Background
Axial myopia is characterised by an elongated eyeball, which can lead to various structural changes within the eye. This white paper explores the nonpathological ocular changes associated with axial myopia. Changes related to mild to moderate myopia are compared to those with high myopia. The authors utilised histomorphometric and clinical studies to analyse the qualitative and quantitative aspects of the myopic eye compared to non-myopic controls.
Orbit and ocular shape
Emmetropic eyes are typically slightly prolate or spherical in shape. However, when the eye undergoes myopic axial elongation, it changes to become more elongated, resembling a prolate ellipsoid. This change occurs primarily in the location between the equator and the posterior pole (retro-equatorial region). Studies have shown that the density of photoreceptors and retinal pigment epithelium (RPE) cells, as well as the overall thickness of the retina, decrease with increasing axial length, particularly in the retro-equatorial region.
The enlargement of the eye wall in myopic eyes is not limited to purely axial elongation. The horizontal and vertical diameters of the eye also increase to a minor extent, along with a slight enlargement of the eye wall in the pre-equatorial region. This finding helps explain why the Bruch’s membrane opening (BMO) of the optic nerve head (ONH) also enlarges in myopic eyes. The strain within Bruch’s membrane (BM), caused by the increased dimensions of the globe, can lead to the expansion of the BMO and the development of secondary BM defects in the macular region.
The myopic enlargement of the eye wall primarily occurs in the retro-equatorial and equatorial regions, consistent with evidence suggesting a feedback mechanism regulating axial elongation in the mid-peripheral region of the eye. This enlargement aligns with clinical observations of a posterior shift of the BMO toward the fovea and explains other characteristics seen in axial myopia, such as BM overhanging into the intrapapillary compartment at the nasal optic disc border, an vertically ovalized optic disc shape, and the absence of BM in the temporal parapapillary region (parapapillary gamma zone).
Optic nerve
In moderately myopic eyes, the shape of the optic disc changes from mostly circular to oval, usually vertically oval. Highly myopic eyes tend to have a larger optic disc and optic nerve head (ONH) canal compared to moderately myopic and emmetropic eyes. Optic disc enlargement in highly myopic eyes is associated with lengthening and thinning of the lamina cribrosa, potentially contributing to glaucoma-like optic neuropathy. In highly myopic eyes, stretching of the lamina cribrosa and flattening of the parapapillary tissue lead to a flattening of the optic cup, making it challenging to detect optic nerve damage. Optic disc enlargement in highly myopic eyes is accompanied by enlargement of the BMO, resulting in the retraction of nasal overhang of the Bruch’s membrane into the parapapillary region and the development of a circular parapapillary gamma zone. The shape of the optic disc in highly myopic eyes shows high interindividual variability, with the longest axis being vertically, obliquely, or sometimes horizontally oriented. In extremely myopic eyes, a backward pull of the optic nerve, potentially due to the optic nerve dura mater, can influence the optic disc shape, leading to a vertically oval shape, rotation of the ONH, and even sagittal rotation toward the fovea.
In highly myopic eyes, there are two parapapillary zones called the gamma and delta zones (see Figure 1).
Figure 1.
From: IMI—Nonpathological Human Ocular Tissue Changes With Axial Myopia Invest. Ophthalmol. Vis. Sci.. 2023;64(6):5. doi:10.1167/iovs.64.6.5
Figure Legend: Clinical photograph of a highly myopic eye with parapapillary gamma zone (green arrows) and parapapillary delta zone (black arrows).
- The gamma zone is an area around the optic nerve head where there is no BM present. It occurs in moderately myopic eyes because the optic nerve canal is misaligned, causing BM to overhang into the nasal side of the optic disc and be absent in the temporal side. With myopia progression, the BMO enlarges, so that in highly myopic eyes the intrapapillarily overhanging part of BM is retracted and a gamma zone is present circular around the optic disc.
- The delta zone is a region within the gamma zone characterized by an elongated and thinned part of the tissue surrounding the optic disc.
The axial elongation-related enlargement of the BMO and enlargement of gamma zone and delta zone lead to an enlargement of the blind spot in the visual field due to absence of photoreceptors in these regions.
Retina
With a longer axial length, there is a decrease in photoreceptor and retinal pigment epithelium (RPE) cell density, especially in the retro-equatorial region. This is coupled with a reduction in total retinal thickness in that region Retinal thickness in the macular region is affected by axial elongation only to a minor part, or not at all.
The prevalence of lattice degeneration and cobblestone degeneration generally increases with a longer axial length.
Choroid and Sclera
Thinning occurs most significantly at the posterior pole with changes in the extracellular matrix and fibroblast activity.
Thinning of the choroid primarily affects the layers of medium and large choroidal vessels, while the choriocapillaris thickness is marginally influenced. The evidence regarding the impact of increased axial length on choroidal blood flow appears to be conflicting.
Vitreous
With increased axial length, the viscosity of the vitreous body decreases, and the prevalence of posterior vitreous detachment increases.
Anterior Segment
Changes in the anterior segment are less marked compared to the posterior segment. The thickness and diameter of the cornea appear to be independent of axial length although the corneal curvature decreases slightly with increased axial length in moderately myopic eyes. Anterior chamber depth and angle increase with increased axial length which reduced this risk of primary angle-closure glaucoma.
Conclusions
This paper highlights that while these myopic changes are nonpathological, they are significant and involve various parts of the eye, such as the retina, choroid, and sclera. Understanding these changes is crucial for recognising the early stages of pathological myopia and potential visual function sequelae.
Clinical Implications
Recognising these changes in myopic patients may inform early intervention strategies and monitoring for pathological progression. The findings underscore the importance of regular ophthalmologic evaluations for myopic patients, particularly those with high myopia, due to the structural changes that could predispose them to further ocular complications.
ACKNOWLEDGMENTS
This IMI White Paper was summarised by Luke Seesink and IMI Program Director Dr Nina Tahhan PhD, MPH, BOptom. A full list of the IMI taskforce members and the complete IMI white papers can be found at myopiainstitute.org. The publication and translation costs of the clinical summary was supported by donations from the Brien Holden Vision Institute, ZEISS, EssilorLuxottica, CooperVision, Alcon, HOYA, Théa, and Oculus.
REFERENCE
Jost B. Jonas, Richard F. Spaide, Lisa A. Ostrin, Nicola S. Logan, Ian Flitcroft, Songhomitra Panda-Jonas; IMI—Nonpathological Human Ocular Tissue Changes With Axial Myopia. Invest. Ophthalmol. Vis. Sci. 2023;64(6):5. doi: https://doi.org/10.1167/iovs.64.6.5
CORRESPONDENCE
Brien Holden Vision Institute Ltd
Level 4, North Wing, Rupert Myers Building, Gate 14 Barker Street,
University of New South Wales, UNSW NSW 2052
