Aging of the human crystalline lens and presbyopia.

The primary function of the crystalline lens is to increase the vergence of light that enters the pupil after passing through the cornea. In an emmetropic eye, the refractive power of the lens will increase the vergence of light to focus on the retina. The young human crystalline lens also serves the function of accommodation whereby the optical power of the lens is increased through the action of a ciliary muscle contraction (see the chapter, “Accommodation and Presbyopia”). Preservation of these two important optical functions throughout life would require that the lens optical and physical properties remain constant. In fact, very little, if anything, about the crystalline lens remains unchanged with increasing age. The rapidity and inevitability of the changes in the lens are evident from both presbyopia, which begins early in life and results in a complete loss of accommodation roughly midway through the human life span, and from the high incidence of cataract in the elderly. Both presbyopia and cataract, in all likelihood, represent part of a larger continuum of events that occur with aging of the eye and lens and are not end points in themselves.