NASA NTRS · 20040091495
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Hamilton, Douglas R., Gloss, David. 2004-03-01. Cases in space medicine. https://ntrs.nasa.gov/citations/20040091495
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Square-wave jerks (SWJs) are small, involuntary saccades that disrupt steady fixation. We report the case of an astronaut (approximately 140 d on orbit) who showed frequent SWJs, especially postflight, but who showed no impairment of vision or decrement of postflight performance. These data support the view that SWJs do not impair vision because they are paired movements, consisting of a small saccade away from the fixation position followed, within 200 ms, by a corrective saccade that brings the eye back on target. Since many returning astronauts show a decrement of dynamic visual function during postflight locomotion, it seems possible that frequent SWJs improved this astronaut's visual function by providing postsaccadic enhancement of visual fixation, which aided postflight performance. Certainly, frequent SWJs did not impair performance in this astronaut, who had no other neurological disorder.
OBJECTIVE: To examine the sensorimotor adaptive response of a 77-year-old man exposed to the gravito-inertial challenges of orbital space flight. STUDY DESIGN: Prospective case study with retrospective comparisons. SETTING: NASA Neurosciences Laboratory (Johnson Space Center) and Baseline Data Collection Facility (Kennedy Space Center). PRIMARY PARTICIPANT: One 77-year-old male shuttle astronaut. INTERVENTION: Insertion into low Earth orbit was used to remove gravitational stimuli and thereby trigger sensorimotor adaptation to the microgravity environment. Graviceptor stimulation was reintroduced at landing, and sensorimotor readaptation to the terrestrial environment was tracked to completion. MAIN OUTCOME MEASURES: Computerized dynamic posturography tests were administered before and after orbital flight to determine the magnitude and time course of recovery. RESULTS: The elderly astronaut exhibited balance control performance decrements on landing day; however, there were no significant differences between his performance and that of younger astronauts tested on the same shuttle mission or on previous shuttle missions of similar duration. CONCLUSIONS: These results demonstrate that the physiological changes attributed to aging do not necessarily impair adaptive sensorimotor control processes.
Recent studies suggest that an underlying mechanism for susceptibility of patients and astronauts to presyncope includes hypoadrenergic responses to orthostatic stress. However, data used to reach this conclusion are open to various interpretations. In this report, maintenance of sympathetic neural activity (MSNA; peroneal nerve microneurography) during -60 mmHg lower body negative pressure (LBNP) was associated with maintenance of orthostatic tolerance, and disappearance of MSNA was associated with hypotension and pre-syncope. However, MSNA was substantially higher during progressive increases of negative pressure in the presyncopal subject, compared to the non-presyncopal subjects. The data from this case report question the notion that orthostatic hypotension occurs due to inadequate sympathetic neural activation during orthostatic stress in apparently normal, healthy subjects.