Ostrin L. Accommodative changes in lens diameter in rhesus monkeys. Invest Ophthalmol Vis Sci 2006

PURPOSE. Some debate surrounds the accommodative mechanism in primates, particularly whether the lens equatorial diameter increases or decreases during accommodation. This study has been undertaken to measure the relationship between changes in lens diameter and refraction during accommodation in rhesus monkeys. METHODS. Photorefraction was used to measure accommodation, and goniovideography was used to measure accommodative changes in lens diameter in the iridectomized eyes of two rhesus monkeys. Accommodation was stimulated through the full amplitude available to each eye by stimulation of the Edinger-Westphal nucleus of the brain. Dynamic measurement of refractive changes followed by dynamic measurements of changes in lens diameter for the same stimulus current amplitudes allow the relationship between refraction and lens diameter to be determined. I t is well established that accommodation is a dioptric change in power of the eye to allow the eye to focus at near distances. Several aspects regarding how this optical change occurs are undisputed. The increase in optical power of the eye is attributed to an increase in power of the lens. The optical change in the lens occurs as a consequence of ciliary muscle contraction and results in an increase in the central lens surface curvatures and thickness. RESULTS. It is less well established exactly how the accommodative increase in lens thickness and surface curvatures occur. The accommodative mechanism proposed by Helmholtz 1 suggests that in the unaccommodated state, resting tension on zonular fibers extends from the ciliary body to the capsule around the lens equator. This resting zonular tension and the capsular molding forces hold the lens in a relatively flattened and unaccommodated state. With a contraction of the ciliary muscle, the apex of the ciliary body moves toward the lens equator to release tension on the zonular fibers. Gullstrand 2 and Fincham 3 suggest that when zonular tension is released, the elasticity of the capsule molds the lens into an accommodated form. Fincham 3 suggests that the capsular molding forces cause a decrease in the lens equatorial diameter, an increase in lens axial thickness, and an increase in the lens anterior and posterior surface curvatures. Many studies address the accommodative changes in the ciliary body, lens thickness, and lens surface curvatures. Few of the basic changes that occur in these aspects of the accommodative mechanism are disputed. Because the lens periphery and the equator are generally hidden behind the iris and are relatively inaccessible, it has been more difficult to observe accommodative changes at the lens equator or to study accommodative changes in lens diameter. However, several studies have considered these. Helmholtz 1 infers that because lens axial thickness increases and the lens volume cannot change, the lens equatorial diameter must decrease during accommodation. Grossmann observes in a 26-year-old patient with congenital aniridia that a lens diameter of 11.25 mm in the untreated eye increased to 12.25 mm when subjected to cycloplegia with homatropine and decreased to 10.25 mm with eserine stimulated accommodation. Fincham 3 observed accommodative changes in lens diameter in a 22-year-old patient who sustained traumatic aniridia. Retroilluminated photographs were taken through the cornea of the unaccommodated and accommodated aniridic eye while the contralateral eye viewed far and near (12.5 cm) fixation targets. Superimposed images consistently showed a decrease in lens diameter of 0.45 mm from 10.2 mm to 9.75 mm with accommodation. The upper edge of the lens was covered by the eyelid, but the remaining two thirds of the lens diameter was clearly visible and showed a concentric decrease with accommodation. Wilson 6 used retroillumination infrared videophotography to observe accommodative changes in the lens diameter during accommodation in a 27-year-old patient with ocular albinism. The lens diameter was measured at rest, during voluntary accommodation, and during pharmacologic cycloplegia and showed a 7.44% decrease in lens diameter with accommodation and an increase in lens diameter during cycloplegia. No indication is provided regarding the extent of the accommodative response achieved. Magnetic resonance imaging was used to measure lens diameters in 25 patients and showed that lens diameter decreased by 604 m (6.57%) with accommodation to an 8 D stimulus in the eight youngest subjects (age range, 22-29.2 years; mean age, 25.2 years). 7 Rhesus (Macaca mulatta) and cynomolgus (Macaca fascicularis) monkeys have been widely used to study the primate accommodative mechanism and the progression of presbyopia. The anatomy of the rhesus monkey accommodative structures and the accommodative mechanism are similar to that of humans