In humans exposed to up to approximately six months of weightlessness, altered visual acuity and structural adaptions of the ocular system have developed, including hyperopic shifts, choroidal folds, and optic disc edema. While the etiology and progression of vision impairment and intracranial pressure (VIIP)/Spaceflight Associated Neuro-ocular Syndrome (SANS) are not yet clearly defined, given the potential implications for mission success and long-term health, potential countermeasures are being investigated in parallel with definition of VIIP/SANS mechanisms. To prepare for future exploration missions, efforts currently are directed to identify factors that contribute to the ocular structural and functional changes in parallel with evaluating possible countermeasures using ground-based analogs of weightlessness. Given that the headward fluid shift associated with weightlessness has been hypothesized to be one of the central contributors to VIIP/SANS, countermeasures that reverse the headward fluid shift, such as those that trap fluid in the legs or lower body, may have the greatest promise. The goal of this project was to determine if venoconstrictive thigh cuffs used during acute head-down tilt (HDT), a space flight analog, mitigate changes in intracranial pressure (ICP), ocular, and intraocular pressure (IOP) associated with a cephalad fluid shift.
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The study was designed to follow a stepwise process and was completed in two years. In the first year, the team tested the efficacy of commercially-available thigh cuffs in healthy normal subjects and developed a prototype enhanced cuff. Dr. Macias' team in collaboration with co-investigator Dr. Huang of Doheny-UCLA, enrolled 20 healthy normal subjects to evaluate pneumatically-controlled thigh cuffs during head-down tilt. Concurrently, Dr. Cole and her team at Clemson University designed and produced novel, prototype thigh cuffs to address previously identified problems with the thigh cuffs currently used in the space, the Russian Braslet. Some of these problems included several layers of material to generate elastic force, bunching of material at the buckle, an inability to fine tune cuff compression. Functional enhancements included usage of lighter weight, more flexible materials in fewer layers; improved closures; and ability to focus compression on the thigh (femoral vein). Dr. Cole focused on achieving enhanced cuff performance by using well-characterized materials that are commercially available.
During the second year, Dr. Cole's team completed fabrication of three sets of enhanced thigh cuffs in four different sizes to accommodate a variety of thigh circumferences and delivered them to the investigator team to support the human testing. Dr. Huang enrolled 20 additional healthy normal subjects to evaluate these enhanced cuffs using the same test protocol as used in the evaluation of the standard thigh cuffs in the first year. Drs. Macias and Kesari also completed tests of three patients from the John Wayne Cancer Institute in whom direct intracranial pressure (ICP) measurements were possible. Measurements were obtained with and without the enhanced thigh cuffs. Further, Dr. Hargens' UCSD research group also identified components that could be made in the development of a combined lower body negative pressure (LBNP) and thigh cuff countermeasure suit.
In the course of this two-year study, the investigators demonstrated the efficacy of thigh cuffs to prevent some ocular and hemodynamic changes associated with a fluid shift induced by acute head-down tilt, designed and constructed enhanced thigh cuffs designed to improve fit, comfort, and performance in comparison to the Russian Braslet, and demonstrated the efficacy of these enhanced cuffs in healthy normal subjects and in patients.
Balasubramanian S, Tepelus T, Stenger MB, Lee SMC, Laurie SS, Liu JHK, Feiveson AH, Sadda SR, Huang AS, Macias BR. "Thigh cuffs as a countermeasure for ocular changes in simulated weightlessness. Ophthalmology
. 2018. March ;125(3):459-60. [DOI]
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