Restoration of accommodation: new perspectives

Abstract

Presbyopia is the loss of accommodative ability that occurs with age. Current accommodative theory postulates that the lens is primarily responsible for the refractive change that allows us to read. Our understanding of this process has grown substantially with the advent of new technologies, including ultrasound biomicroscopy (UBM), endoscopy, optical coherence tomography (OCT), ray-tracing and wavefront analysis. Goldberg's Postulate incorporates all elements of the zonular apparatus into the phenomenon of accommodation (1) . The ciliary body contracts during accommodation. Biometry has shown that the lens thickness increases and the anterior chamber depth decreases (2) . It has also demonstrated the lens capsule steepens, as the posterior-lens surface moves backwards (2) . In addition, there is a decrease in the distance from scleral spur to the ora serrata. UBM identified an attachment zone of the posterior zonules adjacent to the ora, and contraction of these zonules is thought to be the etiology of the decrease in distance found with accommodation. This complex action of the zonules is suspected to be reciprocal. As the same time the anterior zonules relax, reducing their tension on the lens such that the lens changes shape anteriorly, the posterior zonules contract, moving the posterior capsule backward. This vitreal-zonular complex stiffens with age, losing its elasticity (1)(2)(3) . The age-related changes in these structures and their biomechanical interactions with the ciliary-lens complex may contribute to presbyopia (3). It has been newly discovered that there are also changes in extralenticular structures which may have an impact on the loss of accommodation which were previously deemed to be of very little importance, namely the sclera and choroid (2) . All ocular tissues are made of collagen and are impacted like all other connective tissues by age. Ocular rigidity has been correlated with age (4) and the sclera undergoes scleral sclerosis as well as metabolic physiological stress. With the loss of elasticity, the more rigid sclera elicits compression and loading stresses upon underlying structures, specifically those related to accommodative function. Increased ocular rigidity affects other tissues as well, including ocular blood flow through the sclera and optic nerve. It has been correlated to the pathogenesis of macular degeneration (5) and other age-related eye diseases (6) . Ocular rigidity may not only impact the loss of visual accommodation but also have more extensive clinical significance.The impact of age on the lenticular-based model of loss of accommodation is well documented. The amount of accommodation lost with age related to extralenticular apparatus (primarily the zonules, choroid, and sclera) was only recently investigated (7) . It is also now known that the sclera becomes less deformable during accommodation in the nasal area with age (2) . New models suggest up to 2 diopters that might be contributed by ex tralenticular structures (7) . To date, there has been at least up to ...