Corneal Stiffness Parameters Are Predictive of Structural and Functional Progression in Glaucoma Suspect Eyes

Re: Qassim et al.: Corneal stiffness parameters are predictive of structural and functional progression in glaucoma suspects (Ophthalmology. 2020 Nov 25;S0161-6420(20)31116-7. doi: 10.1016/j.ophtha.2020.11.021. Online ahead of print)

“…TO THE EDITOR: The study by Qassim et al 1 was of interest to us. However, the analyses could be greatly improved by correct interpretation of Corvis-ST deformation data.…”

“…Although the first half of the deformation amplitude (before peak deformation amplitude is attained) may have less contribution of whole-eye movement, its effect cannot be neglected without analyses to confirm the same. 1 Thus, the stiffness parameter used by the authors may be an inaccurate measure of corneal stiffness. This could also explain why the correlation between refractive error and corneal stiffness was weak in their study.…”

“…This could also explain why the correlation between refractive error and corneal stiffness was weak in their study. 1 Matalia et al 2 showed that there was a strong correlation between corneal and extracorneal stiffness and refractive error by using a model which segregated the air-puff force into 2 parts: 1 part borne by the cornea and the other by the extracorneal tissues. 2, 3 Qassim et al 1 also did not find any correlation between topical glaucoma medications and corneal stiffnesses measured by them.…”

“…4 Therefore, we suggest that the analyses by Qassim et al be revisited by considering the contribution of whole eye movement on stiffness parameters at first applanation (SP-A1) and by considering anterior chamber depth as a covariate in the analyses of progression. 1 Advanced analytics, which combine corneal anisotropic biomechanical properties, patient-specific tomography, and force decomposition (corneal and extracorneal), are required in the clinic. 5 Such analytics have the potential to perform highly accurate predictions of even postoperative corneal stiffnesses using just preoperative measurements of corneal tomography and corneal deflection only.…”

Reply

“…We would like to clarify that our reported Corvis ST stiffness parameter measured at first applanation (SP-A1) is calculated based on corneal deflection in isolation. That is, any displacement of the extra corneal tissues and whole eye movement (WEM) as a result of the air-puff force are mathematically accounted for (see Figure 1 in the original article, 1 and detailed further by Roberts et al 2 ). The SP-A1 is calculated by dividing the air-puff pressure on the cornea (the external air-puff pressure minus the intraocular pressure) by the isolated corneal displacement (corneal deformation minus WEM), because the stiffness of a material is defined by the extent to which it resists deformation in response to an applied force.…”