Angular Location of Retinal Nerve Fiber Layer Defect: Association With Myopia and Open-Angle Glaucoma

Abstract

Purpose: To compare retinal nerve fiber layer (RNFL) defects' angle measurements determined from the center of the optic disc and Bruch's membrane opening (BMO), as a function of myopia and open-angle glaucoma (OAG) subtypes.

Methods: In total, 118 patients with OAG were grouped by axial length (AL; high myopia, AL >26 mm; mild to moderate myopia, 24 ≤ AL ≤26 mm; nonmyopia, AL <24 mm) and OAG subtype (normal-tension glaucoma [NTG], high-tension glaucoma [HTG]). The disc and BMO centers were determined by a merged image of red-free fundus photography and spectral-domain optical coherence tomography. The angular location of the RNFL defect close to the fovea (angle α) was measured from the disc center and BMO center, respectively (angle αdisc and angle αBMO). The difference between angle αdisc and αBMO (Δα), as well as the RNFL defect width (angle γ), was evaluated.

Results: Angle αdisc was smaller in myopic eyes and correlated significantly with AL (P = 0.001), whereas it did not differ among OAG subgroups. Angle αBMO and angle γ were not different in the myopic and OAG subgroups. The Δ α was larger for eyes with higher degree of myopia and had significant correlation with AL (P < 0.001) and was larger in NTG eyes than in HTG eyes (P = 0.023).

Conclusions: The angular location of the RNFL defect measured from the disc center, but not from the BMO center, was closer to the fovea for glaucomatous eyes with higher values of AL. The present study may facilitate understanding of the characteristic locational pattern of the RNFL defect in myopic glaucomatous eyes.

Figure 1.

Angular parameters of the RNFL photography. (Left) Red-free fundus photograph and infrared fundus image with delineated BMO margin merged. The disc center (green dot), BMO center (red dot), BMO margin (orange dot line), and the RNFL defect margin (yellow dot line) are marked. (Right) Magnified photograph. Two reference lines extend from the fovea to the disc center and BMO center, respectively, (1) foveo-disc axis (a) and (2) foveo-BMO axis (b). The minimum angle between the reference line and a line from the center of the disc or BMO to the margin of the RNFL defect (yellow dot) close to the fovea (angle α), and a line to the margin of the RNFL defect (yellow dot) further from the fovea (angle β), was measured from the two reference lines: (1) angle α_disc and angle β_disc and (2) angle α_BMO and angle β_BMO. Angle γ is the angular width of the RNFL defect, as calculated by the difference between angle β_BMO and angle α_BMO.

Figure 2.

Scatterplots presenting the association with axial length and angular parameters by linear regression analysis. Black lines indicate the best-fit linear regression line. r = correlation coefficient from the fitted linear regression model. (A) Angle α_disc had significant correlation with axial length (r = –0.327, P = 0.001). (B) No significant correlation was demonstrated between angle α_BMO and axial length (r = –0.080, P = 0.39). (C) Angle α difference had significant correlation with axial length (r = 0.471, P < 0.001). BMO = Bruch's membrane opening.

Figure 3.

Representative cases of RNFL defect compared for disc center and BMO center in eyes with nonmyopia, mild to moderate myopia, and high myopia. RNFL photographs merged with spectral-domain optical coherence tomography images locating the disc center (green dot), the BMO margin (red dotted line), the BMO center (red dot), and the RNFL defect (yellow dotted line). Note the position of the disc center relative to the BMO center. (A) Left eye of a 62-year-old man with nonmyopia and AL of 23.6 mm. He was diagnosed with HTG, and the baseline MD was –2.25 dB (angle α_disc: 43.3°, angle α_BMO: 45.7°, Δα: 2.3°, angle γ: 39.4°). (B) Left eye of a 46-year-old woman with mild to moderate myopia and AL of 24.99 mm. She had previously been diagnosed with HTG, and the baseline MD was –4.32 dB (angle α_disc: 39.7°, angle α_BMO: 45.9°, Δα: 5.2°, angle γ: 45.1°). (C) Left eye of a 64-year-old man with high myopia and AL of 26.7 mm. He was diagnosed with normal-tension glaucoma, and the baseline MD was –5.54 dB (angle α_disc: 34.5°, angle α_BMO: 43.4°, Δα: 8.9°, angle γ: 40.5°).

Figure 4.

Schematic summary of the anatomic change with lamina cribrosa shift (LC): (A-1) Before LC shift. (Top row) High viewpoint tilted diagonally. (Middle row) High viewpoint straight down. The overlying layer consists of a healthy RNFL (bright yellow), RNFL defect (dark brown), and the BMO (orange plane). The underlying layer consists of the LC (light-brown plane). *The funduscopically visible disc margin is marked by a dotted line, which does not always consistent with the BMO. (A-2) After LC shift in myopia. (Top row) High viewpoint tilted diagonally. (Middle row) High viewpoint straight down. During axial elongation, the underlying initial LC (light-brown plane) shifts in the red-arrow direction (hatched light-brown plane), which induces a shift (gray arrow) of the vulnerable LC area. Contrastingly, the location of RNFL bundle trajectories relative to the BMO remains relatively stable. The optic disc is indicated as the oval bright orange plane with a dotted line. *The optic disc margin is marked by a dotted line, as it does not correspond to a unique anatomical structure but to a complex interaction of structures.^, Reference lines extending from the fovea (transversal gray dotted lines) to the disc center (green dot) and BMO center (red dot) are lined up with the margin of the RNFL defect close to the fovea, respectively. Note the angular location of the RNFL defect (angle α) compared from the disc and BMO center; angle α_disc (green angle) versus angle α_BMO (red angle). As the LC shifts during axial elongation, the angle α_disc gets smaller, compared to the stable angle α_BMO. The red line passes over the clinically identified optic disc margin (black dotted line). (B, C) Schematic representation of changes occurring on border tissue configuration during LC shift. (B) Border tissue structure before LC shift: internally oblique relative to the underlying sclera. (C) Border tissue structure after LC shift: externally oblique relative to the underlying sclera. Modified from the figure previously published in Reis ASC, Sharpe GP, Yang H, et al. Optic disc margin anatomy in patients with glaucoma and normal controls with spectral domain optical coherence tomography. Ophthalmology. 2012;119:738–747.