Ciliary Body Thickness and Refractive Error in Children

Investigators in the Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) Study have suggested that myopia occurs during two phases of ocular growth: a phase of axial elongation that leads to an increasingly prolate, or less oblate, shape followed by a phase of more uniform global expansion.1 Other investigators have also reported that the myopic globe is relatively more prolate than the emmetropic globe. In a study using magnetic resonance imaging (MRI) for analysis of globe shape, Atchison et al.2 reported that the myopic eye is more relatively prolate than the emmetropic eye, with the myopic eye having dimensions that are more elongated axially than in the horizontal or vertical dimensions. Also in 2004, Logan et al.3 published similar results obtained with A-scan ultrasonography and central and peripheral autorefraction. A relatively more hyperopic or less myopic peripheral refractive error is a correlate of this relatively prolate shape, particularly in the horizontal meridian, among those with a myopic foveal refractive error.4–7 There is renewed interest in this classic observation based on both human clinical data and animal experimentation. A longitudinal study of pilots found that relative peripheral hyperopia in an emmetropic eye is associated with an increased risk of development of myopia.8 Animal experiments also support a potential role for the influence of peripheral input in myopia’s development. The retinal periphery is effective in creating axial myopia when deprived of form vision and in directing recovery once normal input is restored.9 The presence of an intact fovea has been shown to be unnecessary for vision-based alterations to occur in refractive error.10 As has already been shown for axial hyperopic defocus,11–14 peripheral hyperopic defocus may contribute to axial elongation and the development of myopia. A variety of explanations have been proposed to account for the relatively prolate shape of the myopic globe. CLEERE Study investigators have suggested that an internal source of growth restriction might produce the biphasic, increasingly prolate pattern of growth in premyopic eyes, and the crystalline lens has been offered as a potential source of internal growth restriction.1 Conversely, Atchison et al.2 suggested an external source of restriction, stating that the differences in axial versus equatorial dimensions were most likely due to the anatomic constraints of the orbital wall.2 Logan et al.3 suggested that growth in the axial and transverse dimensions of the globe is regulated differently and independently.3 In recent results, Oliveira et al.15 suggest another possible source of the altered globe shape in myopia. They report an association between refractive error and an ocular component in the equatorial region of the globe; adult patients with myopia were found to have thicker ciliary bodies than those without myopia. The finding that the myopic eye has a thicker rather than thinner ciliary body contradicts the choroidal expansion experiments of van Alphen,16 who demonstrated marked thinning of the ciliary muscle structure, with little change in the choroid, when the globe was expanded in vitro. Thickening raises the possibility that there is a physiological response of the ciliary body in myopia rather than simple mechanical stretching. Because data from the CLEERE Study,1 data from Atchison2 MRI studies, data from Logan et al.,3 and work in the monkey9 suggest that the equatorial region of the globe is an important feature in myopia’s development, we sought to determine whether the ciliary body is also thicker in myopic children.