New model for studying spaceflight-associated neuro-ocular syndrome

Many astronauts during long-duration space missions develop optic disc edema, choroidal folds and hyperopic shifts, which has been called spaceflight-associated neuro-ocular syndrome (SANS). This is presumably due to microgravity in space, but is still poorly understood. In this study, the authors document the development of optic disc edema in participants who were placed on strict head-down tilt bed rest (HDTBR) in a mild hypercapnic environment.

Study design

Investigators replicated the International Space Station environment by placing 11 subjects in 30 days of strict 6° HDTBR in a mild hypercapnic environment. OCT and fundus images were obtained before and after HDTBR.

Outcomes

At the end of the study, 4 subjects developed modified Frisén Scale grade 1 optic disc edema, while 1 subject developed grade 2 optic disc edema. In addition, OCT showed a significant increase in the peripapillary total retinal thickness, which supports the development of disc edema after strict HDTBR.

Limitations

The main limitation of the study is that it did not determine whether the optic disc edema originated from the strict nature of the HDTBR or from the mild hypercapnic environment. The authors speculate that the optic disc edema was due to the positioning because the mild hypercapnic environment did not alter the arterial PC0₂ levels. Future studies will be required to confirm this hypothesis.

Clinical significance

This is an important study because it is the first time optic disc edema has been reliably simulated on earth by head down positioning—mimicking the microgravity seen in space. This will hopefully provide new opportunities to investigate the pathophysiology and potential treatment options for optic disc edema seen in our astronauts, which will be important as we prepare for longer duration space flights.