Microgravity-Induced Ocular Changes Assessed by Optical Coherence Tomography

INTRODUCTION: Long-duration space missions pose a risk to visual health due to cephalad fluid shifts in microgravity; this can affect ocular structures, causing decreased astronaut performance in addition to long-term side effects. Understanding these changes is essential for developing effective countermeasures against spaceflight-associated neuro-ocular syndrome. We reviewed how head-down tilt (HDT)-a terrestrial analog of microgravity-affects ocular parameters measured by optical coherence tomography (OCT) and intraocular pressure (IOP). METHODS: Seven databases were searched through May 2025 for studies involving human subjects undergoing HDT at any angle or duration with OCT-based outcomes: total retinal thickness (TRT), retinal nerve fiber layer thickness (RNFLT), and choroidal thickness. Two reviewers extracted data and assessed risk of bias using ROBINS-I v2. RESULTS: There were 11 studies (133 subjects) which met inclusion criteria. Mild HDT (∼6°) caused moderate IOP elevation (+2-4 mmHg) and slight TRT thickening over several days. Prolonged HDT (15-65 d) produced cumulative TRT (+35-54 µm) and RNFLT (+2-7 µm) increases, with occasional choroidal folds. Steeper tilts (-10° to -15°) triggered rapid IOP rises (up to +7 mmHg) and acute choroidal thickness changes. Post-HDT recovery was often incomplete, with delayed normalization of TRT and RNFLT. DISCUSSION: HDT-induced changes in IOP and OCT parameters support the role of cephalad fluid shifts in disrupting cranial venous outflow and causing ocular structural changes. Future research should standardize methodology with longer follow-up periods to improve the translational relevance of findings to spaceflight-associated neuro-ocular syndrome. Roshdy MR, Botros M, Mokhles A, Matwiejuk W, Keriakos AR. Microgravity-induced ocular changes assessed by optical coherence tomography. Aerosp Med Hum Perform. 2026; 97(5):362-371.