Measurement of intraocular pressure by both invasive and noninvasive techniques in rabbits exposed to head-down tilt

Abstract

This study investigates changes in intraocular pressure (IOP) in rabbits during head-down tilt (HDT), which is commonly used as an experimental model to simulate microgravity. IOP was measured by the needle insertion technique (IOP(NEEDLE)) and Tono-pen tonometry (IOP(TONO-PEN)). Although the absolute value of the IOP(TONO-PEN) was significantly smaller than that of the IOP(NEEDLE), a significant correlation (r=0.99) was observed between them. A linear regression analysis yielded an equation as follows: IOP(TONO-PEN)=0.67 IOP(NEEDLE)-0.67. Both the IOP(NEEDLE) and the IOP(TONO-PEN) changed depending on the tilt angle. Tilting from horizontal (0°) to 75°head-down increased the IOP(NEEDLE) and the IOP(TONO-PEN) by 7.3±0.8 (mean±SEM) mmHg and 4.4±1.3 mmHg. The IOP(NEEDLE) elevated from 13.1±1.3 to 16.9±1.0 mmHg immediately after the onset of 45°HDT and then gradually declined. The value of the IOP(NEEDLE) during 8h of HDT was significantly higher than the value in the control animals, which were kept at the horizontal prone position throughout the experiment. Similar findings were observed in the IOP(TONO-PEN). These results suggest that the needle insertion technique and the Tono-pen tonometry are both useful for measuring lOP in rabbits.