Many astronauts experience ocular structural and functional changes during long‐duration spaceflight, including choroidal folds, optic disc edema, globe flattening, optic nerve sheath diameter (ONSD) distension, retinal nerve fiber layer thickening, and decreased visual acuity. The leading hypothesis suggests that weightlessness‐induced cephalad fluid shifts increase intracranial pressure (ICP), which contributes to the ocular structural changes, but elevated ambient CO
2 levels on the International Space Station may also be a factor. We used the spaceflight analog of 6° head‐down tilt (HDT) to investigate possible mechanisms for ocular changes in eight male subjects during three 1‐h conditions: Seated, HDT, and HDT with 1% inspired CO
2 (HDT + CO
2). Noninvasive ICP, intraocular pressure (IOP), translaminar pressure difference (TLPD = IOP‐ICP), cerebral and ocular ultrasound, and optical coherence tomography (OCT) scans of the macula and the optic disc were obtained. Analysis of one‐carbon pathway genetics previously associated with spaceflight‐induced ocular changes was conducted. Relative to Seated, IOP and ICP increased and TLPD decreased during HDT. During HDT + CO
2
IOP increased relative to HDT, but there was no significant difference in TLPD between the HDT conditions. ONSD and subfoveal choroidal thickness increased during HDT relative to Seated, but there was no difference between HDT and HDT + CO
2. Visual acuity and ocular structures assessed with OCT imaging did not change across conditions. Genetic polymorphisms were associated with differences in IOP, ICP, and end‐tidal PCO
2. In conclusion, acute exposure to mild hypercapnia during HDT did not augment cardiovascular outcomes, ICP, or TLPD relative to the HDT condition.
PurposeTo compare ocular outcomes in healthy subjects undergoing 14- and/or 70-day head-down-tilt (HDT) bed rest (BR).MethodsParticipants were selected by using NASA standard screening procedures. Standardized NASA BR conditions were implemented. Subjects maintained a 6° HDT position for 14 and/or 70 consecutive days. Weekly ophthalmologic examinations were performed in the sitting (pre/post-BR only) and HDT positions. Mixed-effects linear models compared pre- and post-HDT BR observations between 14- and 70-day HDT BR in best-corrected visual acuity, spherical equivalent, intraocular pressure (IOP), Spectralis OCT retinal nerve fiber layer thickness, peripapillary and macular retinal thicknesses.ResultsSixteen and six subjects completed the 14- and 70-day HDT BR studies, respectively. The magnitude of HDT BR–induced changes was not significantly different between the two studies for all outcomes, except the superior (mean pre/post difference of 14- vs. 70-day HDT BR: +4.69 μm versus +11.50 μm), nasal (+4.63 μm versus +11.46 μm), and inferior (+4.34 μm versus +10.08 μm) peripapillary retinal thickness. A +1.42 mm Hg and a +1.79 mm Hg iCare IOP increase from baseline occurred during 14- and 70-day HDT BR, respectively. Modified Amsler grid, red dot test, confrontational visual field, color vision, and stereoscopic fundus photography were unremarkable.ConclusionsSeventy-day HDT BR induced greater peripapillary retinal thickening than 14-day HDT BR, suggesting that time may affect the amount of optic disc swelling. Spectralis OCT detected retinal nerve fiber layer thickening post BR, without clinical signs of optic disc edema. A small IOP increase during BR subsided post HDT BR. Such changes may have resulted from BR-induced cephalad fluids shift. The HDT BR duration may be critical for replicating microgravity-related ophthalmologic changes observed in astronauts on ≥6-month spaceflights.
Spectral-domain OCT is an established tool to assist clinicians in detecting glaucoma and monitor disease progression. The widespread use of this imaging modality is due, at least in part, to continuous hardware and software advancements. However, recent evidence indicates that OCT scan artifacts are frequently encountered in clinical practice. Poor image quality invariably challenges the interpretation of test results, with potential implications for the care of glaucoma patients. Therefore, adequate knowledge of various imaging artifacts is necessary. In this work, we describe several factors affecting Cirrus HD-OCT optic disc scan quality and their effects on measurement variability.
Introduction
We evaluated ocular outcomes in a 14-day head-down tilt (HDT) bed rest (BR)
study designed to simulate the effects of microgravity on the human body.
Methods
Healthy subjects were selected using NASA standard screening procedures.
Standardized NASA BR conditions were implemented (e.g., strict sleep-wake cycle,
standardized diet, 24-hour-a-day BR, continuous video monitoring). Subjects maintained a
6° HDT position for 14 consecutive days. Weekly ophthalmological examinations
were performed in the sitting (pre/post-BR) and HDT (in-bed phase) positions.
Equivalency tests with optimal-alpha techniques evaluated pre/post-BR differences in
best-corrected visual acuity (BCVA), spherical equivalent, intraocular pressure (IOP),
Spectral-domain OCT retinal nerve fiber layer thickness (RNFLT), optic disc and macular
parameters.
Results
16 subjects (12 men and 4 women) were enrolled. Nearly all ocular outcomes were
within our predefined clinically relevant thresholds following HDTBR, except near BCVA
(pre/post-BR mean difference: −0.06 logMAR), spherical equivalent (−0.30
D), Tonopen XL IOP (+3.03 mmHg) and Spectralis OCT average (+1.14
μm), temporal-inferior (+1.58 μm) and nasal-inferior RNFLT
(+3.48 μm). Modified Amsler grid, red dot test, confrontational visual
field and color vision were within normal limits throughout. No changes were detected on
stereoscopic color fundus photography.
Discussion
A few functional and structural changes were detected after 14-day HDTBR,
notably an improved BCVA possibly due to learning effect and RNFL thickening without
signs of optic disc edema. In general, 6° HDTBR determined a small
non-progressive IOP elevation, which returned to baseline levels post-BR. Further
studies with different BR duration and/or tilt angle are warranted to investigate
microgravity-induced ophthalmological changes.
Introduction
We report ocular changes occurring in a healthy human subject enrolled in a bed rest (BR) study designed to replicate the effects of a low-gravity environment.
Case report
A 25-year-old Caucasian male spent 30 consecutive days in a 6° head-down-tilt position at the NASA Flight Analogs Research Unit. Comprehensive ophthalmologic exams, optic disc stereo-photography, Standard Automated Perimetry (SAP) and optic disc Spectralis OCT scans were performed at baseline, immediately post-BR (BR+0) and 6 months post-BR. Main outcome measures: changes in best-corrected visual acuity, intraocular pressure (IOP), cycloplegic refraction, SAP and Spectralis OCT measures. At BR+0 IOP was 11 and 10 mmHg in the right (OD) and left eye (OS), respectively (a bilateral 4 mmHg decrease compared to baseline); SAP documented a possible bilateral symmetrical inferior scotoma; Spectralis OCT showed an average 19.4 μm (+5.2%) increase in peripapillary retinal thickness, and an average 0.03 mm3 (+5.0%) increase in peripapillary retinal volume bilaterally. However, there were no clinically detectable signs of optic disc edema. 6 months post-BR, IOP was 13 and 14 mmHg in OD and OS, respectively, and the scotoma had resolved. Spectralis OCT measurements matched the ones recorded at baseline.
Discussion
In this subject, a reduction in IOP associated with subtle structural and functional changes compared to baseline were documented after prolonged head-down BR. These changes may be related to cephalad fluid shifts in response to tilt. Further studies should clarify whether decreased translaminar pressure (i.e., the difference between IOP and intracranial pressure) may be responsible for these findings.
Scan circle displacements occurred in all scans with motions artifacts affecting the optic disc. Average RNFLT and quadrants were more robust than clock-hours. Because motion artifacts may be difficult to detect, clinicians should carefully inspect en face OCT images for their presence and interpret clock-hour results cautiously.
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