of Ocular Biometry and Refraction Data
Cycloplegic refractions using 1% Tropicamide (Akorn), were performed by a NASA Flight Medicine Clinic optometrist, and biometry measurements were collected using an IOLMaster 500 biometer (Carl Zeiss Meditec AG; Germany). Preflight measurements took place between 6 and 21 months prior to flight, while postflight measurements occurred 1 to 6 days after return. Only phakic eyes possessing full sets of preflight and postflight refractive and biometry measurements were included in this study. This yielded 56 eyes from 29 astronauts who had a flight duration of 189.5 ± 60.1 days (mean ± standard deviation). Of these, 26 eyes came from the OH study, 15 eyes came from the FS study, and 19 eyes came from the LSAH study. Four eyes were shared between the OH and FS studies, and the duplicate results were removed from our analysis. For each eye, the preflight-to-postflight changes in spherical equivalent (ΔSE), axial length (ΔAL), average corneal curvature (ΔK), and anterior chamber depth (ΔACD) were calculated. Of note, while SE, AL, and ACD data were previously published for the OH cohort, corneal curvature data had not been reported, and we also now combine the OH results with results from the FS and LSAH studies. Lastly, no change in refraction is defined as a ΔSE = 0 D, a myopic shift is defined as ΔSE < 0 D, and a hyperopic shift is defined as a ΔSE > 0 D. Although NASA typically uses a 0.5 D threshold to consider a change in refraction as significant (so that such a change is less likely within measurement error), this study reports all changes as a continuum in order to examine the relationship between biometric parameters and refraction.
Calculation of Expected Change in Refraction from Biometric Changes
In order to calculate the expected ΔSE induced by each biometric parameter individually and altogether, cataract surgery intraocular lens (IOL) implant power formulas were used. These formulas predict the power of an IOL that is required to achieve a specified outcome in refraction after cataract surgery. Specifically, a combination of the Fyodorov, and Olsen-C, formulas was used. The Fyodorov formula determines the IOL power (P) needed to achieve a specified refractive result based on AL, K, and effective IOL position (ELP). When aimed for emmetropia (desired postsurgical SE = 0), the equation simplifes to:
P=1336AL−ELP−13361336/K−ELP
ELP=ACD+C×LT
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