Aging and age-related diseases of the ocular lens and vitreous body

Abstract

Reduced quality of life and financial burden due to visual impairment and blindness begin to increase dramatically when individuals reach the age of 40. The major causes of age-related vision loss can be traced to changes to the structure and function of the lens, one of the tissues responsible for focusing light on the retina. Age-related nuclear cataracts, which are caused by aggregation and condensation of proteins, diminish vision because they impede the transmission and focusing of light on the retina. In addition to the slow-developing age-related form, cataracts often develop rapidly as a complication of ocular surgery, such as following vitrectomy or as a consequence of vitreous gel degeneration. Posterior capsular opacification, which can develop following cataract removal, is caused by proliferation and inappropriate accumulation of lens epithelial cells on the surfaces of intraocular lenses and the posterior lens capsule. Presbyopia is a loss of accommodative amplitude and reduced ability to shift focus from far to near objects. Onset of presbyopia is associated with an increase in lens hardness and reduced ability of the lens to change shape in response to ciliary muscle contraction. Avenues of promising research that seek to delay or prevent these causes of low vision are discussed in light of our current understanding of disease pathogenesis and some challenges that must be met to achieve success.

Keywords: PCO; cataract; presbyopia; vitreous humor.

Figure 1

A schematic representation of (A) the postsurgical capsular bag containing an IOL and residual LEC at the anterior and equitorial capsule and (B) the extensive growth and capsular wrinkling that gives rise to posterior capsule opacification. (C) A dark-field micrograph of a capsular bag removed from a donor eye that had undergone cataract surgery before death that exhibits light-scattering regions beneath an intraocular lens. Reprinted from Wormstone IM, Wang L, Liu CSC. Posterior capsule opacification. Exp Eye Res. 2009;88:257–269. Copyright 2009, with permission from Elsevier.

Figure 2

Accommodation in the normal eye. In the unaccommodated eye, the lens is held in a flattened shape by zonular fibers under tension. With contraction of the ciliary muscle, the zonular fibers are relaxed, which allows the lens to assume a more rounded shape with a steeper radius of curvature at the anterior surface. These changes allow clear vision at a closer distance. Reprinted with permission from Azar D, Gatinel D, Hoang-Xuan T, ed. Refractive Surgery. Philadelphia: Mosby-Elsevier; 2006:501–510.

Figure 3

The distribution of oxygen (A) in the normal eye and (B) after degeneration or removal of the vitreous body. (A) In an eye with an intact vitreous body, oxygen diffuses into the vitreous gel from vessels near the surface of the retina. Much of this oxygen is consumed by retinal tissue farther from the vessel (curved red arrows). (B) After the vitreous gel is removed during vitrectomy or degenerates and detaches from the retinal surface, much of the vitreous cavity is filled with liquid. This fluid mixes readily (curved black arrows), carrying oxygen away from the retina and distributing it throughout the vitreous cavity. Mixing also delivers more oxygen to the posterior surface of the lens, where it diffuses into the lens, causing nuclear cataract. Reprinted with permission from Beebe DC, Holekamp NM, Siegfried C, Shui YB. Vitreoretinal influences on lens function and cataract. Philos Trans R Soc Lond B Biol Sci. 2011;366:1293–1300.