Neuro-Vestibular Examination During and Following Spaceflight (Vestibular Health)

BACKGROUND
Adaptation to microgravity during spaceflight causes neurological disturbances that are either directly or indirectly mediated by the vestibular system. For example, otolith-mediated reflex gains appear to adapt rapidly during spaceflight and after landing. However, animal studies have shown that structural modifications of the vestibular sensory apparatus develop during long-duration spaceflight. To date, no studies have characterized the severity of vestibular syndromes experienced by astronauts as a function of the duration of spaceflight or whether the effects are caused by changes at the peripheral end organs, midbrain, cerebellum, or vestibular cortex.

OBJECTIVES
We are investigating temporal vestibular changes in International Space Station (ISS) crewmembers (6-12-month duration missions) to identify trends in adaptation of vestibular health and performance in orbit and after landing. The analyses will differentiate between peripheral and central vestibular forms of vertigo and oculomotor disorders.

METHODS
Recordings of eye, head, and body movements, as well as subjective reports of perception of motion, are being used to determine the presence of abnormal eye movements, dysmetria, motion sickness symptoms, and illusions of motion during head or body movements. This includes characterization of temporal trends in central compensation for vestibular (otolith) asymmetry. In-flight examinations are performed early in the mission (Flight Days 1 and 30) and once every 2-3 months thereafter. Postflight examinations are performed after return (R) from the mission on R+0, R+4, R+9, and R+30. The inflight and postflight motion sickness questionnaires are customized to support data sharing across related studies. They include a survey of maximum motion sickness experienced on each flight and return day, respectively, on a scale of 0-20, with 0 indicating no motion sickness and 20 indicating emesis.

RESULTS
As of January 2025, five ISS crewmembers have completed all preflight, inflight, and postflight testing. Two additional crewmembers are currently enrolled, and data collections are ongoing. Thus far, the average motion sickness score reported on Flight Day 1 is 6.8 (range: 0-14) and the average motion sickness score reported on R+0 is 13.8 (range: 7-20). Eye tracking data have been processed for several key tests, including metrics for vestibular ocular reflex (VOR) gains, VOR suppression, gaze nystagmus drift, spontaneous nystagmus drift, and active saccade velocity. While some changes in these metrics are noted early inflight, the greatest changes tend to be observed in the early postflight period, with rapid return to baseline in the following days. Body movement perception data have also been processed, including perceptions of whole-body passive rotations and translations in microgravity and blindfolded walking/stepping accuracy on Earth for the perceptions of egocentric distance, egocentric rotation, and path integration. These preliminary processed data will be presented based on their timelines of completion relative to launch and landing.

RELEVANCE
Depending on the etiology of the vestibular syndromes revealed by these tests, countermeasures will be proposed based on vestibular rehabilitation modalities currently used in patients with vestibular disorders, such as habituation, gaze stabilization, and/or balance training exercises. Further comparisons will be made with the same measures collected on Artemis crewmembers via the Standard Measure project. Artemis 2 crewmembers have completed preflight testing and will repeat these tests after their 10-day lunar fly-by mission. If the observed symptoms are more deleterious after one type of mission than the other, then specific countermeasures/strategies may be needed for each design reference mission.