Optic Nerve Head and Peripapillary Morphometrics in Myopic Glaucoma

Abstract

Purpose.: To investigate morphological characteristics of optic nerve head and peripapillary region with myopia and glaucoma.

Methods.: Ten normal and 17 glaucomatous myopic participants were imaged with a custom 1060-nm swept-source optical coherence tomography system. The three-dimensional images were processed and segmented for inner limiting membrane (ILM), posterior border of retinal nerve fiber layer (RNFL), Bruch's membrane (BM), and posterior border of choroid. Seven shape parameters were measured: nerve fiber layer (NFL) thickness; Bruch's membrane opening (BMO) area, eccentricity, and planarity; BMO and BM depths; and choroidal thickness. The results were analyzed by group and regional sector, and multiple regression was performed on each shape parameter with age, axial length, and glaucoma severity, measured by mean deviation (MD).

Results.: Bruch's membrane opening area (P < 0.001), eccentricity (P = 0.025), and planarity (P = 0.019) were correlated with axial length but not with MD, such that larger, more elliptical, and less planar BMO was associated with longer axial length. Several BMOs displayed a saddle-like shape configuration whose orientation appeared to be aligned with that of the BMO ellipse. All BM showed posterior deformation toward BMO such that BM closer to BMO was more posterior than that farther from BMO. Bruch's membrane depth was correlated with axial length (P = 0.014) and MD (P = 0.040) in intersubject regression, and BMO depth (P = 0.003) and BM depth (P = 0.006) were correlated with MD in intereye regression. Bruch's membrane depth was also associated with age. Choroidal thickness was negatively correlated with age (P = 0.001) and with axial length to a smaller degree (P = 0.034), but not with glaucoma severity.

Conclusions.: Axial length was a significant factor in BMO and BM shape in normal and glaucomatous myopic subjects. Posterior deformation of BM was observed in all eyes and significantly associated with functional glaucomatous damage and age.

Introduction

The pathophysiology of glaucoma, although not fully understood, involves damage to the retinal ganglion cell axons at the level of the lamina cribrosa. ^1–3 Uncontrolled intraocular pressure (IOP) likely triggers several parallel, but interacting, mechanisms including direct axonal damage, disturbances in neurometabolism and microvascular supply, glial activation, and extensive remodeling of the connective tissues of the lamina cribrosa and surrounding tissues throughout the development and progression of glaucoma. ^4–15 The peripapillary tissues that surround the optic nerve itself have also been implicated as possible contributors to glaucomatous changes, in both experimental and modeling studies. ¹⁶ Atrophic features of the peripapillary tissues that appear to be associated with glaucoma can include thinning of the peripapillary scleral tissue ¹⁷ and loss of almost all retinal and deeper layers separating the subarachnoid space from the vitreous cavity. ¹⁸  

Myopia presents unique challenges for the management of glaucoma. Population-based studies have indicated a greater prevalence of glaucoma in myopes. ^19,20 The shallow cupping and pale neuroretinal rim of high myopia make optic nerve head assessment difficult. Myopic individuals can show abnormal results on structural and functional testing because normal databases are composed of individuals with low refractive error. ^21,22 Coexisting pathologies, particularly myopic degeneration, cloud interpretation of visual field changes in advanced glaucoma. Cup-to-disc ratio and retinal nerve fiber layer (RNFL) thickness measured by commercial optical coherence tomography (OCT) and confocal scanning laser ophthalmoscopy (CSLO) were shown to be less effective in discriminating glaucomatous and nonglaucomatous subjects with high myopia, ²³ with several studies reporting RNFL thinning associated with myopia. ^24–26 There are theoretical grounds to suggest that myopic eyes may be more sensitive to a given IOP as a result of the larger globe size and thinner, more compliant tissues. ^27–29  

In studies comparing highly myopic glaucomatous eyes to nonhighly myopic glaucomatous eyes, the former showed significant histological difference in the peripapillary region, including elongation and thinning of the scleral flange. ³⁰ Comparison of color stereo optic disc photography showed more pronounced optic nerve damage, larger and more elongated optic discs, and shallower optic cups in myopic glaucomatous eyes. ^31–33  

To further understand the features of the myopic optic nerve in glaucoma, we have used a custom 1060-nm swept-source OCT system ³⁴ to image the optic nerve and surrounding peripapillary tissues in myopes, both with and without glaucoma, and performed quantitative shape measurement and analysis. 

Materials and Methods

Participants

A total of 27 subjects were recruited for this study: five young healthy controls (10 eyes, mean age = 29.8 ± 3.6 years), five older healthy controls (10 eyes, mean age = 57.0 ± 4.4), seven patients with unilateral glaucoma (14 eyes, mean age = 57.2 ± 12.4), and 10 patients with bilateral open-angle glaucoma (19 eyes, mean age = 55.7 ± 12.6). Ethics review for this study was approved from Simon Fraser University (SFU) and from the University of British Columbia (UBC). The study was conducted in accordance with the guidelines of the Declaration of Helsinki, and informed consent form was obtained from each participant. 

All participants had axial lengths greater than 24 mm. A diagnosis of open-angle glaucoma was made clinically by a fellowship-trained glaucoma specialist (PJM) based on full examination including dilated stereoscopic examination of the optic nerve, analysis of stereo disc photography, and typical reproducible Humphrey SITA-Standard white on white visual field abnormality. No reference to OCT images was made for the purposes of categorizing subjects for the study. Severity of glaucomatous visual field loss was quantified by visual field mean deviation (MD) values. Participant demographics are tabulated in Table 1 with individual subject information in Supplementary Table S1. 

Table 1

 

Demographics and Clinical Characteristics of the Study Subjects by Group

Group	No. of Subjects (No. of Eyes Used)	Age	Axial Length, mm	MD, dB
Young normal	5 (10)	29.8 ± 3.6	25.9 ± 1.4	−0.8 ± 0.6
Older normal	5 (10)	57.0 ± 4.4	25.5 ± 1.0	−0.5 ± 1.1
Glaucomatous, unilateral	7 (14)	57.2 ± 12.4	26.2 ± 0.9	−0.5 ± 0.5
Glaucomatous, bilateral	10 (19)	55.7 ± 12.6	27.1 ± 1.8	−14.6 ± 8.4

Acquisition and Preprocessing

A custom 1060-nm swept-source OCT system, developed by Biomedical Optics Research Group at SFU, was used to image the optic nerve head (ONH). ³⁴ The OCT system included in-house acquisition software that provided real-time en face and cross-sectional images to guide acquisition. The 1060-nm light source, relative to 830-nm light sources in most commercial OCT systems, more clearly visualized deeper structures such as the choroid. The swept-source configuration allowed an A-scan line rate of 100 KHz. 

The acquired three-dimensional (3D) image consisted of 400 B-scans, each with 400 A-scans, and 1024 pixels per A-scan. The imaged region in physical space spanned an axial depth of 2.8 mm and a square area of 5 × 5 to 8 × 8 mm². This area, the image dimension in the lateral direction, was calculated for each eye based on the optics of the acquisition system, scan angle, and axial length of the eye. Resulting voxel dimension was 2.7 μm in the axial direction and 12.5 to 20 μm in the lateral direction. A full volumetric image was acquired in 1.6 seconds. 

Axial motion artifact was corrected using cross-correlation between adjacent frames. ³⁵ Three-dimensional bounded variation smoothing ³⁶ was applied to reduce the effect of speckles while preserving and enhancing edges (Figs. 1a, 1b). 

Figure 1

 

Image processing and segmentation. A B-scan is shown (a) in the original form, (b) smoothed and edge-enhanced, and (c) segmented for inner limiting membrane (ILM, magenta), posterior boundary of retinal nerve fiber layer (NFL, purple), Bruch's membrane (BM, green), Bruch's membrane opening (BMO, red), and choroid–sclera boundary (CS boundary, blue). The CS boundary was defined as the outermost boundary of the choroidal blood vessels, which was consistently visible in all volumes. In (d), the segmented structures are displayed in 3D. Although shown here in a B-scan, the smoothing and segmentation were performed in 3D, not frame by frame.

Segmentation

Inner limiting membrane (ILM), the posterior boundary of nerve fiber layer (NFL), Bruch's membrane (BM), Bruch's membrane opening (BMO), and the choroid–sclera boundary (CS boundary) were segmented for this study (Fig. 1c). The four surfaces (ILM, NFL, BM, and CS boundary) were segmented automatically in 3D using a graph-cut algorithm. ^37–40 Briefly, a graph was constructed for each volume by assigning a node to each voxel and creating arcs between the nodes based on smoothness of target surfaces and distance between the surfaces. We used intensity gradient in the axial direction as the cost function such that the minimum s-t cut of the graph corresponded to smooth edges with strong intensity contrast. Inner limiting membrane and BM, which are imaged with higher contrast, were segmented first, and then posterior NFL and CS boundaries were found by limiting the search region based on the ILM and BM segmentation. Maxflow software (version 3.01, V. Kolmogorov) ⁴¹ was used to compute the minimum cut. 

The automated segmentation result was examined and corrected by a trained research engineer (SXH) using a custom script in Amira (version 5.2; Visage Imaging, San Diego, CA, USA). Segmented surfaces were overlaid on the original grayscale image and viewed simultaneously in three separate orthogonal planes for better visualization of the structural boundaries obscured by blood vessel shadows. The rater was able to scroll back and forth through the volume in any one of the three orientation views while the other two orthogonal views and location pointers were slaved and updated accordingly. 

Bruch's membrane opening was defined as the termination point of the high-reflectance BM/retinal pigment epithelium (RPE) complex on the OCT image (Fig. 1c). This corresponds to a pigmented and thus clinically identifiable structure, or a nonpigmented and thus clinically invisible structure. ^42,43 The BMO was segmented manually by a trained research engineer (SXH) on 40 radial slices extracted from the volume, intersecting at the approximated center of the BMO and spaced at a constant angle of 4.5°. Radial slices were used instead of the original raster scans because ONH is a relatively radially symmetric structure, and the radial slices provide more consistent cross-sectional views of the BMO. ⁴⁴  

Measurements

Seven shape characteristics were defined and measured on the segmented ILM, NFL, BM, BMO, and CS boundary: NFL thickness, BMO area, BMO eccentricity, BMO planarity, ⁴⁵ BMO depth, BM depth, and choroidal thickness. The parameters are graphically described in Figure 2. 

Figure 2

 

Shape parameters. (a) An example B-scan. (b) Nerve fiber layer thickness was measured as the closest distance to ILM from each point on the posterior NFL boundary. (c) Bruch's membrane reference plane was defined as the best-fit plane to BM points 2 mm away from the BMO center. (d) Bruch's membrane opening depth was measured as the normal distance between the BM reference plane and BMO center. (e) Bruch's membrane depth was measured as the normal distance between each point of BM and the BM reference plane. (f) Choroidal thickness was measured as the closest distance to BM from each point on the posterior choroid boundary. Although shown here in a B-scan, all parameters were defined and measured in the full 3D volume.

Nerve fiber layer thickness was measured at each pixel of the posterior NFL surface as the closest distance to the ILM surface. The NFL within 0.25 mm from BMO was excluded because near and inside BMO, NFL changes into vertical fiber bundles. For statistical analysis, NFL thickness was averaged over an elliptical annulus, inwardly bounded at 0.25 mm from BMO and outwardly bounded at 1.75 mm from BMO. This provided a level of anatomical consistency in averaging measurements over multiple eyes with different image and BMO sizes. 

To quantify the BMO shape, an ellipse was fitted to the segmented BMO points by first finding the best-fit plane using principal component analysis (PCA) ⁴⁶ and fitting an ellipse to the projection of the BMO points on the plane by least-squares criterion. Bruch's membrane opening area and BMO eccentricity were calculated from the fitted ellipse. Bruch's membrane opening planarity, or how much the BMO deviates from a plane, was measured as the mean of the normal distance between the segmented BMO points and its best-fit plane. 

For the BM shape parameters, BMO depth and BM depth, a BM reference plane ⁴⁵ was first established by selecting points on BM along a circle 2 mm from the center of the BMO and fitting a plane with PCA. Bruch's membrane opening depth was measured as the normal distance from the BMO center to the BM reference plane and reflects the posterior depth of BMO with respect to the BM reference plane. Bruch's membrane depth was defined as the normal distance from each pixel of the BM surface to the BM reference plane. For statistical analysis, BM depth was averaged over an elliptical annulus, inwardly bounded at BMO and outwardly bounded at 1.75 mm from BMO. Furthermore, BM depth was averaged in regional sectors as shown in Figure 3. Four elliptical annuli, with the innermost boundary of the BMO ellipse and consecutive boundaries at 0.25, 0.75, 1.25, and 1.75 mm from the BMO ellipse, were drawn. Elliptical annuli with fixed distances from the BMO ellipse were chosen over concentric circles centered at the BMO center because of considerable variability in size and eccentricity of BMO among individuals. The annuli were also divided into eight angular sectors: nasal, superior, temporal, inferior (60° each) and superior-nasal, inferior-nasal, inferior-temporal, and superior-temporal (30° each). Such sectorization allowed generating comparable group means for different regions (e.g., superior versus inferior, nasal versus temporal) as well as aggregating measurements of multiple eyes without losing all regional information. 

Figure 3

 

Sectorization. Elliptical annuli were drawn at 0.25, 0.75, 1.25, and 1.75 mm from Bruch's membrane opening (BMO). The annuli were divided into 60° angular sectors of superior (S), nasal (N), inferior (I), and temporal (T) and 30° angular sectors of superior-nasal (SN), inferior-nasal (IN), inferior-temporal (IT), and superior-temporal (ST).

Choroidal thickness was measured at each pixel of the posterior choroid boundary (CS boundary) as the closest distance to the BM surface. The choroid within 0.25 mm from BMO was excluded because choroid termination near sclera canal was often unclear and indistinguishable. Similarly to BM depth, for statistical analysis, choroidal thickness was averaged in the elliptical annulus and regional sectors as described above. 

Analysis

Subjects were divided into four groups: young normal (five subjects, 10 eyes, mean age = 29.8 ± 3.6 years), older normal (five subjects, 10 eyes, mean age = 57.0 ± 4.4), glaucoma suspect (seven subjects, seven eyes, mean age = 57.2 ± 12.4), and glaucoma (17 subjects, 26 eyes, mean age = 55.7 ± 12.6). The suspect group consisted of the apparently normal contralateral eyes of the patients with unilateral glaucoma. ^47,48 For all analysis and graphical presentation, left eyes were flipped horizontally into right-eye orientation. 

Nerve fiber layer thickness, BM depth, and choroidal thickness were mapped on two-dimensional (2D) en face color maps. Bruch's membrane depth and choroidal thickness were further plotted in the regional sectors described in the previous section. All seven shape characteristics (NFL thickness, BMO area, BMO eccentricity, BMO planarity, BMO depth, BM depth, choroidal thickness) were scatter plotted and compared between groups and against age, axial length, and MD. For BM depth and choroidal thickness, regional sectors were also compared by averaging measurements in each sector for all eyes in each group. Multiple linear regression was performed on the shape parameters against age, axial length, and MD. For the regression, only the right eye of each subject was selected to avoid artificial reinforcement of a trend due to intereye correlation. Outliers were excluded from the regression by a threshold of two standard deviations. IBM SPSS Statistics Version 19 (IBM Corp., Armonk, NY, USA) was used to perform multiple linear regression on each of the seven dependent variables against the three independent variables, as the following equation:  

 

 

Regression was performed with all eyes and repeated for two subsets: normal eyes (young normal and older normal, 10 subjects) and older eyes (all subjects age 50 or older, 18 subjects). Lastly, multiple regression was performed on intereye difference of the shape parameters with intereye difference of axial length and MD. 

Results

Out of 53 eyes, three eyes from two subjects were excluded from NFL and choroid analysis because the layer boundaries could not be segmented with confidence, either automatically or manually. Table 2 summarizes the performance of the automated segmentation by (1) percentage of correction, calculated by dividing the number of corrected pixels by the total number of pixels (400 × 400 pixels), and (2) amount of correction, calculated by the difference between the automated segmentation and manual correction averaged over all corrected pixels. Out of 202 automatically segmented surfaces, 10 surfaces (three ILM, two posterior NFL boundary, two BM, three CS boundary) from six volumes had a manual correction rate greater than 50% and were categorized as unsuccessful. These cases were attributed to severe pathological deformation, image artifact, and poor image contrast. 

Table 2

 

Performance of the Automated Segmentation of Peripapillary Structures

	ILM	NFL	BM	CS Boundary
Corrected region, %	8.0 ± 12.3	8.6 ± 10.5	7.8 ± 8.8	12.2 ± 15.3
Mean amount of correction, pixels	28.6 ± 31.0	7.6 ± 12.2	7.4 ± 13.4	18.6 ± 12.5
Mean amount of correction, mm	0.077 ± 0.084	0.020 ± 0.033	0.020 ± 0.036	0.050 ± 0.033

Figure 4 illustrates the NFL thickness mapping for all subjects by group and demonstrates the characteristic hourglass pattern of thicker NFL in superior and nasal regions relative to temporal and nasal regions. The accompanying scatter plot shows the distribution of mean NFL thickness, averaged over the region between 0.25 and 1.75 mm from BMO, between groups. As expected, NFL thickness decreases in the glaucomatous group. In multiple regression (Table 3), NFL thickness was significantly correlated with MD, and the intereye difference in NFL thickness was significantly correlated with the intereye difference in MD (Table 4). 

Figure 4

 

Peripapillary retinal nerve fiber layer (NFL) thickness. All thickness color maps are in scale and right-eye orientation. The region within 0.25 mm from Bruch's membrane opening (BMO) was excluded. The graph plots the NFL thickness averaged over the region between 0.25 and 1.75 mm from BMO, with outliers in each group marked by red circles.

Table 3

 

Multiple Regression Analyses of Shape Parameters With Age, Axial Length, and Mean Deviation (MD): Mean Nerve Fiber Layer Thickness, BMO Area, BMO Eccentricity, BMO Planarity, BMO Depth, Mean BM Depth, and Mean Choroidal Thickness

			F (Sig.)	Part Correlation Coefficients (P Value)
				Age	AL	MD
Mean nerve fiber layer thickness
All data	25	0.92	38.55 (<0.001)	-	-	0.89 (<0.001)
Normal, YN, and ON	-	-	-	-	-	-
Age-matched, 50+	16	0.86	11.95 (0.001)	-	-	0.75 (<0.000)
BMO area
All data	25	0.71	7.18 (0.002)	−0.34 (0.038)	0.69 (<0.001)	-
Normal, YN, and ON	10	0.86	5.47 (0.038)	-	0.83 (0.008)	-
Age-matched, 50+	16	0.78	6.13 (0.009)	−0.45 (0.030)	0.59 (0.007)	-
BMO eccentricity
All data	26	0.59	3.94 (0.022)	-	0.42 (0.025)	-
Normal, YN, and ON	10	0.80	3.54 (0.088)	-	0.59 (0.053)	-
Age-matched, 50+	-	-	-	-	-	-
BMO planarity
All data	25	0.69	6.70 (0.002)	-	0.40 (0.019)	-
Normal, YN, and ON	-	-	-	-	-	-
Age-matched, 50+	-	-	-	-	-	-
BMO depth
All data	-	-	-	-	-	-
Normal, YN, and ON	-	-	-	-	-	-
Age-matched, 50+	-	-	-	-	-	-
Mean BM depth
All data	-	-	-	-	-	-
Normal, YN, and ON	-	-	-	-	-	-
Age-matched, 50+	16	0.67	3.30 (0.058)	-	0.62 (0.014)	−0.49 (0.040)
Mean choroidal thickness
All data	24	0.75	8.82 (0.001)	−0.59 (0.001)	-	-
Normal, YN, and ON	10	0.84	4.92 (0.047)	−0.81 (0.010)	-	-
Age-matched, 50+	16	0.76	5.59 (0.012)	−0.56 (0.011)	−0.45 (0.034)	-

 Sig., significance; YN, young normal; ON, older normal.

Table 4

 

Multiple Regression Analyses of Intereye Difference in Shape Parameters With Intereye Difference in Axial Length and Mean Deviation (n = 26)

		F (Sig. F)	Part Correlation Coefficients (P Value)
			AL—IED	MD—IED
NFL thickness—IED	0.83	21.5 (<0.001)	-	0.745 (<0.001)
BMO area—IED	-	-	-	-
BMO eccentricity—IED	-	-	-	-
BMO plane error—IED	-	-	-	-
BMO depth—IED	0.61	5.98 (0.009)	-	0.610 (0.003)
BM bowing—IED	0.56	4.90 (0.018)	-	−0.554 (0.006)
Choroidal thickness—IED	-	-	-	-

 IED, intereye difference.

Figure 5 illustrates the delineated BMO points overlaid on the sum-voxel, en face view of the image volumes for all subjects. Red points indicate where the BMO is positioned posterior (into the page) to its plane (best-fit plane to all BMO points), and green points indicate where the BMO is positioned anterior (out of the page) to the plane. The variable correspondence between the BMO, delineated from 3D OCT image, and clinical disc margin can be seen. In Figure 6, three BMO shape parameters (area, eccentricity, mean planarity) are plotted by group and against age, axial length (AL), and MD. In multiple regression of the same parameters with age, AL, and MD (Table 3), all three parameters of BMO area, eccentricity, and planarity were significantly correlated with AL. 

Figure 5

 

Bruch's membrane opening disc margin correspondence and planarity of BMO. Bruch's membrane opening points overlaid on en face images generated by summing the 3D OCT volumes in the axial direction. Red points indicate where the BMO is posterior (into the page) to reference plane, and green points indicate where the BMO is anterior (out of the page) to the reference plane.

Figure 6

 

Bruch's membrane opening shape measurements. (A) Bruch's membrane opening area, (B) BMO eccentricity, and (C) mean BMO planarity, distributed by (i) group and versus (ii) age, (iii) axial length, and (iv) MD. Outliers in each group are marked by red circles in plots (i).

Figure 7 illustrates BM depth with respect to the BM reference plane at every point across the whole BM for each subject. Warm colors, or positive distance values, indicate that the BM surface is posterior to the reference plane; cool colors, or negative distance values, indicate that the BM surface is anterior to the reference plane. In Figure 8, BMO depth and mean BM depth are plotted by group and against age, AL, and MD. Bruch's membrane opening depth captures the degree of posterior deviation of BM at BMO. A larger BMO depth reflects a greater degree of steepness between from the BM reference plane and BMO. Mean BM depth is the mean of the absolute BM depth value, averaged over the region between BMO and 1.75 mm from BMO, and reflects the degree to which the overall BM shape deviates from a plane. A larger mean BM depth indicates a less planar BM, while a smaller mean BM depth indicates a flatter BM. In multiple regression of BMO depth and mean BM depth with age, AL, and MD (Table 3), some correlation existed between BM depth and AL and MD. More significant correlations were observed between the intereye difference of BMO depth and intereye MD difference, and also between the intereye difference in mean BM depth and intereye MD difference. 

Figure 7

 

Peripapillary BM depth. All depth maps are in scale and right-eye orientation. The BM depth is measured with respect to the BM reference plane at each point on BM.

Figure 8

 

Bruch's membrane opening and mean BM depth measurements. (A) Bruch's membrane opening depth and (B) mean BM depth distributed by (i) group and versus (ii) age, (iii) axial length, and (iv) MD (C). Intereye differences of BMO depth and BM depth are also plotted versus intereye MD difference. Outliers in each group are marked by red circles in plots (i).

Figure 9 presents the sectoral analysis of BM depth with sectors divided by both radial distance from BMO and angular sectors. In Figure 9A, BM depth is shown averaged in each sector across each group. A greater overall BM depth, indicating a greater degree of global deviation from a plane, can be seen in the older normal and glaucomatous groups compared with the young normal group. In Figures 9B and 9C, BM depth is plotted by angular sectors only, starting from temporal and proceeding clockwise to inferior-temporal region. In Figure 9B, all BM depth points in the same angular sectors were averaged, each sector extending from BMO to 1.75 mm from BMO. In Figure 9C, only the BM depth points between BMO and 0.25 mm from BMO were averaged by angular sectors. In both cases, BM depth is smaller in young normal eyes compared to older normal and glaucomatous eyes, with a general pattern of smaller BM depth in the nasal region. 

Figure 9

 

Bruch's membrane depth sectoral analysis. (A) Sectorized group averages of Bruch's membrane (BM) surface depth. The color in each sector indicates the mean absolute magnitude of the normal distance between BM and its fitted plane. Elliptical annuli are drawn at 0.25, 0.75, 1.25, and 1.75 mm from Bruch's membrane opening (BMO). The annuli are divided into 60° angular sectors of superior (S), nasal (N), inferior (I), and temporal (T), and 30° angular sectors of superior-nasal (SN), inferior-nasal (IN), inferior-temporal (IT), and superior-temporal (ST). (B) Average BM depth by angular sector for the whole BM surface. (C) Average BM depth by angular sector for the inner annulus only (0–0.25 mm distance from BMO).

Figure 10 illustrates choroidal thickness at every point across the whole choroid for each subject. Compared to NFL thickness or BM depth, there is a larger individual variability in the magnitude and spatial pattern of choroidal thickness. There is, however, visible similarity between fellow eyes. The young normal group generally exhibited thicker choroid. 

Figure 10

 

Peripapillary choroidal thickness. All thickness color maps are in scale and right-eye orientation. The region inside and within 0.25 mm from Bruch's membrane opening (BMO) was excluded.

In Figure 11, mean choroidal thickness, averaged over the region between 0.25 and 1.75 mm from BMO, is plotted by group and against age, AL, and MD. In multiple regression with age, AL, and MD (Table 3), choroidal thickness was significantly correlated with age, and within the age-matched group (50+), also with axial length. 

Figure 11

 

Choroidal thickness measurements. Choroidal thickness distributed by (i) group, and versus (ii) age, (iii) axial length, and (iv) MD.

Figure 12 presents the sectoral analysis of choroidal thickness, similarly to Figure 9 for BM depth. In Figure 12, all groups display the thickest choroid in the superior or superior-nasal sector and the thinnest choroid in the inferior sector. 

Figure 12

 

Choroidal thickness sectoral analysis. (A) Sectorized peripapillary choroidal thickness. Elliptical annuli are drawn at 0.25, 0.75, 1.25, and 1.75 mm from Bruch's membrane opening (BMO). The annuli are divided into 60° angular sectors of superior (S), nasal (N), inferior (I), and temporal (T), and 30° angular sectors of superior-nasal (SN), inferior-nasal (IN), inferior-temporal (IT), and superior-temporal (ST). (B) Average choroidal thickness by angular sector.

Discussion

In this study we have examined RNFL thickness, BMO shape, BM planarity, and choroidal thickness in myopic subjects both with and without glaucoma. Most patients had asymmetric glaucoma, which allowed us to also compare the intereye difference in the degree of glaucoma and axial length with the intereye difference in the shape parameters, thus controlling for large intersubject differences. We found that the dimension and shape of BMO tended to change with axial length but not with age or degree of glaucoma. Peripapillary BM position was associated with distance from BMO such that BM was more posterior closer to BMO. Large variability in BM depth was noted between subjects; but within subjects, our analysis revealed an association between degree of glaucoma and BM depth. Finally, choroidal thickness appeared to decrease with age but not with the presence of glaucoma. 

In displaying images in en face view and NFL thickness, BM depth, and choroid thickness color maps, we chose to use the same size scale for all images rather than scaling the images to the same presentation size. Differences in scan size were due to the axial length differences between the eyes, and a common scale allowed a truer, in-scale visual comparison, including that of varying BMO size and level of BM stiffness among the subjects. 

In the sectoral analysis, the sectors were divided by angles measured with reference to the acquired image frame. A more anatomically consistent approach would be using the axis between the center of the BMO and fovea (foveal–BMO axis), which aligns with the direction of the nerve fiber bundles. ⁴⁹ It has been shown that there is intrasubject and intersubject variability in the correspondence between the acquired image frame and the foveal–BMO axis ^43,50 ; using the foveal–BMO axis in future studies will reduce the effect of individual variability and result in more anatomically equivalent and comparable sectors across multiple individuals. 

The eyes that were grouped as “suspect” in this study were the normal-appearing fellow eyes of subjects with glaucoma. No reference to OCT imaging was made during diagnosis. 

Outliers in the dataset were included in the scatter plots but excluded from the multivariate regression analyses. The most extreme example was a glaucomatous eye with axial length of 31.3 mm, which was also a full millimeter longer than that of its fellow (glaucomatous) eye. At such extreme axial lengths of pathologic myopia, different mechanisms may be at play than generally seen in the rest of the dataset. With the exception of this eye, our axial lengths were between 24 and 30 mm, and the results are applicable only to this range. 

Nerve Fiber Layer Thickness

In nonmyopic eyes, characteristic ONH features are more apparent. In myopic eyes, however, the myopic tilt, shallow laminar position, decreased contrast of neuroretinal rim tissues, and peripapillary degenerative changes can make preperimetric glaucoma detection more difficult. Optical coherence tomograpy imaging of the NFL can be particularly beneficial in these patients. 

As expected, we observed decreasing NFL thickness in subjects with glaucoma and significant negative correlations between NFL thickness and severity of glaucoma quantified by MD (Fig. 4; Table 3). These findings agree with a large body of research on NFL in glaucoma ^51–54 and provided an internal control for imaging, segmentation, and analysis method in our study. Among the studies on NFL thickness of normal myopic subjects, Bendschneider et al. ⁵⁴ and Budenz et al. ⁵⁵ found significant correlations in NFL thickness with both age and axial length, whereas Leung et al. ⁵⁶ and Rauscher et al. ⁵⁷ reported no correlation with age but with axial length. In age-matched studies, several groups ^24–26 reported significant correlation between NFL thickness and axial length, while the last saw large variability in the correlation depending on the quadrant, with the inferior quadrant showing the highest correlation and the temporal quadrant showing the weakest correlation. Hoh et al. ⁵⁸ found no significant correlation between NFL thickness and axial length among 132 young males. Our measurement of NFL thickness was not correlated with age or axial length. The varying results of previous studies possibly indicate that age and axial length, compared to MD, are more subtle and easily confounded factors; and the small sample size and presence of glaucoma patients in this study likely made it difficult to detect meaningful influence by age or axial length. 

Bruch's Membrane Opening Shape

We quantified BMO shape by three parameters: area, eccentricity, and planarity. Figure 5 illustrates the large variability in BMO shape between myopic patients with and without glaucoma. The figure also demonstrates the large variability in the correspondence between the clinical disc margin and BMO between myopic patients, again with and without glaucoma. This supports an increasing body of evidence that the clinical disc margin is clinically heterogeneous, even within an individual eye, ^{42,43,59–63} and suggests that similar issues also exist in myopes. 

We observed larger BMO area with increasing axial length (Fig. 6A; Table 3). This finding is not artifactual since the pixel dimension was corrected for axial length, and it is consistent with previous studies showing increasing disc area with increasing axial length. ^64–66 We also noted a small but significant (P < 0.05) negative correlation between age and BMO size. This may represent type I error in our population and needs to be further investigated. However, if confirmed, we speculate that this could reflect the presumed greater compliance of younger tissues, resulting in outward expansion of BMO as seen in nonhuman primate (NHP) experimental glaucoma. ^3,67–69 Bruch's membrane opening eccentricity increased with increasing axial length; that is, as axial length increased, the BMO tended away from a circular to a more elliptical shape (Fig. 6B; Table 3). Regarding the orientation of the BMO ellipse, or the direction of its major axis, 61%, or 32 out of a total 52 BMOs, were oriented in the nasal-temporal direction; 23%, or 12 BMOs, were oriented in the superior-nasal–inferior-temporal direction. Seven BMOs were oriented in the superior-temporal–inferior-nasal direction, and only 1 BMO was oriented in the superior-inferior direction. This pattern can be seen in Figure 5. Bruch's membrane opening planarity (by which we measure how much BMO deviates from a plane) also appeared to increase slightly with axial length (Fig. 6C; Table 3). Bruch's membrane opening planarity can be related to a small but consistent saddle configuration of BMO we observed (Fig. 5). In this saddle configuration, BMO tended to be posteriorly displaced along its long axis and anteriorly displaced along its short axis. Furthermore, BMO planarity was indeed correlated with BMO eccentricity (Pearson correlation = 0.613, P = 0.001), such that a more elliptical BMO was correlated to more deviation from a plane. It should be noted this BMO saddling is small in magnitude (∼0.03 mm on average) relative to the length of BMO (∼1 mm on average), and BMO is still a relatively planar structure. We are unsure whether the BMO saddling is a feature unique to myopes, how it reflects underlying stresses and strains on BM, or whether it corresponds to local variability in deep ONH morphology such as the recently reported horizontal laminar ridge. ⁷⁰ We are currently analyzing the 3D morphology of BM in more detail in a greater number of subjects and hope to investigate these questions in future studies. 

In summary, BMO appeared to become larger, more elliptical, and less planar with increasing axial length. No relationship was seen between glaucoma severity (as measured by MD) and BMO shape, which agrees with previous studies. ^71,72 The change in BMO shape associated with longer axial length in this study may be a ramification of global structural change in myopia, including elongation of the eye due to growth and remodeling mechanism driven by visual error signal. 

Peripapillary Bruch's Membrane Shape

All eyes showed increasing posterior deformation of BM with increasing proximity to BMO (Figs. 7, 9B). Within the same subject, there appeared to be a good intereye correspondence in the degree of posterior deformation. In BMO depth and mean BM depth, no significant group difference was observed between the nonglaucomatous and glaucomatous eyes (Figs. 8A, 8A). However, a significant correlation existed between the intereye differences of BMO depth and mean BM depth with the intereye MD difference (Fig. 8C; Table 4), suggesting that when intersubject variance is partitioned, greater BMO depth and BM depth are associated with a greater degree of glaucomatous damage. Similar change was also observed in a longitudinal study of experimental glaucoma in NHPs. ⁴⁵ These data are illustrated in a sectoral analysis in Figure 9, which presents, with small but consistent associations with the presence of glaucoma, a visible BM depth increase with age. The statistical significance of the group mean difference between the young normal (n = 5) and older normal subjects (n = 5) was 0.084 with the right eyes and 0.037 with the left eyes, suggesting a relationship that would possibly be more apparent with a larger dataset. Taken together, the results suggest that in the myopic subjects under study, despite the large intersubject variability in BM position, there is an association of BM depth with age and a smaller but consistent association with degree of glaucoma. Axial length was also correlated with BM depth among the older, age-matched normal and glaucoma subset of the data. The exact shape and regional pattern of this deviation requires further investigation to analyze the slope, change of slope (bending/bowing), and differences and similarities in the effect of myopia and effect of glaucoma. We are currently using a combined surface and volume registration technique to further characterize these differences. ⁷³  

The increase in BM depth may be a direct mechanical result of increased IOP or a secondary deformation resulting from tissue remodeling of deeper structures, such as the lamina cribrosa and peripapillary sclera, in a complex interplay of aging, myopia, and glaucoma. The peripapillary sclera was shown to become posteriorly deformed and displaced in early glaucomatous monkey ONH, ¹⁷ and the peripapillary scleral thickness near the scleral canal and optic nerve meninges was shown to decrease significantly with increasing axial length. ⁷⁴ Posterior cupping of the lamina cribrosa (LC) and posterior migration of the laminar insertion in the connective tissue remodeling in response to elevated IOP in glaucoma ^14,17,75 may also influence posterior deformation of BM. Ocular elongation and IOP both influence ONH remodeling, and it is an important challenge to understand the combined impact and mechanism, particularly in the context of higher glaucoma susceptibility among people with advanced myopia. 

Myopia, especially high myopia, has been a complicating factor in glaucoma in that it is associated with structural changes in the peripapillary region. Optic disc area and area of the peripapillary region with chorioretinal atrophy were correlated with degree of myopia along with disc elongation, ^64,76–80 and the LC was shown to be thinner in highly myopic eyes than in nonhighly myopic eyes. ⁸¹ In highly myopic glaucomatous eyes, compared to nonhighly myopic glaucomatous eyes, optic disc area, elongation, cup length, and peripapillary atrophy were significantly larger, ^32,33 LC was thinner, ⁸¹ and rim loss was greater. ³¹ In our study of myopic glaucomatous subjects, we aimed to observe, in relation to myopia and glaucoma, not only the changes in RNFL but also their effect on Bruch's membrane and the opening. These are relatively robust structures not directly subject to glaucomatous atrophy, and thus better indicators of mechanical or pressure-related deformation associated with both myopia and glaucoma. A recent study by Johnstone et al. ⁸² suggests posterior migration of BMO with age in relation to age-related choroidal thinning. This questions the appropriateness of the BMO as a reference structure in shape measurement. However, in establishing the reference plane in this study, we used BM points 2 mm outward from the BMO centroid. ⁴⁵ A typical model of ONH deformation in glaucoma, originating from the LC region and including cupping, also suggests that the BM may be more mechanically stable at its peripheral region than at its opening. In summary, our study demonstrated large differences between myopic subjects in peripapillary measurements with smaller but consistent peripapillary changes associated with glaucoma. 

Choroidal Thickness

Thinner mean choroidal thickness was associated with increased age, and also with axial length among older, age-matched subjects (Table 3). Mean deviation was not a significant factor in choroidal thickness. These results are in agreement with several studies that have reported macular choroidal thinning with both age ⁸³ and high myopia. ⁸⁴ Maul et al. ⁸⁵ found that peripapillary choroidal thickness was associated with age, axial length, central corneal thickness, and also diastolic ocular profusion pressure in glaucoma suspects and patients. In recent studies, choroidal thickness was not correlated with glaucomatous damage. ^85–87 In our sectoral analysis (Fig. 12), we observed a regional pattern of the thickest choroid at superior or superior-nasal region and the thinnest in inferior region. 

Conclusions

We have demonstrated a computational pipeline for shape analysis in normal and glaucomatous myopic eyes. We found that in myopes with axial lengths between 24 and 30 mm, increasing axial length was associated with deviation from a circular, planar BMO, but BMO shape was not associated with age or functional glaucomatous damage. Posterior deformation of the peripapillary BM was seen in all myopes, although highly variable between subjects, and associated with the degree of functional glaucomatous damage. 

Supplementary Materials

pdf S1 

Acknowledgments

Supported by Canadian Institutes of Health Research (CIHR), Natural Sciences and Engineering Research Council of Canada (NSERC), and Michael Smith Foundation for Health Research (MSFHR). 

Disclosure: S. Lee, None; S.X. Han, None; M. Young, None; M.F. Beg, None; M.V. Sarunic, None; P.J. Mackenzie, None