Eye, Aging-Related Diseases
EYE, AGING-RELATED DISEASES
The sense of sight, said Aristotle is preferred "to everything else. The reason is that this, most of all the senses makes us know and brings to light many differences between things" (Metaphysics, 98A225).
If one looks an eye straight on, several important structures are recognizable, including the pupil, which is the black circle in the center of the eye that dilates and constricts in response to light; the iris, which is the colored muscular structure that allows the pupil to change its size; the transparent cornea, which is really the window of the eye; and the lens. The latter focuses light through the pupil onto the retina at the back of the eye, much as a lens in a camera focuses images onto film. Images are then transformed into electrical signals that are transmitted to the visual cortex at the back of the brain. The space in the eyeball between the lens and the retina is filled with a viscous clear liquid known as vitreous humor (see Figure 1).
With age, many changes (some virtually universal, and others arising from age-related disease) threaten the quality of vision. For instance, tear glands often work less well, leading many older adults to suffer from dry eyes (easily treated with artificial tears). Lax muscles around the eye can result in the lower lid margin rotating away from the eyeball (ectropion) or an inward migration of the eyelashes and eyelid toward the globe (entropion). In the conjunctiva (the skin on the inside of the lid and covering the white part of the eye), tiny blood vessels can rupture, giving rise to localized accumulation of blood akin to a bruise (subconjunctival hemorrhage). Though unsightly, it is not serious and usually does not signify any underlying disorder. It can clear up spontaneously in six to eight days.
Particularly in people who are near-sighted, "floaters" (tiny, condensed debris floating in the vitreous humor) are common and of little consequence. However, any sudden increase in the number of floaters, particularly if accompanied by flashing lights, can herald the onset of serious retinal problems, and requires a thorough eye examination as soon as possible.
Other age-related eye problems can also threaten vision and require evaluation by an ophthalmologist. Corneal ulcers, tumors on or near the eye, inflammation of the structures of the eyeball, occlusion of retinal vessels, and any sudden loss of vision are among them.
Three common sight-threatening disorders are described here in more detail. Each can result in problems sufficient to meet the World Health Organization's definition of visual impairment (i.e., visual acuity less than 20/60 in the better eye). Overall, about 2–4 percent of the population age seventy and over suffers from such impairment.
Glaucoma is a chronic disorder that when left untreated, will lead to blindness in most cases. The clinical picture usually, but not always, includes elevated intraocular pressure that leads to damage of the optic nerve (optic neuropathy). There are several types of glaucoma: acute and chronic, open- and closed-angle, and secondary. In all cases, treatment is directed toward reducing the intraocular pressure with eye drops, laser, and/or surgery, depending upon the severity of the case and the response to the medical therapy.
Chronic open-angle glaucoma, the most common form, usually comes about insidiously and progresses chronically. Since it is peripheral visual field loss that first occurs, damage can be extensive before the person affected becomes aware of important visual loss.
Theories about the nature and causes of glaucoma are being reevaluated. Early views of open-angle glaucoma as a simple matter of a problem in aqueous outflow have given way to the concept of a more complex neurodegenerative disorder of the optic nerve. New modalities of treatment are being evaluated, including medications that may protect the optic nerve, but the current treatment remains aimed at lowering pressure inside the eyeball. However, it has become clear that a number of other factors play important roles in determining whether a specific level of intraocular pressure will be harmful to a given eye. As these additional factors are identified, new modalities of treatment will arise to address their influence whether these be genetic, vascular, or neuronal.
Age-related macular degeneration
Macular degeneration (the macula is the very central part of the seeing eye) is a common cause of impaired vision in older adults, and often progresses to cause legal blindness. Two types of macular degeneration predominate. The acute "exudative" (wet) form results from proliferation of blood vessels in the macula that subsequently bleed, causing a hemorrhage and scarring in this area. This acute process has devastating effects on vision. In a small percentage of cases (10–15 percent), treatment with either a "hot laser" or, more recently, a combination of a "cold laser" and photodynamic dye, can stabilize the condition and preserve vision.
The more common form of macular degeneration is the slowly progressive atrophic type. While there is no treatment for this form, the clinical development is much slower, and a reasonable degree of vision is often maintained for many years despite the presence of the atrophic lesions in the macula.
Since the macula is necessary for central (in contrast to peripheral) vision, decreased visual acuity and, especially, distortion of the central part of the visual field are important symptoms of this disorder.
A cataract is a clinical condition that describes the opacification (clouding) of the natural lens of the eye. A cataract is not a growth, but a change in the tissue structure of the lens. These changes are very much age-related, and almost physiological, although they occur at much younger ages in some patients than in others, and there are also many ways in which the tissue changes. The most common and physiological change, described as sclerosis of the lens, is similar to a yellowing of plastic after many years of exposure to sunlight. This type of cataract often improves near vision because it helps magnify items, but it reduces distance vision.
Other types of cataracts appear in a spoke-like fashion in the lens or in a membrane type of opacification known as a posterior subcapsular cataract. In all types of cataracts, vision, usually for distance and near, is reduced. Patients often have problems with glare from sunlight, and streetlights and car headlights, and eventually their quality of life deteriorates. As these changes occur, changes in the refractive error, or the power that is needed for eyeglasses, also changes, and in the early stages, a change in lenses can compensate for some of the changes in the eye. Eventually, however, this no longer suffices and more drastic therapy is needed.
Because the lens is an isolated structure with no blood vessels, it has long been known that it could be removed, rather safely, from the eye. Current cataract techniques are called removal by phacoemulsification, and introduction of an intraocular lens. Phacoemulsification is a technique in which a very high-frequency ultrasound probe is introduced into the lens. The lens tissue is then pulverized, as it were, and a second system sucks out the material so that the lens tissue is removed completely from its very thin membranous sac, known as the capsule. An intraocular lens is then inserted to replace the natural lens. The current standard is to insert a lens that has been folded on itself and will unfold in the eye when it is in place. These techniques allow cataract surgery to be done through a 3 millimeter incision in the clear part of the cornea, and does not require any stitching. Healing is more rapid and safe with this technique. Currently, cataract surgery is an outpatient procedure with only numbing drops put on the eye rather than any other anesthesia or medication being used.
The cataract surgery is one of the great surgical success stories of the late twentieth century. It has permitted millions of people the world over to be rehabilitated to a more functional status and a better quality of life. Nonetheless, cataracts remain the number one cause of blindness in the developing world, and it is simply a matter of resources to get enough cataract surgery done in the developing world to reverse that statistic.
Raymond Leblanc Kenneth Rockwood
See also Assisted Living; Brain; Driving Ability; Functional Ability; Images of Aging; Neurodegenerative Diseases; Reaction Time; Vision and Perception.
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Jaenicke, R., and Slingsby, C. "Lens Crystallins and Their Microbial Homologs: Structure, Stability, and Function." Critical Reviews in Biochemistry and Molecular Biology 36 (2001): 435–499.
Lichter, P. R.; Musch, D. C.; Gillespie, B. W.; Guire, K. E.; Janz, N. K.; Wren, P. A.; Mills, R. P.; and the CIGTS Study Group. "Interim Clinical Outcomes in the Collaborative Initial Glaucoma Treatment Study Comparing Initial Treatment Randomized to Medications or Surgery." Ophthalmology 108 (2001): 1943–1953.
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