Kazemi, Arash

Precis: Ocular biomechanics were compared between treated glaucoma patients and healthy subjects matched for age, intraocular pressure (IOP), and axial length. There was no difference in corneal wave propagation speed, but ocular rigidity was lower in glaucomatous eyes. Purpose: Ocular biomechanical properties are important in understanding glaucoma pathogenesis but the affected tissues are unclear. In this study, we compared corneal wave speed (a measure of corneal elasticity) and ocular rigidity coefficient between glaucomatous and normal eyes. Materials and Methods: Twenty glaucomatous eyes from 10 patients and 20 normal eyes from 13 controls, matched for age, IOP, and axial length were included. Ocular rigidity was calculated based on the difference in supine IOP by pneumatonometry with and without a 10-g weight. Corneal wave speed was determined by ultrasound surface wave elastography. A small, 0.1-second harmonic vibration at 100 Hz was generated through the closed eyelids. Wave propagation was captured by an ultrasound transducer, and wave speed was determined from the phase change with distance. Comparisons were performed using generalized estimating equation models. Results: There were no significant differences in corneal wave speed between glaucomatous and normal eyes (2.16±0.25 vs. 2.07±0.16 m/s, P=0.17). However, ocular rigidity was significantly lower in glaucomatous eyes (0.0218±0.0033 vs. 0.0252±0.0050/μL, P=0.01). Corneal wave speed was not correlated with age and IOP in either group (P≥0.23) but was correlated with ocular rigidity (R=0.48, P=0.02) and inversely correlated with axial length (R=−0.53, P=0.01) in glaucomatous eyes. Conclusion: Glaucomatous eyes tend to have lower ocular rigidity than healthy eyes with similar age, IOP, and axial length. However, the lack of a difference in corneal wave speed suggests that corneal tissue may not be significantly affected, and scleral changes likely play a more important role in glaucoma.

Papilledema is optic nerve swelling caused by increased intracranial pressure, which has the potential to cause significant vision loss. Papilledema is typically bilateral and symmetric but can sometimes be asymmetric and even unilateral. The cause for this asymmetry is unknown. The purpose of this study was to utilize ultrasound vibro-elastography (UVE) to assess for biomechanical differences in eyes with papilledema. Nine patients with papilledema and 9 age-matched controls were enrolled. An external harmonic vibration was used to generate wave propagation through the eyelid with three excitation frequencies of 100, 150, and 200 Hz. A 6.4 MHz ultrasound probe was used to noninvasively measure the wave propagation in the posterior sclera to provide shear wave speeds and viscoelasticity (fit with Voigt model). The magnitudes of the shear wave speed and viscoelasticity of the idiopathic intracranial hypertension patients’ posterior sclera were significantly higher than those of healthy subjects. Moreover, for patients with unilateral papilledema, the magnitudes of wave speed and viscoelasticity of the posterior sclera were statistically higher in eyes with papilledema than in the contralateral eyes without papilledema. UVE provides a noninvasive technique to measure the viscoelastic properties of the posterior sclera, which is stiffer in eyes with papilledema.

Biomechanical properties of the eye are important in understanding glaucoma. However, specific tissues that may be affected are unclear. In this study, we compared glaucomatous and normal eyes for differences in corneal tissue elasticity (as indicated by wave speed) and global ocular rigidity. Both eyes of 10 glaucoma patients and 10 normal controls, matched for age and intraocular pressure (IOP), were included. The ocular rigidity coefficient was calculated from supine IOP measured with and without a 10 g weight added to the tonometer. The wave speed in the cornea was measured by ultrasound surface wave elastography. With this technique, a spherical-tipped probe (4 mm diameter) was placed on the closed eyelid and vibrated at 100 Hz for 0.1 s. The wave speed was calculated using the phase gradient method. Measurements for normal and glaucoma eyes were compared using generalized estimating equation models. The corneal wave speed was similar between normal and glaucomatous eyes (P = 0.4). However, ocular rigidity was significantly lower in glaucomatous eyes (P < 0.001) compared with normal eyes. There was no difference in age or IOP (P > 0.3). Lower ocular rigidity in glaucomatous eyes suggests that a more compliant ocular shell may predispose to glaucoma. However, the lack of difference in the corneal wave speed suggests that corneal tissue is not significantly affected, and changes likely involve the sclera.