Ophthalmic and Visual System Changes in Human Spaceflight: A Review of Mechanisms, Measurement, and Countermeasures.

Background: Long-duration spaceflight (LDSF) poses unique challenges to ocular health as microgravity, radiation, and environmental changes can cause lasting visual and structural impairments that affect astronaut performance. Objective: This review synthesises current evidence on in- and post-flight ocular complications. It integrates clinical findings, terrestrial analogues, animal studies, and theoretical models to characterise the pathophysiology, risk factors, and countermeasures associated with spaceflight-induced ocular changes. Methods: A review of peer-reviewed literature was conducted, focusing on dry eye disease, corneal edema, ocular biometric shifts, spaceflight associated neuro-ocular syndrome (SANS), and radiation-induced cataractogenesis. Data from in-flight imaging, post-flight assessments, and ground-based analogues were analysed. Results: Spaceflight induces multifactorial ocular changes, including tear film instability, optic disc edema, posterior globe flattening, and hyperopic refractive shifts. These effects are thought to result from cephalad fluid shifts compartmentalised cerebrospinal fluid pressure, venous congestion, and impaired glymphatic system. Long-term risks, such as cataractogenesis, are linked to radiation exposure and genetic susceptibility. Although several countermeasures are being explored, no single approach fully prevents these complications. Conclusions: Ocular complications during LDSF remain a significant challenge for astronaut health and mission performance. A multimodal approach combining mechanical, nutritional, and diagnostic strategies will be essential for future exploration-class missions. Further research is needed to refine countermeasures and preserve astronauts' visual function.