The Eyes Optical Components Cornea

The cornea is the anterior portion of the eye, and its curved surface together with the watery layer of tears is responsible for most of the refraction of the light rays. During aging, the cornea becomes thicker and less curved, mainly due to an increase in the horizontal diameter of the eye. These changes alter the refractive properties of the cornea, leading to "against the rule" astigmatism, a condition characterized by defective corneal curvature and diffusion of light rays (8). The cornea is also highly sensitive to irritable stimuli, a protective function for the eye (8). Corneal sensitivity declines by nearly one half between youth and old age (8).

Other conditions in the cornea associated with aging are the arcus senilis (or senescent arc), the Hudson-Stahli line, and spheroidal degeneration. The arcus senilis increases in frequency and density with aging, particularly after 60 years. It is a yellowish-white ring around the cornea's outer edge, formed by cholesterol ester deposits derived from plasma lipoproteins (lipid arcus). In the dilated pupil, this ring would interfere with passage of light rays; however, because of partial pupillary constriction in the elderly, arcus senilis is not detrimental to visual function (7-10).

The Hudson-Stahli line is a horizontal brown line formed by iron deposits in corneal basal cells. Its frequency increases from 2% at 10 years to 14% at 30 years and 40% at 60 years but has no detrimental effect on vision. Spheroidal degeneration occurs in cornea's Bowman's layer. It is observed frequently in aged populations exposed to high levels of ultraviolet (UV) radiation or ambient light reflected from snow or sand (8).

The corneal endothelial cells number about one million per cornea at birth; this number declines to 70% by 20 years and to 50% and 30% by 60 and 80 years, respectively. Normally, the pumping action of corneal cells removes water and helps keep the cornea transparent. Because these cells do not divide after birth, their loss due to aging or injury after surgical treatments of the cornea or lens can lead to a decline in corneal transparency. Endothelial cells also secrete the cornea's basement membrane. With aging, warts (cornea guttata) appear in this membrane mainly in the cornea's periphery and cause marked increase in corneal permeability. Guttatas are observed with increasing frequency with aging: 20% in youth, 60% in the sixth decade, and nearly 100% in very old age (8).

Recent Studies on Corneal Aging: Biochemical and Structural Changes

Biochemical studies revealed a gradual decline in high-energy metabolism of the aging cornea as shown by linear decreases in phosphomonoesters, phosphocreatine, and ATP, accompanied by decreased inorganic orthophosphate (14). Corneal aging is accompanied by a linear loss of keratocytes paralleling loss in endothelial cell density (15). A three-dimensional expansion of collagen fibrils along the axial direction occurs in corneal stroma with aging. The expansion is due to decreased molecular tilting angle within fibrils. It reflects an increased number of fibrils due to expansion of intermolecular Bragg spacing caused by glycation-induced cross-linkages (16). Previously described morphological aging changes in cornea have been confirmed and better described. Human corneas become less symmetrical with age; increased pupillary dilation and wavefront aberration become more pronounced with aging (17). Steepening of corneal curvature with aging, as shown by a decrease in vortex radius and an increase in P-value, reflects a shift to a more spherical surface (18).

Corneal and Conjunctival Sensitivity

In a recent study, Acosta et al. (19) found that corneal thresholds to mechanical and chemical stimuli increased with age. Premenopausal women were more sensitive to corneal stimulation than men of similar ages, but overall differences in mechanical and chemical threshold between men and women were not significant. Also corneal mechanical threshold depended on age and iris color. Similar results were also noted by Roszkowska et al. (20) with regard to corneal sensitivity.

FIGURE 1 Increase in the incidence of visual and hearing impairments in males and females 65 years of age and older in the general U.S. population. Note the higher rate of sensory impairments in males, particularly for hearing. Source: From Ref. 2.

Asphericity and Cell Density

A flattening (more horizontal than vertical) of both anterior and posterior corneal surfaces of the cornea was reported in a recent study (21). With aging, the asphericity of both corneal surfaces change significantly, causing a peripheral thinning of the cornea. As a result, the astigmatism (i.e., defective curvature of the cornea resulting in blurred and imperfect image) of the posterior corneal surface compensates for that of the anterior one. According to another recent study by Roszkowska et al. (22), both central and peripheral endothelial cell densities decrease with age in the cornea, with higher peripheral decrements in older subjects. Old subjects showed significant differences in cell densities between corneal center and periphery but not the young adults. Similar cell density changes were noted with aging but differences were not found in corneal endothelial cell density values between emmetropic (i.e., normal accommodation and refraction), myopic (i.e., shortsighted), and hyperopic (i.e., far-sighted) subjects (23).

anterior ciliary chamber process conjunctiva sclera canal of Schlemm ciliary body

canal of Schlemm

Human Power

FIGURE 2 Schematic drawing of the human eye and lens. Source: From Refs. 5 (eye illustration B), 6 (lens illustration A).

FIGURE 2 Schematic drawing of the human eye and lens. Source: From Refs. 5 (eye illustration B), 6 (lens illustration A).

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