Nasa – fluid shifts before, during and after prolonged space flight and their association with intracranial pressure and visual impairment c gastronomie vitam

More than half of American astronauts have developed ocular refraction changes after long duration space flights. Findings have also included structural changes of the eye, as well as signs of elevated intracranial pressure. While short duration space flight is also characterized by vision disturbances, these are generally transient and do not appear to have lasting impacts on the structure or function of the eye.

Changes in vision and eye structure are thought to be the result of prolonged exposure to space flight-induced headward fluid shifts and elevated intracranial pressure. However, this hypothesis has never been systematically tested. The purpose of this investigation is to characterize the space flight-induced fluid shift, including intra- and extravascular shifts, intra- and extracellular shifts, changes in total body water and lower vs. upper body shifts. Noninvasive techniques are used to assess arterial and venous dimensions and flow parameters, ocular pressure and structure, and changes in intracranial pressure.

More than half of American astronauts have developed ocular structural or functional changes during or after long duration space flight on the International Space Station (ISS). Findings have includedoptic disc edema, globe flattening, choroidal folds, dilation of the optic nerve, as well as imaging signs and lumbar puncture data indicative of elevated intracranial pressure (ICP). This pattern is referred to as the Spaceflight Associated Neuro-ocular Syndrome (SANS, formerly the visual impairment and intracranial pressure, or VIIP, syndrome). While short duration space flight is also characterized by vision disturbances, these are generally transient and do not appear to have lasting impacts on the structure or function of the eye. Changes in vision, eye and adnexa morphology, are hypothesized to be the result of space flight-induced cephalad fluid shifts and elevated intracranial pressure. This hypothesis, however, has not been systematically tested. In earlier anecdotal publications, ICP elevation in long-duration space flight has been inferred, but without association with structural or functional changes of the eye. Furthermore, while fluid shifts and compartmentalization during short duration space flight (Space Shuttle missions) have been studied, the fluid distribution patterns and their effects on intracranial pressure or the structure and function of the sensory organs in the course of long-duration space flight are not well known.

This experiment examines a variety of physiologic variables in 13 long-duration ISS crewmembers before, during and after long duration space flight. Measures include: (1) fluid compartmentalization (total body water by 2H2O, extracellular fluid by NaBr, intracellular fluid by calculation, plasma volume by CO rebreathe, interstitial fluid by calculation); (2) forehead/eyelids, tibia, calcaneus tissue thickness (by ultrasound); (3) vascular dimensions by ultrasound (jugular veins, cerebral and carotid arteries, vertebral arteries and veins, portal vein) and venous pressure using a VeinPress during ultrasound; (4) vascular dynamics by MRI (head/neck blood flow, cerebrospinal fluid pulsatility); (5) ocular measures (optical coherence tomography, intraocular pressure, 2-dimensional ultrasound including optic nerve sheath diameter, globe flattening, and retina-choroid thickness, Doppler ultrasound of ophthalmic and retinal arteries, and veins); (6) cardiac variables by ultrasound (inferior vena cava, tricuspid flow and tissue Doppler, pulmonic valve, stroke volume, right heart dimensions and function, four-chamber views); and (7) ICP measures (tympanic membrane displacement, distortion-product otoacoustic emissions, and optic nerve sheath diameter). On the ground, acute head-down tilt induces cephalad fluid shifts, whereas LBNP opposes these shifts. Controlled Mueller maneuvers are used to manipulate cardiovascular variables. Through interventions applied before, during, and after flight, the relationship between fluid shifts and the SANS syndrome can be fully evaluated. ^ back to top

Fluid Shifts investigates the causes for severe and lasting physical changes to astronaut’s eyes. Because the headward fluid shift is a hypothesized contributor to these changes, reversing this fluid shift with a lower body negative pressure device is being investigated as a possible intervention. Results from this study may help to develop preventative measures against lasting changes in vision and eye damage.

Results from the Fluid Shifts investigation are expected to improve scientists’ understanding of how blood pressure in the brain affects eye shape and vision, which could also benefit people confined to long-term bed rest, or suffering from disease states that increase swelling and pressure in the brain.

Each in-flight Fluid Shifts session consists of three data collection modalities (Dilution Measures, Baseline Imaging Measures, and Imaging Measures with Chibis) spread across four days. Dilution Measures are scheduled +/- 15 days of FD45 and R-45 (also FD150 for one-year mission crewmembers). The Imaging Measures baseline activity is planned within 10 days of each performance of Dilution Measures collections and the Lower Body Negative Pressure (LBNP) Imaging Measures are completed within 30 days of Dilution Measures operations. There is a requirement for real-time data/video downlink for remote guidance for all ultrasound, OCT, and Intraocular Pressure data collections. Additional medical monitoring requirements are in place for the LBNP collections using the Russian Chibis. An additional crewmember acting as an on-orbit medical monitor and real-time monitoring of physiological data on the ground are required during these sessions. Also, LBNP is limited to no more than 60 minutes at a time, resulting in the split of the LBNP collections over 2 days in-flight.