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Editors Selection IGR 18-3

Clinical Forms of Glaucoma: Corneal biomechanics in NPG

Cynthia Roberts

Comment by Cynthia Roberts on:

70916 Corneal Biomechanical Parameters and Asymmetric Visual Field Damage in Patients with Untreated Normal Tension Glaucoma, Li BB; Cai Y; Pan YZ et al., Chinese Medical Journal, 2017; 130: 334-339


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The authors are to be congratulated for a well-designed study to investigate the potential contribution of biomechanics to glaucomatous damage in asymmetric normal-tension glaucoma (NTG). Two important features of the design include that subjects were newly diagnosed, so the results are not influenced by treatments which may affect biomechanical response, and that intraocular pressure (IOP) was not different between fellow eyes. Higher IOP leads to a stiffer response simply due to the nonlinear properties of the cornea, and not necessarily the presence or absence of disease.1 The device used was the Corvis ST, which produces a large number of dynamic corneal response (DCR) parameters which are extracted from 140 images captured during a 30 ms air puff induced deformation of the cornea.2

The authors have correctly interpreted the differences in DCR's between the worse eye and the better eye. However, it is recommended to use more standard biomechanical terminology. Stiffness defines resistance to deformation. Greater resistance is a stiffer eye, and lower resistance is a more compliant eye, or a softer eye. Therefore, where the authors conclude that the shape of the cornea is more easily changeable in the worse eye, it is recommended to instead state that the worse eye was more compliant or softer than the better eye, in the presence of similar IOP and central corneal thickness.

The corneal biomechanical deformation response is influenced not only by the properties of the cornea, but also the properties of the sclera due to displaced fluid when the cornea becomes concave

It is important to note that the corneal biomechanical deformation response is influenced not only by the properties of the cornea, but also the properties of the sclera due to displaced fluid when the cornea becomes concave. Stiffer boundary conditions limit corneal deformation, which leads to the conclusion that a stiffer sclera will be associated with stiffer corneal behavior.3 Therefore, it is difficult to determine whether the cornea, or the sclera, or both are softer in the worse eye. In addition, it is not yet known how these biomechanical differences influence the development of NTG. However, the current study is an important contribution to this field. A novel stiffness parameter for glaucoma will be reported at the 2017 World Glaucoma Conference.4

References

  1. Roberts CJ. Concepts and Misconceptions in Corneal Biomechanics. J Cataract Refract Surg 2014;40(6):862-869. PMID: 24857435.
  2. Roberts CJ, Mahmoud AM, Bons JP, et al. Introduction of Two Novel Stiffness Parameters and Interpretation of Air Puff Induced Biomechanical Deformation Parameters with a Dynamic Scheimpflug Analyzer. J Refract Surg (In press).
  3. Metzler K, Mahmoud AM, Liu J, Roberts CJ. Deformation Response of Paired Donor Corneas to An Air Puff: Intact Whole Globe vs Mounted Corneoscleral Rim. J Cataract Refr Surg 2014;40(6):888-96. PMID: 24857437.
  4. Roberts CJ, Mahmoud AM, Stead RE, Halim WH, Basta M, Shah S, Nessim M. Novel Biomechanical Stiffness Parameter in the Evaluation of Glaucoma. 7th World Glaucoma Congress, Helsinki Finland, June 2017.


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