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Editors Selection IGR 13-4
Intraocular Pressure: IOP in space
Comment by Marco Vizzeri on:
48269 Diurnal pattern of intraocular pressure is affected by microgravity when measured in space with the pressure phosphene tonometer (PPT), Chung KY; Woo SJ; Yi S et al., Journal of Glaucoma, 2011; 20: 488-491
The effects of microgravity on the human body are not yet fully understood. For example, does exposure to microgravity during spaceflights cause IOP changes? In this case report, Chung et al. (1815) attempt to answer this intriguing question by comparing pre-flight/ground to in-flight/space IOP values measured on a Korean astronaut with the Pressure Phosphene Tonometer (PPT). The authors report significantly higher IOP values in space along with the existence of a different diurnal IOP pattern on the ground compared to the pattern registered in space, suggesting that cephalad body fluid shifts related to microgravity may be responsible for these findings. Although there is raised concern that body fluid shifts may exert a profound influence on ocular and brain structures and function, the process has yet to be characterized.
Although there is raised concern that body fluid shifts may exert a profound influence on ocular and brain structures and function, the process has yet to be characterized
This study fails to provide any meaningful data. The finding that IOP is higher in space than on the ground is in conflict with previous reports indicating that IOP increases during the first days of spaceflight with subsequent reduction and normalization to pre-flight levels. One possible explanation is that the reliability of PPT may be affected by the microgravity environment. In the absence of gravity, increased force against the sclera might be needed to generate a phosphene spot, resulting in artifactual IOP overestimation. Also, previous studies noted that phosphenes are frequently seen in space and this phenomenon could potentially interfere with PPT measurements. The finding that IOP is higher in space than on the ground is in conflict with previous reports indicating that IOP increases during the first days of spaceflight with subsequent reduction and normalization to pre-flight levels. With regard to the 24-hour IOP pattern, one wonders if it is even possible to identify a 'diurnal' cycle in space. Also, a six-hour time difference between the sleep-work schedule in space and on the ground may have further limited the validity of the experiment. However, careful analysis of the results does not indicate a difference in IOP patterns. In fact, considered separately, both ground and in space 'diurnal' IOP measurements are comprised within a narrow range not exceeding 2 mmHg, with regression coefficients close to 0 (p values not reported). Given the complexity of the experimental setup in space, a ground-based analog for microgravity may help better characterize the effects of microgravity on ocular structures, including changes in IOP and other important variables.
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