We read with great interest the article by Sun and associates, and also the corresponding letter to their article by Shimmyo. Sun reported on recovery of corneal hysteresis (CH) after reduction of intraocular pressure (IOP) in chronic primary angle-closure glaucoma. The authors concluded that due to a decreasing IOP, the biomechanical properties of the cornea changed and therefore CH increased. Shimmyo reported on similar results in 16 consecutive primary open-angle glaucoma eyes. The author proposed a new theory to explain this finding. So far, low CH has been explained by general weakness of the collagen, as for example indicated by a decreasing CH following laser in situ keratomileusis, or a secondary material fatigue of collagen due to chronic exposure to high IOP. Shimmyo explained the increase of CH by a probably shallower indentation of the eyeball for limitations of the maximum air force available by the Ocular Response Analyzer (ORA), especially in eyes with higher IOP levels.

Findings from our group support this novel theory. We examined the dependency of the CH on “intraocular pressure” in an in vitro model. We therefore constructed an artificial anterior chamber (Barron) connected to an infusion system. Nine human donor corneas were clamped into this artificial anterior chamber, and the pressure in the artificial anterior chamber was changed manometrically to levels of 10, 20, 30, 40, and 50 mm Hg, respectively. We found a significant dependency of the CH from the adjusted pressure level. At 10 mm Hg, CH was 14.3 mm Hg (SD ± 3.9 mm Hg); at 20 mm Hg, 15.7 mm Hg (SD ± 3.6 mm Hg); at 30 mm Hg, 13.1 mm Hg (SD ± 3.6 mm Hg); at 40 mm Hg, 5.0 mm Hg (SD ± 3.4 mm Hg); and at 50 mm Hg, 1.7 mm Hg (SD ± 2.5 mm Hg). An analysis of variance (ANOVA) revealed a statistically significant decrease of CH between 10 to 40 and 50 mm Hg, between 20 to 40 and 50 mm Hg, and between 30 to 40 and 50 mm Hg. Interestingly, corneal resistance factor showed no significant dependency on the adjusted pressure and ranged from 13.8 to 18.6 mm Hg. CH showed no dependency on central corneal thickness (CCT), which was determined at the beginning, in the middle, and at the end of each measurement; nor did CCT change significantly during the measurement (589 μm ± 86 μm at 10 mm Hg and 595 μm ± 106 μm at 50 mm Hg).

As CH is said to be a biomechanical property of the cornea, it should be independent of intraocular pressure. We experimentally disproved this assumption for high IOPs. ORA seems to be incapable to properly determine CH at higher IOP levels. Our results therefore confirm the theory of Shimmyo. The measurement might be limited by the force of the maximum air jet force and, therefore, a diminished indentation of the cornea as the corneal fibers at higher IOP levels are stretched to a maximum. We agree with the author that a better understanding of the mechanical aspect of the ORA is necessary to classify the clinical standing of this measurement device.