Simulated-Weightlessness Alters Retinal Foveal Structure and Microvasculature that Is Reversed by a Venocontrictive Thigh Cuff Countermeasure

Abstract

Purpose : Astronauts experience ocular changes during long-duration spaceflight. We hypothesize that these changes may result from a chronic headward fluid shift that alters the retinal blood flow or the retinal structure. We sought to determine if the use of head-down tilt as a spaceflight analog would induce foveal structural and vasculature alterations, and further if the use of venoconstrictive thigh cuffs (VTCs) could reverse these changes.

Methods : In this IRB-approved prospective comparative study, 20 healthy subjects were studied in the following sequential postures (10 min each): seated, supine, 15° head-down tilt (HDT), and 15° HDT with bilateral VTCs at ∼50 mmHg (HDTC, head-down tilt cuff) as a countermeasure to a headward fluid shift. We quantified foveal morphological changes based on foveal diameter (FD) and foveal pit depth (FPD) using optical coherence tomography (OCT) (Fig 1A), and foveal vascular changes based on foveal avascular zone (FAZ) area (Fig 1B), perfusion density (PD) (Fig 1C), and vessel density (VD) (Fig 1D) at the superficial capillary plexus using optical coherence tomography angiography (OCT-A). Linear regression models with repeated measures were used to test for mean change in each response. Values are shown as mean ± SE.

Results : Mean FD decreased from seated to HDT (p = 0.001) with apparent reversal during HDTC (p = 0.35 vs. seated) (seated: 2304.4 ± 42.2 µm; supine: 2190.7 ± 47.3 µm; HDT: 2197.9 ± 47.0 µm; HDTC: 2326.6 ± 45.4 µm). There were no significant differences in mean FPD across the four conditions. Mean FAZ area also decreased from seated to HDT (p = 0.001), also with apparent reversal during HDTC (p = 0.35 vs. seated) (seated: 0.29 ± 0.02 mm²; supine: 0.26 ± 0.02 mm²; HDT: 0.262 ± 0.02 mm²; HTDC: 0.29 ± 0.02 mm²). There was no evidence of differences in mean PD or VD across the four postures.

Conclusions : Acute head-down tilt (a space-flight/weightlessness analog) and resulting headward fluid shifts lead to an overall reduced FD and a smaller FAZ area with reversal when using a bilateral VTC countermeasure.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

(A) Foveal diameter (FD) and Foveal pit depth (FPD), (B) Foveal avascular zone (FAZ), (C) Perfusion density (PD), and (D) Vessel density (VD).