Non-invasive in vivo measurement of ocular rigidity: Clinical validation, repeatability and method improvement

Ocular rigidity (OR) is thought to play a role in the pathogenesis of glaucoma, but the lack of reliable non-invasive measurements has been a major technical challenge. We recently developed a clinical method using optical coherence tomography time-lapse imaging and automated choroidal segmentation to measure the pulsatile choroidal volume change (ΔV) and calculate OR using Friedenwald's equation. Here we assess the validity and repeatability of this non-invasive technique. We also propose an improved mathematical model of choroidal thickness to extrapolate ΔV from the pulsatile submacular choroidal thickness change more accurately. The new mathematical model uses anatomical data accounting for the choroid thickness near the equator. The validity of the technique was tested by comparing OR coefficients obtained using our non-invasive method (OR_OCT) and those obtained with an invasive procedure involving intravitreal injections of Bevacizumab (OR_IVI) in 12 eyes. Intrasession and intersession repeatability was assessed for 72 and 8 eyes respectively with two consecutive measurements of OR. Using the new mathematical model, we obtained OR values which are closer to those obtained using the invasive procedure and previously reported techniques. A regression line was calculated to predict the OR_IVI based on OR_OCT, such that OR_IVI = 0.655 × OR_OCT. A strong correlation between OR_OCT and OR_IVI was found, with a Spearman coefficient of 0.853 (p < 0.001). The intraclass correlation coefficient for intrasession and intersession repeatability was 0.925, 95% CI [0.881, 0.953] and 0.950, 95% CI [0.763, 0.990] respectively. This confirms the validity and good repeatability of OR measurements using our non-invasive clinical method.