In our study, mean CCT and GAT IOP, IOPcc, and IOPg were significantly higher in diabetic patients than in healthy control subjects. This result was consistent with those in large population-based studies of diabetic patients in which a consistently higher IOP was found.
32 33 34 35 However, there is still controversy about whether diabetic patients really have higher IOPs in light of the finding that thicker corneas yielded higher IOP measurements with various tonometers. As mentioned, corneal collagen cross-linking may lead to biomechanical changes, which can lead to an overestimation of the true IOP with GAT.
1 4 5 6 7 The altered collagen structure may be additive to the effect of a thicker cornea, thereby further increasing the measured IOP. This additional effect was illustrated in a recent study in which the cross-linking of corneal collagen was observed experimentally.
36 In this study, a pneumotonometer and a hand-held tonometer (Tono-Pen; Reichert) measured IOP at approximately the same level in a cadaveric eye when the eyes were held at a constant pressure of 30 mm Hg. The pressure was monitored after cannulation of the vitreous chamber with a transducer. After cross-linking, both devices measured the IOP at more than twice that level. Liu and Roberts
9 analyzed the effect of CCT, corneal radius, and Young’s modulus on GAT IOP measurements through a mathematical model and showed that variation in the elasticity of the cornea within a range predicted to occur in a normal population would result in an IOP measurement error of as much as 17 mm Hg. If the impact of these altered corneal properties in diabetic patients is extrapolated with regard to IOP measurement, the results of these large epidemiologic studies could certainly be looked at from a different perspective. The majority of these studies, such as the Beaver Dam Eye Study,
37 the Los Angeles Latino Eye Study,
38 and the Blue Mountains Eye Study,
39 demonstrated that diabetes is a risk factor for the development and progression of glaucoma. At the same time, other large studies, including the Rotterdam Study,
40 the Baltimore Eye Study,
32 and the Barbados Eye Study,
41 showed that diabetes was not a significant risk factor for development and progression of glaucoma. At least one explanation can be postulated for the protective effect of diabetes in the development of glaucoma. As mentioned, glucose-mediated corneal collagen cross-linking may have been responsible for the overestimation of IOP. If we consider the possibility that the true IOP in these patients is statistically lower, then it is possible that the threshold for significant pressure-dependent damage to the optic nerve may have not been reached.
42 Therefore, the exact relationship between of IOP and glaucoma to diabetes remains controversial.