Intraocular accommodative movements in monkeys; relationship to presbyopia

Abstract

Our goal was to quantify the age-related changes in the dynamic accommodative movements of the vitreous and aqueous humor in iridic, aniridic, phakic and aphakic primate eyes. Six bilaterally iridic and four bilaterally iridectomized rhesus monkeys, ranging in age from 6 to 25 years, received a stimulating electrode in the midbrain Edinger-Westphal nucleus to induce accommodation, measured by a Hartinger coincidence refractometer. One of the four iridectomized monkeys underwent unilateral extracapsular and another monkey underwent intracapsular lens extraction. Eyes were imaged utilizing specialized techniques and contrast agents to resolve intraocular structures. During accommodation the anterior hyaloid membrane and the posterior lens capsule bowed backward. Central vitreous fluid and structures/strands moved posteriorly toward the optic nerve region as peripheral vitreous, attached to the vitreous zonule, was pulled forward by ciliary muscle contraction. Triamcinolone particles injected intravitreally were also observed in the anterior chamber and moved from the anterior chamber toward the cleft of the anterior hyaloid membrane and then further posteriorly into the vitreous-filled cleft between the vitreous zonule and the ciliary body pars plana. These accommodative movements occurred in all eyes, and declined with age. There are statistically significant accommodative movements of various intravitreal structures. The posterior/anterior fluid flow between the anterior chamber and the vitreous compartments during accommodation/disaccommodation represents fluid displacement to allow/facilitate lens thickening. The posterior accommodative movement of central vitreous fluid may result from centripetal compression of the anterior tips of the cistern-like structure attached to the vitreous zonule, and posterior displacement of the central trunk of the cistern during ciliary muscle contraction and centripetal muscle movement. The findings may have implications for presbyopia.

Keywords: Accommodation; Choroid; Iris; Lens; Monkey; Presbyopia; Vitreous.

Figure 1.

Ultrasound biomicroscopy (UBM) Images of 6 and 19-year-old rhesus monkeys in the unaccommodated and accommodated states. A/P distances (green arrows) were measured perpendicular to the anterior plane of the iris (yellow lines) near the region of the circumlental space (red arrows). During accommodation, the iris bows backward in the region of the circumlental space—more so in the older eye than in the young eye.

Figure 2.

Accommodative backward bowing of the anterior hyaloid with respect to the scleral spur vs accommodative amplitude in one eye of each of 4 rhesus monkeys, aged 8, 8, 13 and 19-years old. Accommodative backward bowing of the anterior hyaloid was significantly related to accommodative amplitude; as the accommodative amplitude increased the backward bowing of the anterior hyaloid increased.

Figure 3.

Accommodative posterior movement of the central vitreous with respect to the retina plotted vs accommodative amplitude (left panel) or age (right panel) in 7 rhesus monkeys. Triamcinolone particles were not visualized in the central vitreous in one monkey post injection, thus n=7.

Figure 4.

Accommodative forward movement of the peripheral lacunae (cistern) tip with respect to the scleral spur plotted vs accommodative amplitude (left panel) or age (right panel) in 8 rhesus monkeys.

Figure 5.

Ultrasound biomicroscopy (UBM) images in iridectomized rhesus monkey eyes aged 7-years-old (left panel) and 17-years-old (right panel) following intravitreal injection of triamcinolone. With age, vitreous fibers aggregate and build up peripherally in the ora serrata region in close juxtaposition and attachment to the vitreous zonule, as evidenced by the accumulation of triamcinolone particles.

Figure 6.

Typical ultrasound biomicroscopy (UBM) image of the accommodated capsule shape following extracapsular lens extraction (ECLE).