Human eye dimensions and retinal shape in myopia: effects of refractive error and ethnicity studied with 3D high resolution MRI at 3T
- 1. Queensland University of Technology, Chemistry, Physics and Mechanical Engineering, Brisbane, Australia
- 2. Queensland University of Technology, Institute for Health and Biomedical Innovation, Brisbane, Australia
- 3. Queensland University of Technology, School of Optometry, Brisbane, Australia
Myopia (short-sightedness) is associated with axial elongation of the eye, causing the image of a distant object to be focussed in front of the retina. It is the most common refractive anomaly in young people and is particularly prevalent in Chinese communities. High myopia is a leading cause of blindness through increased risk of retinal disease, retinal detachment and glaucoma. While the causes of myopia have long been debated, the mechanism of elongation of the eye remains unclear. Both environmental and genetic factors are associated with the onset and progression of myopia.
Much attention has been given to the central retina and the state of focus along the visual axis, but since this forms a small part of the visual field, peripheral retinal areas may be of importance in driving refractive status. Hoogerheide et al. (1) reported different patterns of peripheral refraction in emmetropes and myopes, with emmetropes generally showing relative peripheral myopia and myopes usually showing relative peripheral hyperopia. Recent studies (2) also suggest that relative peripheral hyperopia may result in myopia. Retinal shape may play a role in this, with steep retinas likely to be a source (at least in part) of the problem (3).
Standard optical methods for biometry of the eye are restricted to the paraxial region and are not suited to studying the shape and dimensions of the eye as a whole. We have employed 3D high resolution MRI to study the dimensions of the eye and shape of the retina in 60 young subjects (38 female and 22 male, mean age 22.5 ± 3.1), comprising 30 myopes, (with refractive errors M in the range -1.25D to -8.25D) and 30 emmetropes (M = +0.75D to -0.62D). Because of the high prevalence of myopia in Chinese communities, the subject group was selected to contain equal numbers of East Asians (30) and Caucasians/South Asians (24/6) in order to compare results for these groups.
Results show that the dimensions of the eye increase with degree of myopia, with eye length increasing more rapidly than eye width. Retinal shapes were well fitted by (non-axially symmetric) ellipsoids. The shape of the retina, which tends to be oblate in emmetropes, becomes approximately spherical in high myopes. Vertex radii decrease slightly with increasing myopia, consistent with the overall eye shape becoming less oblate, despite the eyes on average becoming larger. There were no significant differences between the eye shapes of East Asians and those of Caucasians/South Asians, and the trends in eye dimensions and shape with increasing myopia appear similar.
References:
1. Hoogerheide J et al. 1971. Ophthalmologica, 163, 209-215.
2. Charman WN & Radhakrishnan H 2010. Ophthalmic and Physiological Optics, 30, 321-338.
3. Verkicharla PK et al. 2012. Ibid, 32(3): 184-199.