Glaucomatous changes in lamina pores shape within the lamina cribrosa using wide bandwidth, femtosecond mode-locked laser OCT

Abstract

Purpose: The lamina cribrosa (LC) is known to play a critical role in the pathogenesis of glaucoma. Although it has been reported that striae-shaped or slit-shaped lamina pores are more frequent in eyes with primary open angle glaucoma (POAG), this observation is based only on fundus photography. The primary object of this study is to perform layer-by-layer comparisons of the shape of lamina pores within the LC in vivo.

Design: Cross-sectional study.

Methods: Optic nerve head B-scans were obtained using custom-made broad-wavelength optical coherence tomography with a mode-locked laser. A total of 300 single B-scans per eye were obtained and three-dimensional images were rendered from these image sequences to obtain 2-μm thin-slice en face images of the LC. Elongation indices (EIs) of the lamina pores were measured from the anterior surface (AS) of the LC to the deeper layers in 40-μm increments.

Results: Thirteen eyes from 10 primary open angle glaucoma (POAG) patients of mean deviation -15.2 (-16.5, -12.9) (median [25,75 percentile]) dB and 10 eyes from 7 normal controls were studied. Although the EI value was not significantly different between the superior, temporal and inferior regions of the LC at any depth level in either group, it was greater at the AS than at the 40 μm and 80 μm depth levels (P < .001) in both groups, and was greater in the POAG group only at the AS and 40 μm depth level (P ≤ .05). After adjustment for age and refraction, the effects of depth and presence of POAG on the EI value remained significant. Also, the severity of glaucoma and depth were significant factors associated with EI in multivariate analysis.

Conclusions: Elongation of lamina pores was significantly more evident at the anterior surface and the 40-μm depth level of the LC in POAG eyes than in normal eyes, suggesting that nerve fiber bundles passing through the LC were under greater stress in the anterior layers of the LC.

Fig 1. Diagram depicting the creation of an en face optic disc image from A-scan and B-scan images obtained with the optical coherence tomography device.

Fig 2. Schematic diagram of the 3 measurement regions in a left eye.

The dotted line connecting the centroid of the disc margin and the fovea was designated as the reference line. A measurement sector at 0° to ±45° relative to the fovea center of disc axis was defined as the temporal region and the 45° to 135° circumferentially superior and inferior directions as the superior and inferior regions.

Fig 3. Schematic explanation of the method of acquisition of en face images.

(A) Photographic image of a fundus (B) The anterior surface (AS) of the lamina cribrosa (LC) was determined from a B-scan image. Depths of 40 μm and 80 μm were determined from the AS images. (C–E) Each en face image of the AS of the LC (C), 40 μm from the AS (D) and 80μm from the AS (E).

Fig 4. Schematic explanation of the method for measuring the elongation index (EI) for the lamina pores in the lamina cribrosa (LC).

(A) En face image and line scan (red arrow). (B) The reflectivity of the lamina pore is tabulated using the Image J program. The boundary of the lamina pore (red arrow) was defined as the point where its reflectivity is lower than the mean lamina beam reflectivity (blue dot line). (C) En face image (left) and magnified view (right). Red dot square corresponds to a magnified view. The red dot line is the fovea-disc line and blue dot line divides each region. (D) Representative pore analysis. In this case, the longest diameter is 20.4 pixels (a) and the shortest diameter is 9.3 pixels (b) for one pore (red arrows), which means that the EI is 2.19. For another pore, the longest diameter is 22.2 pixels (c) and the shortest diameter is 7.5 pixels (red arrows) (d), which means that the EI is 2.96.

Fig 5. Box plot for the elongation index (EI) and region analysis.

Fig 6. Box plot for the elongation index (EI) and depth analysis.