Effects of short-term mild hypercapnia during head-down tilt on intracranial pressure and ocular structures in healthy human subjects.

Steven S Laurie,Michael B Stenger,Gianmarco Vizzeri,Giovanni Taibbi,Robert Ploutz-Snyder,Scott M Smith,Connor R Ferguson,Xiao Hu,Sara R Zwart,Stuart M C Lee

doi:10.14814/phy2.13302

Abstract

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.

Highlights

Astronauts completing long-duration spaceflight of up to ~6 months have developed ocular structural and functional changes including choroidal folds, optic disc edema, globe flattening, optic nerve sheath distension, retinal nerve fiber layer (RNFL) thickening, and visual acuity decrements (Mader et al 2011; Alexander et al 2012), not all symptoms have developed in all crewmembers
Stroke volume increased and heart rate (HR) decreased from seated upright breathing room air (Seated) to head-down tilt (HDT), but there was no significant difference between the two HDT conditions
Reduce the emphasis on intracranial pressure, the medical operations community within NASA is in the process of renaming Visual Impairment and Intracranial Pressure (VIIP) to the Spaceflight Associated Neuro-ocular Syndrome (SANS)

Summary

Introduction

Astronauts completing long-duration spaceflight of up to ~6 months have developed ocular structural and functional changes including choroidal folds, optic disc edema, globe flattening, optic nerve sheath distension, retinal nerve fiber layer (RNFL) thickening, and visual acuity decrements (Mader et al 2011; Alexander et al 2012), not all symptoms have developed in all crewmembers. The initial report of these findings in 2011 suggested that postflight lumbar puncture opening pressures were slightly elevated based on measurements in five of six astronauts who developed optic disc edema, no preflight measurements were available for Published 2017. This article is a U.S Government work and is in the public domain in the USA It was hypothesized that increased intracranial pressure (ICP) may be responsible for the above ocular findings, perhaps secondary to the weightlessness-induced headward fluid shift, ICP has never been measured in human subjects during spaceflight

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