Comparison of Corneal Biomechanics Among Primary Open-Angle Glaucoma with Normal Tension or Hypertension and Controls
Glaucoma is a progressive optic neuropathy characterized by changes in the optic nerve head (ONH) and corresponding visual field defects, with or without elevated intraocular pressure (IOP). Normal tension glaucoma (NTG) is a unique type of primary open-angle glaucoma (POAG) where glaucomatous optic nerve (GON) damage occurs despite normal IOP levels. In contrast, POAG with hypertension (HTG) is associated with elevated IOP. The interaction between IOP-related stress and the biomechanical properties of the ONH is believed to play a significant role in determining the extent of optic nerve damage. This may explain why some patients are susceptible to glaucomatous damage even under normal IOP levels. Recent studies suggest that corneal biomechanics, which reflect the deformability of the cornea, may provide insights into the susceptibility of the ONH to glaucomatous damage. This study aimed to compare corneal biomechanical properties among NTG, HTG, and control groups and to analyze the correlations between these properties and known glaucoma risk factors.
The study was conducted as a cross-sectional observational study involving 49 NTG patients, 45 HTG patients, and 50 control subjects. All participants underwent a comprehensive ophthalmic evaluation, including slit-lamp biomicroscopy, fundus examination, IOP measurement using Goldmann applanation tonometry (GAT), and gonioscopy. Central corneal thickness (CCT) was measured using a Pentacam, and axial length (AL) was measured using an IOL-Master 500. Corneal biomechanical properties were assessed using corneal visualization Scheimpflug technology (CST), which provides dynamic corneal response parameters (DCRs). The main DCRs analyzed in this study included the maximum inverse concave radius (MICR), deformation amplitude ratio at 2 mm (DAR 2 mm) and 1 mm (DAR 1 mm), integrated radius (IR), and stiffness parameter at the first applanation (SP-A1). Higher values of MICR, DAR 2 mm, DAR 1 mm, and IR indicate a more deformable cornea, while a lower SP-A1 value suggests increased corneal deformability.
The results showed significant differences in corneal biomechanical properties between the NTG group and the control group. Specifically, the NTG group had higher MICR (NTG: 0.18 [0.17, 0.20] mm⁻¹; control: 0.17 [0.16, 0.18] mm⁻¹; P = 0.033), DAR 2 mm (NTG: 4.87 [4.33, 5.39]; control: 4.37 [4.07, 4.88]; P < 0.001), and DAR 1 mm (NTG: 1.62 [1.58, 1.65]; control: 1.58 [1.54, 1.61]; P < 0.001) values compared to the control group. These findings indicate that the cornea in NTG patients is more deformable than in the control group. Additionally, the NTG group exhibited higher IR (NTG: 8.40 ± 1.07 mm⁻¹; HTG: 7.64 ± 1.31 mm⁻¹; P = 0.026) and DAR 2 mm (NTG: 4.87 [4.33, 5.39]; HTG: 4.44 [4.12, 5.02]; P < 0.007) values and lower SP-A1 (NTG: 91.23 [77.45, 107.45]; HTG: 102.36 [85.77, 125.12]; P = 0.007) values compared to the HTG group. These results suggest that the cornea in NTG patients is also more deformable than in HTG patients. However, no significant differences in DCRs were observed between the HTG and control groups.
The study also analyzed the correlations between DCRs and known glaucoma risk factors, including age, CCT, IOP, and AL. In univariate analysis, CCT was negatively correlated with MICR, DAR 2 mm, DAR 1 mm, and IR (r = -0.502, -0.752, -0.673, and -0.736, respectively; all P < 0.05) and positively correlated with SP-A1 (r = 0.791; P < 0.05). IOP was negatively correlated with DAR 2 mm, IR (r = -0.560 and -0.611, respectively; both P < 0.05), and positively correlated with SP-A1 (r = 0.672; P < 0.05). Multivariate regression analysis confirmed these correlations, showing that CCT was negatively correlated with MICR, DAR 2 mm, and IR (r = -0.238, -0.520, and -0.529, respectively; all P < 0.05) and positively correlated with SP-A1 (r = 0.560; P < 0.05). IOP was also negatively correlated with IR (r = -0.650; P < 0.05) and positively correlated with SP-A1 (r = 0.642; P < 0.05). No significant correlations were found between age or AL and DCRs in either univariate or multivariate analysis.
The findings of this study suggest that corneal biomechanical properties differ significantly between NTG and HTG patients, with NTG patients exhibiting more deformable corneas. This increased corneal deformability in NTG patients may reflect greater susceptibility of the ONH to IOP-related stress, even at normal IOP levels. The results also indicate that corneal deformability is influenced by CCT and IOP, with thinner corneas and lower IOP associated with increased deformability. These findings align with previous studies suggesting that corneal biomechanics may serve as a surrogate marker for the biomechanical properties of the ONH, which are difficult to measure directly in vivo.
The study has several limitations. First, as a cross-sectional study, it can only establish associations between corneal biomechanical properties and glaucoma subtypes, not causality. Longitudinal studies are needed to determine whether corneal biomechanics can predict the development or progression of glaucoma. Second, the sample size was relatively small, which may limit the generalizability of the findings. Larger studies are needed to confirm these results and explore the potential clinical applications of corneal biomechanics in glaucoma management.
In conclusion, this study demonstrates that NTG patients have more deformable corneas compared to HTG patients and control subjects. These differences in corneal biomechanics may reflect underlying differences in the susceptibility of the ONH to glaucomatous damage. The findings also highlight the importance of considering corneal biomechanical properties, along with traditional risk factors such as CCT and IOP, in the assessment and management of glaucoma. Further research is needed to explore the potential role of corneal biomechanics in predicting glaucoma risk and guiding treatment decisions.
doi.org/10.1097/CM9.0000000000001399
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