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Case report: what gives the myopic tilted disc an oval appearance?
BMC Ophthalmology volume 20, Article number: 20 (2020)
Myopic tilted disc, observed as an oval disc, has been alleged to be a funduscopic en-face manifestation of excessive optic nerve head (ONH) sloping or tilting. Here, we report the case of a myopic child showing a developing oval disc in fundus photos during axial elongation, but without progressive tilting in spectral-domain optical coherence tomography (SD-OCT) images.
By merging B-scan SD-OCT images of the ONH and macula, the curvature of the posterior pole, including both the fovea and ONH, was reconstructed and compared before and after 2 years of axial elongation. Despite the marked increase of disc ovality, the posterior polar curvature was rarely changed. The preponderance of optic disc change was induced by the shift of the temporal disc margin in the nasal direction. This shifting alone imitated an increase of tilt angle but one that was still far smaller than the required degree of tilt for ONH-tilt-based disc ovality. To clarify, we calculated the required extent of axial elongation to obtain a substantial degree of ONH tilt when considering the adjacency of the fovea and the ONH. Without a focal increase of posterior polar curvature, which is to say posterior staphyloma, such change is not possible until the axial length increases extraordinarily.
The most prominent change in the development of myopic tilted disc, which change gives it an oval appearance and imitates a tilt when measured, is actually not a tilt but rather a shift of the temporal disc margin.
As an eyeball is a sphere, the fundus is a curved plane, and both the optic nerve head (ONH) and fovea are on the surface of that curved plane, with some sloping of the former relative to the latter. Myopic tilted disc has been alleged to be a funduscopic en-face manifestation of excessive ONH sloping or tilting in this curved plane [1,2,3]. In the Boramae Myopia Cohort Study [4,5,6], however, we theoretically showed that disc ovality cannot be explained by the aspect of tilt. We found that the inner retinal structure of the posterior polar area including the Bruch’s membrane opening was relatively preserved during axial elongation, while the outer load-bearing structure expanded. This expansion of the sclera and consequent shift of the lamina cribrosa from the preserved Bruch's membrane opening may result in the change of ONH shape that is seen in myopia [4,5,6]. Herein, we report a case developing oval disc in fundus photos, but without progressive tilting in spectral-domain optical coherence tomography (SD-OCT) images.
This girl had been enrolled in the Boramae Myopia Cohort Study when she was 8 years old, and was observed for 2 years. Her best-corrected visual acuity was 20/20 in the right eye throughout the entire study period, while her refractive error worsened from − 7.5 Dsph = − 0.50 Dcyl × 180 A to − 8.75 Dsph = − 1.50 Dcyl × 180 A. During the same period, her axial length increased from 24.62 mm to 25.7 mm, and the horizontal optic disc diameter had been reduced to 0.772 of its original size (Fig. 1). If tilting or sloping was the reason, its’ angle should have been 39.5° by arccosine (0.772) . Here, to show the change of posterior polar curvature, we merged the B-scan SD-OCT images of the macula and optic nerve head. Except for small differences in the nasal curvature, the posterior polar curvatures of Bruch’s membrane and the anterior sclera were nearly identical between the initial and final visits (Fig. 1c). There was almost no progressive ONH tilting or sloping of the ONH. The majority of optic disc change was induced by the nasal elongation of the scleral layer uncovered by the Bruch’s membrane: γ-zone parapapillary atrophy.
The absence of progressive tilting is obvious when we consider how closely the ONH and the fovea are located in comparison with the whole eyeball size on MRI (Fig. 2a). The ONH is very close to the fovea (within 12° from the external view)  relative to the horizontal diameter of the eye. If axial growth is modeled mathematically in the rectangular coordinate system, the ONH tilt angle could be back-calculated from tangential lines at the points of the optic disc (d1 and d2) of a sphere and an ellipsoid (Fig. 2b), which reveals that a unit sphere has to be elongated 2 times for the initial tilt angle of α to be 2 α (Appendix). This means that this degree of tilt would not be acquired until the axial length has increased from 24 to 36 mm, even if we assume that axial growth occurs only in the posterior half of the eyeball. The disc tilt angle (angle α) of emmetropic eyes was reported to be 2.4° in a multicenter cohort study . Therefore, even extraordinary growth can produce only a negligible degree of additional tilt in this axial growth model.
Moreover, many myopic eyes have a growth pattern that is focused on the equatorial region while preserving the posterior curvature [8, 9]. This would make optic disc tilt almost impossible. Instead, we think that shifting of the temporal disc margin might imitate progressive tilting (Fig. 1c). During axial elongation, the temporal ONH border is rotated from internally oblique to externally oblique, and then, the externally oblique border is elongated nasally [4,5,6]. In the area of the externally oblique border, the temporal disc margin is reported to coincide with the anterior scleral opening . Therefore, increased disc ovality is related to nasal shifting of the temporal disc margin. Theoretically, a tilt angle  increases as a result of a temporal disc margin shift (from point A to point B in Fig. 2d, shown exaggeratedly) and a stable nasal disc margin (point D in Fig. 2d). This is due to the fact that the angle would be measured more obliquely on the periphery of the curvature (Fig. 2d). In our case, the disc margin shift alone imitated a tilt of 4.4° (Fig. 1c), similar to the tilt angle difference of 3.6° between myopia and emmetropia groups in a multicenter study . More tilt will necessitate a steeper curvature (=smaller radius) in the posterior pole: posterior staphyloma (Fig. 2d, orange dotted line) .
In describing myopic oval disc thus far, we have been preoccupied by the concept of tilt and have pointed to the eyeball’s spherical shape as a cause. However, almost no progressive change of posterior polar curvature was observed despite huge change of disc ovality, and considering the adjacency of the fovea to the optic disc, the ONH tilt angle cannot exceed a very limited range. Therefore, the most prominent change in development of myopic oval disc is not a tilt rather a shift.
Availability of data and materials
The data supporting the conclusions of this article are contained within the manuscript. The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Optic nerve head
Spectral-domain optical coherence tomography
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This work was supported by a clinical research grant-in-aid from the Seoul Metropolitan Government Seoul National University (SMG-SNU) Boramae Medical Center (grant no. 03–2019-3). The funders had no role in study design, data collection and analysis, interpretation of data, decision to publish, or preparation of the manuscript.
Ethics approval and consent to participate
The data from this study belong to the study that was approved by the Institutional Review Board of Seoul National University Boramae Medical Center and conformed to the tenets of the Declaration of Helsinki. All of the participants’ parents provided written informed consent before enrollement.
Consent for publication
Written informed consent was obtained from the patient’s parent for publication of this case report and any accompanying images. A copy of the written consent is available for review by the Editor of this journal.
The authors declare that they have no competing interests.
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Let us imagine a unit sphere and an ellipse elongated c times along the y axis to simulate prolate axial growth.
The equations of the unit sphere (blue line) and the ellipse (red line) are
The optic disc locations before and after the axial elongation are designated as d1 and d2, respectively. Since the foveo-Bruch’s membrane distance is preserved during axial elongation, d1 and d2 would share the same x values: (k, m) and (k, n).
The tangential line at d1 is:
The tangential line at d2 is:
Now, since d1 (k, m) and d2 (k, n) are on the sphere and ellipse, respectively,
If |x| ≪ 1, we can approximate tan x to x, because the Taylor series expansion for the tangent function is as follows.
The ONH is very close to the fovea relative to the horizontal diameter of the eye (k ≪ 1 ).
∴|k| ≪ |m| < |n|, and |tanα| < |tanβ| ≪ 1
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Lee, K.M., Kim, M. & Kim, S.H. Case report: what gives the myopic tilted disc an oval appearance?. BMC Ophthalmol 20, 20 (2020). https://doi.org/10.1186/s12886-020-1305-9
- Myopic tilted disc
- Boramae myopia cohort study