Corneal hysteresis changes in diabetic eyes.

In their recent article, Goldich et al. found that corneal hysteresis (CH), corneal resistance factor (CRF), and central corneal thickness were significantly higher in diabetic eyes than in healthy eyes and CH was claimed to be related to the corneal stiffness. However, we found significantly lower CH values in diabetic eyes than in eyes of healthy subjects. We believe that the statements that equate a higher CH with increased corneal stiffness are at best speculative in this context. Corneal hysteresis can increase or decrease with stiffening depending on the behavior of the viscous material element, so the change in CH alone has too many undefined degrees of freedom to say anything more than that CH is increased. Hysteresis has been shown to decrease during aging, when the cornea is known to stiffen, as well as after the cornea has been stiffened by crosslinking techniques (Noguera GE, et al. IOVS 2007;48:ARVO E-Abstract 1860). Since diabetes is a well-known cause of crosslinking, it may be that CH decreases in diabetes, as in aging. A recent paper by Glass et al. addresses this complexity. The output of the 3-element viscoelastic model demonstrated behavior consistent with clinical data of increasing or decreasing hysteresis with stiffening of the cornea and low hysteresis with low elastic modulus (as in keratoconus) or high elastic modulus (as in advanced age). This model illustrates how changing viscosity and elasticity affect the hysteresis measurement in various ways. We think issues surrounding the samples are driving the findings in both papers; that is, how similar the groups are in terms of possible confounders of CH and CRF. In the Goldich et al. study, the diabetic patients were not described in terms of severity. We think it essential to investigate the role of HbA1c and disease duration on CH and CRF as it may help explain the discrepancies. From this point of view,we believe that larger cohorts studying the role of severity of diabetes, disease duration, and HbA1c are needed to explain the inconsistencies.