Real-time In Vivo 2-D Deformations of the Optic Nerve Head Using OCT Imaging

Abstract

Purpose : To quantify the real-time, in vivo, IOP-dependent mechanical deformations of the optic nerve head (ONH)

Methods : The ONH of 7 eyes from 5 normal nonhuman primates (NHPs) was imaged with Spectralis OCT (Heidelberg Eng.). The OCT was equipped with research software that allowed continuous recording of a single B-scan over a period of 30 seconds at 5Hz while IOP was manipulated via anterior chamber manometry. At time=2s after the acquisition start the IOP was increased from 10 to 30 mmHg by switching a valve connecting 2 PBS manometry reservoirs. A collection of 150 vertical B-scans of the same cross-section (Fig. 1d) was processed with a custom algorithm that estimates the in-plane displacement field in each scan. The equivalent von Mises strain (Strain_VM, Fig. 1b and 1c) was computed for each of the B-scans in the 4 regions shown in Fig. 1a

Results : The Strain_VM was particularly high and quickly variable in the RNFL and near the PpScl - peripheral lamina junction, because of high bending deformations. The ONH tissues quickly deformed under increasing IOP but deformations steadily increased even after IOP reached 30 mmHg. A tight negative correlation between the ppScl and RNFL average strain was observed (p=0.0096, Fig. 1e). No significant correlations were observed for the strain in other regions

Conclusions : We estimated in vivo, real-time 2-D strain of the ONH tissues via OCT. The inverse correlation between the average strain in the peripapillary sclera and RNFL suggests that the stiffer sclera in the elderly and African descent populations may aggravate the biomechanical environment in the peripapillary axonal layer

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.

 

Time-dependent variation of the real-time 2D strain in the ONH in 4 regions (1a): RNFL (green), prelaminar tissue (yellow), lamina cribrosa (red), and sclera (blue). Equivalent strain varied over the 30 sec imaging time among the 7 tested eyes both in the RNFL (1b) and peripapillary sclera region (1c) in the cross-section shown in 1d

 

The Strain_VM maps for one cross-section at 3 different time points. At 5 sec the ONH already shows some deformations in the ppScl and RNFL. At 15 sec the IOP reached the final pressure of 30mmHg in every eye. A comparison between the strain map at 30 and 15 sec shows that the ONH kept on deforming despite the IOP was no further incremented. That indicates that viscous effects (time-dependent deformations) are present in the ONH tissues