Interocular symmetry of ocular rigidity and related ocular parameters in isometropia

Driven by scleral stiffness, ocular rigidity (OR) is emerging as a critical factor in refractive development. Although a common assumption is that OR is symmetric in a healthy individual's eyes, empirical data remains scarce. This study directly tests this assumption by quantifying intereye OR differences in healthy, isometropic subjects.

Isometropia was defined as intereye differences in spherical equivalent refraction (SER) of <0.50 diopters (D) and axial length (AL) of <0.15 mm. OR was measured using a validated method based on video-rate optical coherence tomography imaging and dynamic contour tonometry. A neural network approach was used to segment the choroid and obtain the pulsatile choroidal thickness (CT) and the volume (ΔV) change. OR was estimated based on Friedenwald's pressure-volume relationship using the ΔV and the ocular pulse amplitude. In addition to OR, other acquired measurements included the aqueous depth (AD), central corneal thickness, Pascal intraocular pressure (IOP), Goldmann-correlated IOP, corneal-compensated IOP, corneal hysteresis, and corneal resistance factor (CRF).

Thirty-six subjects aged 17 ± 6 years were enrolled, 33% of whom had isomyopia. Mean (SD) SER was -0.69 (1.48) D in OD and -0.68 (1.41) D in OS (p = 0.88), and mean AL was 23.79 (0.94) mm in OD and 23.78 (0.94) mm in OS (p = 0.36). No difference in OR was found between the right 0.017 (0.011)/μL and left 0.018 (0.012)/μL eyes (p = 0.74; mean difference = -0.0003/μL). No significant intereye differences were observed for most parameters, although right eyes had slightly higher values for AD (0.03 mm, p < 0.001), CRF (0.3 mmHg, p = 0.006), and subfoveal CT (11.5 μm, p = 0.010).

Our findings primarily show that OR displays strong interocular symmetry in healthy individuals with isometropia. Establishing normative symmetry of OR in living human eyes lays the groundwork for the biomechanical basis of refractive development and myopia, indicating that intereye differences in OR may reflect pathological change.