An improved mathematical model for OCT-based ocular rigidity measurements: clinical validation.

Abstract

Purpose : The rigidity of the corneoscleral shell is an important biomechanical property of the eye. It is relevant in elucidating the pathophysiology of glaucomatous optic neuropathy.
While no technique is currently available to reliably and non-invasively measure ocular rigidity (OR) in living human eyes, our group has recently developed such a technique. This clinical method uses optical coherence tomography (OCT) imaging and the automated segmentation of the choroid to measure pulsatile choroidal volume change (ΔV) and calculate OR using Friedenwald’s equation (Beaton et al., 2015).
We hereby propose a more anatomically accurate mathematical model to extrapolate ΔV from the pulsatile submacular choroidal thickness change (ΔCT) to further improve measured OR coefficients.

Methods : The proposed mathematical model was designed according to anatomical data accounting for the choroid shape near the equator as measured experimentally. OR values obtained using our method and the new model were compared to the old model, to those obtained using an invasive procedure in the same eyes, and to those reported in manometric studies in the literature.

Results : In the new mathematical model, ΔV is estimated as (π/2) × (AL_adj + CT)² × ΔCT, compared to π × (AL/2)² × ΔCT in the original model. In these equations, CT is the average choroidal thickness at the macula and AL is the biometry measured ocular axial length, AL_adj is the adjusted AL and excludes the anterior chamber depth. The average of newly calculated values of ΔV and OR is 9.4±3.6 µL and 0.027±0.013 µL^-1 respectively.The old model underestimated ΔV and overestimated OR values compared to the new model.

Conclusions : A new mathematical model of choroidal thickness is presented here. It is shown to be more anatomically accurate and to further improve the measurement of OR using our non-invasive, clinical method. Compared to the original model, this new model yields values of OR which are highly correlated and closer to those obtained invasively.This method permits large scale investigations of OR in glaucoma and other ocular diseases.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.