January 07, 2007

Aging and your Eye


As the eye ages certain changes occur that can be attributed to solely the aging process. Most of these anatomic and physiologic processes follow a gradual decline. Although these processes are distinct from the aging eye diseases, the vision changes they produce may be similar, albiet smaller. Physicians and researchers do not usually pay much attention towards understanding these aging related decline in eye functioning, perhaps because of the enormous focus on the effort to understand and treat the blinding aging eye diseases like glaucoma and macular degeneration.

Vision changes

With aging, the quality of vision worsens due to reasons independent of aging eye diseases. While there are many changes of significance in the nondiseased eye, the most functionally important changes seem to be a reduction in pupil size and the loss of accommodation or focusing capability. The area of the pupil governs the amount of light that can reach the retina. The extent to which the pupil dilates also decreases with age (Br J Ophthalmol 2000;84:1173-6). Because of the smaller pupil size, older eyes receive much light at the retina. The light adapted eye of a 20 year old receives six times more light than that of an 80 year old. In dark adapted conditions, the 20 year old eye receives about 16 times more light. In comparison to younger people, it is as though older persons were wearing medium-density sunglasses in bright light and extremely dark glasses in dim light.

To maximize the light entering the eye, make sure that your eye glasses have anti-reflective coating. Anti-reflective lens coatings significantly reduce surface reflectance and allow greater light transmission through the lens. For CR-39 lenses, approximately 8% of transmitted light is lost to reflectance on the lens surfaces – 4% on the front surface, and 4% on the back surface. Only about 92% of light entering the lens is transmitted to the lens wearer’s eyes. A high index material with a refractive index of 1.6 is transmitting only about 90% of incident light, with 10% lost to reflectance. However, when AR coating is applied to both the front and back surfaces of a lens, the percentage of transmitted light for both CR-39 and high index lenses increases to approximately 99%.

For any detailed visually guided tasks on which performance varies with illumination, older person requires extra lighting. Aging causes a dramatic slowing in dark adaptation that can be attributed to delayed rhodopsin regeneration in the retinal photreceptors. This age related delay in dark adaptation may also contribute to night vision problems commonly experienced by the elderly (Vision Res 1999;39:3975-82). Color vision and contrast sensitivity are also affected by aging. Color vision changes cause some reduction of ability to discriminate blues and blue-greens. The yellowing of the lens is believed to be responsible for this effect. The aging lens and cornea causes glare by light scattering, especially for shorter wavelengths. Stray light and lenticular fluorescence washes out contrast. (Reference: Work, Aging and Vision. National Academy Press; 1987). Contrast sensitivity shows a significant age-related decline (Ophthalmic Physiol Opt 2000;20:323-34). Wavefront aberrations of the cornea also increase with age, leading to poor vision quality especially when the pupil dilates as in the dark (Invest Ophthalmol Vis Sci 1999;40:1351-5)

The most aggravating aspect of vision in an older person seems to be the feeling that it does not work as effortlessly or as quickly as it did in younger days. They must concentrate harder and require higher levels of illumination than they formerly did in order to have the same perceptual results. Although plain seeing in simplified situations, as in routine vision testing, is as good and as quick as ever, perceiving a complex, changing scene is more difficult and slower. The older person sees parts almost as well as they ever did but organizing of the perception as a whole is more time consuming and require more attention.

Lighting the way:
A key to independence. Everyone's vision deteriorates with age, but there are ways to lessen its impact. Good lighting can make the difference between seeing and not seeing for older adults. AgingEye Times highly recommends these guidelines and suggests that you read and implement the suggested lighting solutions in your homes, stairways, kitchen, living rooms etc.

Here are some home lighting tips for the elderly from CNN.com health: Naked bulbs: These are worst for older eyes, producing a glare that can be disorienting and painful. Chandeliers are problematic for the same reason. Task lighting: It makes sense to have a place in the house to do paperwork. Light should be concentrated on the work, not toward the eyes. Adjustable lamps with shades that direct light are recommended. A good choice is a compact fluorescent light with an electronic ballast that starts up without flickering. Lights should be installed over countertops and stoves. Bathroom lighting: Lots of light should be provided overhead and even in the shower, where accidents are particularly common. 'Hollywood' lights around the mirror are a poor choice; better is a rectangular fluorescent lamp with an opaque cover to block glare and distribute light upward and outward. Bulb choice: Be cautious of cheap fluorescent bulbs. Look for phrases such as "high color" or "excellent color rendering," since older eyes often have difficulty defining colors. An 80 or above 'cri' (color rendering index) is recommended. An advantage of fluorescent lighting is its 'blue' energy, which helps older people see colors.

Anatomic changes

Aging causes laxity and downward shift of eyelid tissues and atrophy of the orbital fat. These changes contribute to the etiology of several eyelid disorders such as ectropion, entropion, dermatochalasis,and ptosis. The higher eyelid skin crease and ptosis may be due to age related disinsertion of the levator muscle aponeurosis, and to involutional atrophy of the orbital fat. The horizontal eyelid fissure shortens by about 10% with aging (Br J Ophthalmol 1999;83:347-352).With aging a prominent white ring develops in the periphery of the cornea- called arcus senilis. The number of corneal endothelial cells gradually decrease in number.

The vitreous gel undergoes liquefaction and its opacities - visible as floaters gradually increase in number.

Age, Eyes, and Crime
Some properties of the eye change roughly linearly with age and present opportunity for non-invasive assessment of age. Three of these properties are: 1)lacrimation; 2) fluorescence of the cornea and lens; and 3) accommodation. The first two are objective, whereas the third requires cooperation from the individual. Lacrimation and accommodation decrease with age. Fluorescence of the cornea and lens increase with age. Although specialist equipment and a clear understanding of what one is doing are essential, the tests lend themselves to rapid assessments for incorporation in a set of other age estimates (Weale R. Lancet 2001;358:1644)

Adapted from: TheEyeDigest