Age-Related Changes in the Anterior Segment Biometry During Accommodation

Contemporary and classic theories for the accommodative mechanism are based on the concept that the contraction of the ciliary muscle is the initial event that leads to the reshaping of the crystalline lens, an idea initially developed by von Helmholtz.1 The key elements of accommodation, such as the elasticity of the crystalline lens and the activity of the ciliary muscle, decline with age, which degenerates the capacity of accommodation.2–7 Restoring the accommodative ability has been a substantial challenge. The implantation of accommodating intraocular lenses (AIOL) has been shown to restore less than 2 diopters (D) of objective accommodation,8,9 partly due to the difficulty in predicting the dynamic response of the accommodative apparatus during accommodation. The dynamic relationship between ciliary muscle contraction and its ability to reshape the crystalline lens during the real-time processes of the human eye has not been fully studied in vivo, which may further limit the improvements in the restoration of accommodation. Hess10 and Gullstrand11 developed a lenticular theory, which stated that presbyopia was caused by a decrease in the lens reshaping capacity. The ciliary muscle normally contracts in the region where the lens still is capable of a response, but it cannot produce an accommodative response above this level due to the lens hardness, even though muscle still is capable of contracting. In contrast, the extralenticular theory of Duane12,13 showed that a slight weakening of the ciliary muscle decreased the accommodative amplitude. Fincham14,15 proposed a reconciliation of these two extremes, suggesting that the lens is responsible primarily for the loss of accommodation and that the contraction of the ciliary muscle required for lens reshaping increases with age. Although these early theories all predict the age-related changes in muscle-lens relationship, none of them is fully supported. The in vivo assessment of the dynamic response of the crystalline lens to ciliary muscle contraction is critical to understand the mechanism of accommodation and the age-related decline of accommodation. The difficulty in simultaneously imaging the entire anterior segment, including the crystalline lens and ciliary muscle, has limited in vivo studies of accommodation. In nonhuman primate models with iridectomies, the movement of ciliary processes and the lens equator during accommodation has been visualized using goniovideography under the stimulation of the Edinger-Westphal nucleus.16,17 This method may not be applicable to human subjects. Others have used magnetic resonance imaging (MRI), which allows the entire eye (including the ciliary muscle and entire lens) to be imaged, to visualize accommodation.18,19 The low scanning speed limits the application of dynamic measurements in accommodation research. To date, to our knowledge no study has simultaneously assessed the biometric changes in the ciliary muscle and crystalline lens as the function of age in real time in a cohort of human subjects. Optical coherence tomography (OCT) allows the noninvasive and rapid imaging of the ocular anterior segment in the static and dynamic states.20–28 Previously, we developed a system that combined two spectral-domain OCT (SD-OCT) devices.27 One has extended the scan depth to approximately 12 mm in air for the imaging of the entire anterior segment through the pupil, while the other was equipped with a light source that was centered at a wavelength of 1310 nm to image the ciliary muscle in real time. The objective of this study was to investigate the age-related changes in the dynamic responses of human accommodative elements, and the relationship between lens reshaping and muscle contraction during accommodation using the synchronized SD-OCT system.