There is a need to understand why the homeostatic mechanisms regulating normal ocular growth between 6 and 15 years of age should fail and as a consequence produce myopia in a high proportion of children.
1–4 Once developed, myopia is a condition that is likely to extend over at least six decades and during that time will carry a significantly increased risk of ocular pathology even for moderate levels.
1 Studies have demonstrated that both pharmaceutical and optical interventions are able to slow progression of myopia.
5,6
Current optical strategies, based principally on animal myopia experiments,
7,8 reduce the degree of relative peripheral hyperopia while maintaining clear central vision and have resulted in the application of dual focus spectacle
9 and contact lens devices
10–12 and orthokeratology
13–16 to the management of juvenile-onset myopia. A consensus has yet to emerge on the etiological significance of the peripheral refraction in human myopia,
17–20 but its dependence on eye shape
21 has prompted further evaluation of three-dimensional (3D) structural correlates of myopia and its precursor, emmetropia.
1
Magnetic resonance imaging (MRI) can provide composite images independent of an eye's optical properties and is therefore ideally suited for studies of ocular structure. Atchison et al.
22,23 presented two comprehensive studies on shape of the eye and shape of the retinal surface in myopia and emmetropia with reference to T1-weighted MRI (i.e., two-dimensional [2D] anatomical representations). The first study
22 measured three linear dimensions (internal length, height, and width) by placement of single 3-mm-thick slices in axial and sagittal planes. Using the same data set, the second study
23 fitted nonrotationally symmetrical ellipsoids to the retinal surface using transverse axial and sagittal images derived from MR images. Substantial variation was found between and within subjects in both studies: for eye shape, neither a global expansion model nor an axial elongation model was sufficient to define the entire myopic population,
22 and for retinal shape, retinas of subjects with myopia generally exhibited an oblate rather than prolate shape, although to a lesser degree than in emmetropia.
23
In contrast to T1-weighted MRI, T2-weighted MRI is able to illustrate internal eye shape by high-contrast delineation of the vitreous–retina interface. Our laboratory was the first to report on the use of T2-weighted MRI to depict in vivo and in 3D the posterior vitreous chamber of the adult human eye for a range of refractive errors.
24
Several subsequent studies have used T2-weighted MRI to investigate variations in eye shape in children and adults. Ishii et al.
25 in a study of 105 Japanese children aged 1 month to 19 years used elliptic Fourier descriptors to describe eye shape in emmetropia and myopia and defined an oblate-to-prolate pattern of growth that varied with age. The study used a 1.5-T MRI (head coil) and 1.2- to 3-mm slices for the horizontal meridian only. Lim et al.
26 demonstrated in 67 Chinese children aged between 6 and 7 years that eye shape is different in myopic and nonmyopic children even at a very young age, with the former exhibiting asymmetric axial globe elongation and the latter global expansion. The study used a 32-channel 3.0-T MRI (head coil) and in-plane resolution was 1 by 1 mm. Three further investigations,
27–29 based in Japan, have been carried out by the same department on eye shape in adult patients with emmetropia and with high myopia (>8 D; axial length > 26.5 mm) and associated ocular pathology. Comprehensive qualitative and quantitative analyses were made of the ocular distortions that occur in the posterior pole in high myopia
27,28 and their association with sclera thickness and contour using a swept-source optical coherence tomography prototype instrument.
29 The range of refractive error used to assign patients to a control group of patients with emmetropia was wide: ±1 D
27,28 and ±3 D.
29 The studies used an 8-channel 1.5-T MRI (head coil) that provided a 1.2-mm slice thickness. A recent paper by Lim et al.
30 compared the degree of oblateness and prolateness in 173 full-term newborn Singaporean children based on measurements of eye length, width, and height. Although oblate eyes were identified, mean values indicated that eyes were generally prolate at birth. The study used an 8-channel 1.5-T MRI (head coil) that provided a 1 by 1 by 1 mm resolution.
There are no studies to date that have used T2-weighted MRI to compare ocular dimensions in adults with emmetropia (as generally defined; i.e., mean spherical error (MSE) ≥ −0.55; <+0.75 D) and myopia without associated manifest ocular pathology. The present study uses an 8-channel 3.0-T (head-coil, 1 by 1 by 1 mm resolution) T2-weighted 3D MRI technique
24 to analyze posterior vitreous chamber dimensions of both eyes in adults with emmetropia and myopia. The 3D data are plotted in two dimensions to signify the composite shape of nasal, temporal, superior, and inferior quadrants. The aim of the study is to make a detailed comparison of the degree to which the posterior vitreous chamber of the human myopic eye differs in shape from the emmetropic eye and to consider whether structural differences are likely to have etiological and clinical significance in terms of the onset, development, and treatment of myopia.