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

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.

Hypercapnia augments resistive exercise‐induced elevations in intraocular pressure in older individuals

“…Whereas Laurie et al. (2017) observed a significant effect of CO 2 during resting conditions (supine 6° HDT), we did not (prone 6° HDT). Rather, we observed the hypercapnic augmentation of IOP during the static exercise.…”

“…Our results confirm the findings of Laurie et al. (2017) of an elevation in resting IOP during supine 6° HDT. However, in the present study, the prone 6° HDT elevated IOP much more than was observed in the supine position.…”

Spaceflight associated neuro-ocular syndrome (SANS) and the neuro-ophthalmologic effects of microgravity: a review and an update

“…Likewise terrestrial head down studies with and without hypercapnia have demonstrated increased retinal nerve fiber layer on OCT and might still prove to be a suitable terrestrial analog to test hypotheses and potential countermeasures to SANS. [73][74][75][76][77][78][79][80][81][82] These countermeasures include metabolic and pharmacologic treatments 69,[83][84][85][86][87] in the one carbon pathways; 63,69 lower body negative pressure; [74][75][76] and swim goggles to affect the translaminar pressure gradient. While current data does not seem to support prolonged, significant elevations of ICP to the levels seen in IIH, throughout LDSF, Lawley et al proposed that even mild elevations of ICP for a prolonged duration may contribute to the structural changes seen in SANS.…”

“…Table 3 outlines some of the proposed areas for continued research in SANS. 60,78 CONCLUSION In summary, novel and unique neuro-ophthalmic findings have been documented in astronauts during and after LDSF and have been termed SANS. Although a single unifying and overreaching mechanism has yet to be proven, 1,2,[4][5][6][7][8][9]19,40,41,44,[55][56][57] and a multifactorial pathogenesis may be involved, it is likely that SANS may be the end result of cephalad fluid shifts to the brain and orbit brought about by extended MG exposure.…”

Optical Coherence Tomography Analysis of the Optic Nerve Head and Surrounding Structures in Long-Duration International Space Station Astronauts

“…11,12 However, as the role of ICP remains unclear, the syndrome has since been renamed spaceflight-associated neuro-ocular syndrome. 13,14 Because optic nerve pathology could result in irreversible vision loss, it has been a concern for long-duration missions (eg, 6-month International Space Station missions). The clinical standard for assessing disc edema has been the modified Frisén grading system, either quantified from dilated indirect evaluation or fundus photography of the optic nerve head (ONH).…”