Cromwell, R. L.

Ploutz-Snyder, R.

Taibbi, G.

Vizzeri, G.

Yarbough, P. O.

Zanello, S. B.

NASA Technical Reports Server

NASA Flight Analogs Research Unit (FARU) Standardized Conditions:
• During in-bed phase: subject reclined and monitored 24 hours/day
• Vitals, body weight, fluid intake/output measured daily
• Awake time: 6:00 am – 10:00 pm
• Standardized diet to maintain weight within 3% of initial weight
BACKGROUND
Bed Rest Platform:
 6° head down-tilt (HDT) bed rest
70 DAYS OF 6 DEGREES‐HEAD DOWN TILT BED REST AND ITS IMPACT ON 
OCULAR PARAMETERS
RL Cromwell1, SB Zanello1, PO Yarbough1, R. Ploutz‐Snyder1, G Taibbi2, G Vizzeri2
1Universities Space Research Association, 3600 Bay Area Blvd., Houston, Texas, 77058 2 Department of Ophthalmology and Visual Sciences, The University of Texas Medical Branch, 301 University Blvd., Galveston, Texas, 77555
RESULTSRESULTS
 Ocular changes such as visual impairment and intracranial pressure (VIIP) 
have been observed in astronauts returning from long-duration 
spaceflight.
 Changes may be attributed to the shift in body fluids due to microgravity.
 It remains to be determined whether 6° head-down bed rest is a good 
analog for VIIP.
PURPOSE
 Investigate the effects of 6° head-down bed rest on ocular structures and 
function.
 Determine if 6° head-down bed rest is a suitable ground-based analog for 
modeling and studying the VIIP Syndrome.
CONCLUSION
ACKNOWLEDGEMENTS
NASA Flight Analogs Research Unit (FARU) personnel, NASA Flight Analogs
Project funding 516724.03.04.01
DISCLOSURE
Taibbi, G None; Cromwell, RL None; Zanello, SB None; Yarbough, PO 
None; Vizzeri, G None
Visual Impairment and Intracranial Pressure (VIIP) Syndrome: 
May include the following symptoms: 
• Hyperopic shift 
• Cotton wool spots
• Edema of the Optic disc (papilledema)
• Choroidal folds
• Optic nerve sheath distention
• Globe flattening
 16 subjects (15M, 1F, age range 29-54 years): healthy, normotensive, non-
smoker, normal weight 
 Participated in 70 days of 6° HDT bed rest
 Control group and exercise group
 Measurements used in this study:
o Best corrected visual acuity (BCVA)
o Spherical equivalent
o Intraocular pressure (IOP)
o Retinal nerve fiber layer (RNFL) thickness by OCT
 Statistical methods:
o Completely repeated measures experimental design 
oData collected longitudinally at several time points before, during and 
after bedrest, and for subjects’ left and right eyes at each time point. 
o Repeated measures data submitted to a mixed-effects linear regression 
with a-priori contrasts comparing subjects’ closest pre-bedrest
observation to each subsequent observation available per outcome.  
METHODS
Fig. 2. Spherical Equivalent changes during pre-/in-/post-bedrest with the Retinomax device.
• Mean Spherical Equivalent significantly decreased from pre-BR to post-BR (p<0.001) with both 
the Retinomax and Reichert devices.
• Spherical Equivalent Retinomax was significantly different from the last BR day and post-BR, 
but not from the first or middle BR days.
• No in-bed measurements were taken for Spherical Equivalent Reichert.
DISCUSSION
 Both far and near best corrected visual acuity (BCVA) improved from pre-BR 
to post-BR. We speculate that this may be due to the learning effect of 
using the visual chart weekly during the study. 
 Spherical equivalent decreased significantly from pre-BR to post-BR with 
both the Retinomax and Reichert device, which indicates a slight shift to 
myopic biometrics, which might be due to adjustment to near-vision 
activities during bed rest confinement.
o Reichert device is non-handheld and only used pre- and post-BR.
 There was a significant increase in IOP from pre-BR to all in-BR and post-BR 
time points.
o This increase in IOP associated with the positional change may be 
reversible with more time in the post-bed rest phase.
 Average retinal nerve fiber layer (RNFL) thickness by Spectralis OCT 
increased significantly from pre-BR to post-BR.
o Possibly due to interstitial edema from fluid redistribution.
Fig. 1. IOP changes during pre-/in-/post-bedrest.
• IOP significantly increased from a mean of 12.90 mmHg in pre-BR to 13.86 mmHg in post-BR.
• IOP in pre-BR is significantly different from all the in-BR and post-BR time points.
• Subjects’ closest pre-bedrest observations were between 3 and 6 days pre-bedrest.
• Subjects’ first bedrest observation was usually day 3 of bedrest (BR3).
• Subjects’ mid-bedrest observation was usually day 30 of bedrest (BR30).
• Subjects’ last bedrest observation was usually day 65 of bedrest (BR65).
• Subjects’ closest post-bedrest observations were between 2-3 days post.
 Although no clinically significant changes have been observed, IOP 
increase, spherical equivalent decrease, and thickening of the retina due to 
fluid redistribution in bed rest justify further investigation of HDT bed rest 
as an analog to model and study the VIIP syndrome.
 Nomenclature
o Pre-bedrest phase: BR -14 to BR -1 for control subjects, BR -21 to BR -1 
for exercisers
o In-bedrest phase: BR1 to BR70
o Post-bedrest phase: BR+0 to BR+13
Although statistically significant changes were found, none were clinically 
significant.
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Fig. 3. BCVA Far and Near changes during pre-/post-bedrest.
• Mean BCVA far and near significantly increased from pre-BR  to post-BR (p 
< 0.001).
• BCVA FAR and NEAR are significantly different from all of the in-BR and 
post-BR time points.
For the following variables, “Pre/Post” was defined as the CLOSEST Pre and Post 
observations to bedrest. Other in-bed time points were also utilized:
• Mean Average RNFL Thickness by Spectralis optical coherence tomography (OCT) significantly 
increased from a mean of 100.84µm in pre-BR to 102.03 µm in post-BR (p < 0.001).
• No in-bed measurements were taken for RNFL Thickness.
IOP (mmHg)
Day Pre-BR BR3 BR30 BR65 Post-BR
Mean 12.90 14.66 14.25 14.52 13.86
CI (+/- 95%) (61.76, 64.24) (62.86, 65.33) (64.20, 66.67) (64.80, 67.27) (63.73, 66.20)
p (vs. pre-BR) - <0.001 <0.001 <0.001 <0.009
Spherical Equivalent Retinomax (diopters)
Day Pre-BR BR3 BR30 BR65 Post-BR
Mean 0.15 0.11 0.09 -0.07 -0.04
CI (+/- 95%) (-0.39, 0.70) (-0.43, 0.66) (-0.45, 0.64) (-0.61, 0.47) (-0.59, 0.50)
p (vs. pre-BR) - 0.496 0.289 <0.001 <0.001
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BCVA Far (# of letters 
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BCVA Near (# of letters 
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Day Pre-BR Post-BR Pre-BR Post-BR
Mean 63.00 64.96 77.40 79.59
CI (+/- 95%) (61.76, 64.23) (63.73, 66.20) (76.37, 78.43) (78.56, 81.09)
p (vs. pre-BR) - <0.001 - <0.001
https://ntrs.nasa.gov/search.jsp?R=20140008716 2019-08-29T13:52:25+00:00Z


70 Days of 6 Degrees-head Down Tilt Bed Rest and its Impact on Ocular Parameters

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