Relationship between corneal biomechanics and lamina cribrosa shape in low and high myopes

Abstract

High myopia is an important risk factor for open-angle glaucoma that is possibly caused by weakened ocular biomechanics. The cornea and lamina cribrosa both develop from mesoderm with a similar extracellular matrix and they might share similar tissue biomechanics. The lamina cribrosa is recognized as the site of glaucomatous damage, but its biomechanical properties are difficult to measure. Several research groups have used optical coherence tomography (OCT) during a transient intraocular pressure (IOP) elevation to study lamina cribrosa deformation. Other research groups have measured the shape of the lamina cribrosa using a parameter termed the "lamina cribrosa curvature index" (LCCI) to indicate its biomechanical properties. Glaucomatous eyes were found to have a higher LCCI than healthy eyes. This study aimed to evaluate whether or not high myopes with weakened corneal biomechanics would have a weakened lamina cribrosa To answer this question, the first study was conducted to confirm imaging of lamina cribrosa using the OCT during a transient IOP elevation as an appropriate method to be used. The second study evaluated the association between corneal biomechanics and lamina cribrosa. IOP elevation has been used to examine lamina cribrosa biomechanics. A pilot study was conducted to determine if the IOP could be maintained at a stable high level through ocular compression while the lamina cribrosa was imaged. Inspired by the results of the pilot study, a decreasing trend of IOP during ocular compression and an extended study was conducted. The study included thirty high myopes with spherical equivalent ≤ -6.00 D and thirty low myopes with spherical equivalent from -0.50 D to -3.00 D. The ocular compression phase and the recovery phase were 2 and 10 minutes, respectively. It was found that low myopes had slightly faster IOP declining rates during ocular compression than high myopes (LM: -3.25 mmHg/min; HM: -2.58 mmHg/min, p = 0.053). High myopes took longer than low myopes for IOP to sustainably return to their baseline levels (at 510 seconds versus at 360 seconds). This study concluded that IOP was unstable during ocular compression. Therefore, imaging the lamina cribrosa during the ocular compression did not apply to studying the shape of the lamina cribrosa. Given that the high and low myopes demonstrated different IOP changes during and after the ocular compression, it could be hypothesized that this phenomenon could be related to the different aqueous outflow facilities of the two groups. Further study is required to confirm this hypothesis. To study the association between lamina cribrosa shape, LCCI was used. Thirty-two low (spherical equivalent from -0.625D to -3.00D) and thirty-two high (spherical equivalent ≤ -6.00D) myopes were recruited. The lamina cribrosa shape was imaged using a spectral domain OCT (Spectralis, Heidelberg, Germany) to derive the LCCI. A corneal indentation device was used to measure the corneal tangent modulus. Other corneal biomechanics parameters were measured using the Ocular Response Analyzer (ORA, Reichert Inc., USA). Low myopes had higher corneal tangent modulus (LM: 0.518 MPa; HM: 0.434 MPa, p < 0.001) and higher corneal hysteresis (CH) (LM: 10.45 mmHg; HM: 9.53 mmHg, p = 0.012), but higher LCCI (LM: 7.84; HM: 6.37, p < 0.001) values than high myopes. Although previous studies found a higher LCCI in glaucomatous eyes, LCCI may not be a good indicator of glaucoma risk of high myopes. This could be due to the lamina cribrosa being stretched during the axial elongation in myopic development.