Repeatability of the in vivo mechanical strain in optic nerve head (ONH) by video OCT

Abstract

Purpose : Elevated ONH tissue mechanical deformations have been hypothesized to play a role in glaucoma. Quantitative assessment of mechanical strain has been elusive and their repeatability has never been investigated. We quantified the inter-session repeatability of the strain response to controlled acute IOP elevation in vivo by video OCT imaging and a novel virtual extensometer strain quantification method.

Methods : The ONHs of 5 eyes from 5 Rhesus macaques (NHPs) were imaged by Spectralis OCT (Heidelberg Engineering). A single B-scan was continuously recorded for 60 seconds at 9Hz during manometer-controlled IOP elevation from 10 to 30 mmHg. Imaging was repeated over 3 imaging sessions at least 2 weeks apart. The time-varying 2D displacement and strain were computed by template-matching. ANOVA was used to assess inter-animal, inter-session and inter-region variability (Fig. 1)

Results : (A) Between group (across NHPs) variation was higher within-animal variation. (R = 14.61, p < 0.0001; Fig 2). (B) Mean strain values did not differ significantly between imaging sessions for the same NHP. (R = 1, p = 0.293; Fig 2) (C) Strain varied significantly between regions within the same eye. (R = 69.77, p < 0.0001; Fig 2)

Conclusions : These results indicate that virtual extensometer strain quantification by video OCT: (A) was sensitive to the eye-specific mechanical response to IOP; (B) did not significantly vary across imaging sessions and is thus a repeatable measure of tissue deformation; (C) was sensitive to the specific mechanical response to IOP of the different regions of the ONH.

This is a 2020 ARVO Annual Meeting abstract.

 

Figure 1. Measurements of strain by virtual extensometers on a vertically-oriented OCT B-scan. The yellow line shows the extensometer to measure the change in distance between BMO points. The blue line, extensometer to measure the compression of the prelaminar tissue. The light blue lines, extensometers to measure changes in distance between BMO and Minimum Rim Width on both sides. The green and yellow boxes show the strain gauges in the choroidal regions. Red box, strain gauge in the central Lamna Cribrosa

 

Figure 2. Box-whisker plots of the compressive strain distribution in response to IOP elevation. (A) Variation of the strain among across the five NHPs. (B) Averaged across the eyes of the strain change across the 3 imaging sessions. (C) Averaged across eyes of the variation across the different virtual extensometers placed in the ONH