IOP-related Mechanical Deformation is Concentrated in the Anterior Optic Nerve Head and Peripapillary Sclera

Abstract

Purpose : To apply a 3D ultrasound elastography technique to map and evaluate the through-depth and regional variation of tissue deformation in the optic nerve head (ONH) and the surrounding peripapillary sclera (PPS) during intraocular pressure (IOP) elevation.

Methods : Inflation tests were performed in 15 normal human donor (20-78 years old) globes while IOP was raised from 15 to 30 mmHg. A 3D volume of the posterior eye centered at the ONH (10 mm × 10 mm) was scanned at each pressure step using a 50MHz ultrasound probe (MS700, Vevo2100, VisualSonics). A correlation-based 3D speckle tracking algorithm was used to compute tissue displacements. Radial, circumferential, meridional and shear strains in spherical coordinates were obtained. ONH and PPS were manually segmented on 3D ultrasound images and divided into equal anterior and posterior layers. Average strains at 30 mmHg were calculated for each region and compared using linear mixed models with unconstrained covariance matrix to account for associations of different regions of the same eye.

Results : Strain maps (Figure 1) showed higher magnitudes of radial (ε_rr) and out-of-plane shear (ε_φr) strains in the anterior ONH and PPS. Quantitative analysis showed that ε_rr and ε_φr were significantly greater in the anterior PPS than posterior PPS (-4.80±0.84% vs -1.42±0.74% and 1.53±0.43% vs 0.76±0.21%, p’s<0.001; Figure 2). Similarly, ε_rr and ε_φr were significantly greater in the anterior ONH than posterior ONH (-4.15±0.83% vs -0.10±0.61% and 2.45±0.93% vs 1.30±0.42%, p’s<0.001). Anterior ONH had larger shear but smaller radial strain compared to anterior PPS (p=0.0003 and 0.0001).

Conclusions : Despite a small total thickness, the ONH and PPS exhibited significant depth-dependent variation in IOP-related mechanical deformation. Radial and shear strains were largely concentrated in the anterior region, possibly underlying glaucomatous damage such as tissue tear and disc hemorrhage in this region.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.

 

Figure 1: 3D deformation in a donor eye showing through-depth (anterior vs. posterior) variations for radial and out-of-plane shear strains in ONH and PPS. Dashed circles indicate the border of the ONH. S = superior, N = nasal, I = inferior, T = temporal.

 

Figure 2: Radial and out-of-plane shear strains were significantly greater in the anterior ONH and PPS than posterior ONH and PPS. *: p < 0.05