<i>In Vivo</i> Changes in Lamina Cribrosa Microarchitecture and Optic Nerve Head Structure in Early Experimental Glaucoma

<div><p>The lamina cribrosa likely plays an important role in retinal ganglion cell axon injury in glaucoma. We sought to (1) better understand optic nerve head (ONH) structure and anterior lamina cribrosa surface (ALCS) microarchitecture between fellow eyes of living, normal non-human primates and (2) characterize the time-course of <i>in vivo</i> structural changes in the ONH, ALCS microarchitecture, and retinal nerve fiber layer thickness (RNFLT) in non-human primate eyes with early experimental glaucoma (EG). Spectral domain optical coherence tomography (SDOCT) images of the ONH were acquired cross-sectionally in six bilaterally normal rhesus monkeys, and before and approximately every two weeks after inducing unilateral EG in seven rhesus monkeys. ONH parameters and RNFLT were quantified from segmented SDOCT images. Mean ALCS pore area, elongation and nearest neighbor distance (NND) were quantified globally, in sectors and regionally from adaptive optics scanning laser ophthalmoscope images. In bilaterally normal monkeys, ONH parameters were similar between fellow eyes with few inter-eye differences in ALCS pore parameters. In EG monkeys, an increase in mean ALCS Depth (ALCSD) was the first structural change measured in 6 of 7 EG eyes. A decrease in mean minimum rim width (MRW) simultaneously accompanied this early change in 4 of 6 EG eyes and was the first structural change in the 7^th EG eye. Mean ALCS pore parameters were among the first or second changes measured in 4 EG eyes. Mean ALCS pore area and NND increased in superotemporal and temporal sectors and in central and peripheral regions at the first time-point of change in ALCS pore geometry. RNFLT and/or mean ALCS radius of curvature were typically the last parameters to initially change. Survival analyses found mean ALCSD was the only parameter to significantly show an initial change prior to the first measured loss in RNFLT across EG eyes.</p></div

Early changes in ONH parameters, RNFLT, and ALCS pore geometry in the EG eye of monkey OHT-66.

<p>Values for (A) mean ALCSD, (B) mean RoC, (C) mean MRW, and (D) RNFLT are shown as a function of study time for all measured time-points (black circles) before and after the initial laser treatment (day 0). The black horizontal line in each plot indicates the baseline value for each parameter while the gray shaded region represents the 95% confidence interval for each parameter calculated from data measured in the fellow control eye. Yellow circles represent the time-point of first significant change in each plotted parameter, while vertical red lines represent the time-point of first significant change in ALCS pore geometry. The first parameter to significantly change from baseline values was (A) mean ALCSD (49 days after the initial laser treatment). The next parameter to significantly change from baseline was (B) mean RoC (130 days after the initial laser treatment), followed by simultaneous significant changes in ALCS pore geometry and mean MRW (168 days after the initial laser treatment). RNFLT was the last measured parameter to significantly change (182 days after the initial laser treatment).</p

IOP data for control and EG eyes throughout the duration of the study.

<p>IOP data for control and EG eyes throughout the duration of the study.</p

LC pore parameters were analyzed on global and local levels in all normal eyes.

<p>(Rows 1, 3, 5) AOSLO montages of the ALCS scaled, registered, and overlaid on the corresponding SLO images in right and left eyes of 6 normal monkeys. (Rows 2, 4, 6) After manually marking LC pores (filled in white), pores were examined globally, in central and peripheral regions (separated by green boundaries) and in 60° sectors (divided by fuchsia meridians). Scale bar: 100 μm.</p

Mean (± st. dev.) pore parameters across all eyes of 6 bilaterally normal monkeys on global, regional, and sector scales.

<p>Mean (± st. dev.) pore parameters across all eyes of 6 bilaterally normal monkeys on global, regional, and sector scales.</p

Distinct changes in LC beam and pore structure were observed in early EG.

<p>(A-F) AOSLO montages of the ALCS were constructed, registered, and averaged across multiple time-points in the same EG eye for each monkey and overlaid on the corresponding SLO images to show the LC before (left) and after (right) the first statistically significant changes were seen in ALCS pore geometry in early experimental glaucoma. Large differences in beam and pore structure can be seen in 6 of 7 EG eyes over time. (A: OHT-63; B: OHT-64; C: OHT-65; D: OHT-66; E: OHT-68; F: OHT-69.) (G) ALCS pore structure did not change significantly over time in the EG eye of monkey OHT-67. Scale bar: 300 μm.</p

RNFLT and mean ONH parameters averaged across all study time-points for each control eye.

<p>‘—’—ONH parameters were not quantified in the control eye of OHT-65.</p><p>RNFLT and mean ONH parameters averaged across all study time-points for each control eye.</p

The first significant changes in ALCS pore geometry (relative to baseline time-points) occurred simultaneously with the first significant change measured in mean ALCSD in the EG eye of monkey OHT-64.

<p>Images of (A-C) ALCS microarchitecture and (G-I) the ONH were each acquired at baseline (left column), a representative follow-up time-point (middle column, 210 days after the initial laser treatment) that showed no significant change in ALCS pore geometry or mean ALCSD, and the time-point corresponding to the first significant change (*) in ALCS pore geometry and mean ALCSD (right-most column, 238 days after the initial laser treatment). (Note: This figure does not include all imaging time-points for this eye.) (D-F) After marking ALCS pores, mean ALCS pore geometry was quantified in central and peripheral regions (green boundaries) and in 60° sectors (fuchsia meridians). Significant increases in ALCS pore geometry were first measured in the central region and superotemporal and temporal sectors at 238 days after the initial laser treatment. (G-I) SDOCT maximum intensity projection images of the ONH showing marked ALCS points (yellow dots) from all B-scans and the BMO reference plane (red line) for the corresponding time-points in (A-C). (I) A significant increase in mean ALCSD was measured simultaneously with (F) the first significant change in pore geometry. A white asterisk (*) indicates the time corresponding to the first significantly measured change in ALCS pore or ONH geometry. Scale bar: 350 μm.</p

Longitudinal changes in IOP, RNFLT, and ONH parameters in control eyes and eyes with experimental glaucoma.

<p>Plots of (A) cumulative IOP difference, (B) RNFLT, (C) mean ALCSD, (D) mean RoC, and (E) mean MRW as a function of study time for all monkeys. The legend presented in panel (A) denotes which filled colored circles correspond to each EG eye and is also applicable to panels (B)–(E). Open white circles represent control eyes. (In [A], open white circles represent control IOP data corresponding to the right-most axis). Progressive increases were measured in cumulative IOP difference and mean ALCSD with increasing study duration, whereas progressive decreases were measured in RNFLT, mean RoC, and mean MRW in nearly all EG eyes.</p

Values of mean (± SD) pore parameters measured at baseline (No change) and at the first time-point of statistically significant change in LC pore geometry (First change) on regional (central, peripheral) and sector scales.

<p>‘*’—Statistically significant difference in a LC pore parameter compared to previous baseline time-points with no change (<i>P</i> < .05).</p><p>Values of mean (± SD) pore parameters measured at baseline (No change) and at the first time-point of statistically significant change in LC pore geometry (First change) on regional (central, peripheral) and sector scales.</p