January 09, 2006.
Scientific American - USA.
David Zacks, a retina specialist and assistant professor of ophthalmology and visual sciences at the University of Michigan Kellogg Eye Center, explains.
Many of us think that as we grow older our eyesight is destined to deteriorate. We talk about "tired" or "old" eyes as if we are fated to have the gift of vision taken from us simply because we have aged. For example, in what might be the earliest reference to cataracts in the Bible (Genesis, Chapter 27, Verse 1) we learn that, "And it came to pass, that Isaac was old, and his eyes were dim..." The truth is that, with today's treatment options, there is no intrinsic reason for our vision to worsen with time. In theory, we should be able to retain undiminished our capability to accept, process and experience visual sensory input.
But, as with most conventional wisdom, there is an element of truth to the notion that age-related processes can affect our eyesight. The good news is that identifying and treating these processes can often result in the preservation or restoration of excellent vision for an entire lifetime.
A good understanding of vision loss requires a brief foray into the basic structure of the eye. Although it might be considered a cliché, it is still useful to compare the eye to a video camera. Thus, as light enters the eye--or the camera--it travels through four main structures. By understanding this basic eye anatomy we can begin to understand the ways in which the light signal can be degraded or distorted, resulting in poor vision.
The entry point for light is the cornea, the transparent tissue in the very front of the eye that functions as the window through which all light has to pass on its way to forming a visual perception. Next, the light signal encounters the lens, which focuses it finely on the third structure of the eye: the retina. The retina is analogous to the film in the back of the camera. It is on the retina that the light is converted into a neural signal that is ultimately interpreted by the brain as an image. Finally, the optic nerve, which carries these signals to the brain, functions like a cable that connects the video camera to the television screen. The deterioration of our vision with age is invariably the result of a problem with one of these four structures.
The integrity of the corneal surface is very important for the reception of a clear image. If its surface is not smooth and intact the image will suffer, much like the image from a scratched camera. A very thin layer of tears coats the surface of the cornea. This tear surface, produced by various cells, some of which line the inner surface of the eyelid, is vital to maintaining the cornea's smooth surface and precise optical characteristics. Any condition that disrupts this tear film can lead to a breakdown of the cornea's surface and, thus, degradation of the image.
With age, this smooth surface can be damaged by conditions such as blepharitis, an inflammation of the eyelids. If the inflammation is severe enough, then the cells that produce the tears can be damaged, with an attendant impact on the tears coating the cornea. In this situation an individual may experience symptoms such as the sensation of a "film" over the eye, which is usually transient and relieved with blinking. More serious symptoms can also occur, such as blurry vision, which is due to evaporation of the tears and drying out of the corneal surface. This condition can become chronic and is known as dry eyes. Treatment options for blepharitis and dry eyes depend on the cause of the inflammation of the lid surface but can include warm compresses, artificial tears, oral antibiotics and even topical immunosuppressive agents to decrease the inflammatory response leading to the degraded tears.
The lens is also subject to the aging process, and the resulting conditions--cataracts and presbyopia--will eventually affect just about everyone. When we are born our lenses are crystal clear, meaning that the image entering the eye is not degraded by any opacity. They are also very pliable, meaning that the lens can change its shape to help focus images that are very far away as well as very close. Unfortunately, as we grow older two things happen to the lens: it clouds up and it becomes less pliable. The clouding of the lens--or cataract--forces an image to travel through a distorted medium, resulting in diminished vision. Similarly, when the lens loses its pliability it also loses its ability to focus over a wide range of distances. A hardened lens becomes more fixed on distant objects and cannot focus as well on nearby objects or vice versa. This hardening often occurs in middle age, when people notice the need to hold the newspaper farther away in order to read the print, and is known as presbyopia from the Greek words presbus, meaning old man, and opia, meaning eye.
Fortunately, there are treatments for both these conditions. A lens that has developed a cataract can be removed surgically and replaced by an artificial one. The implanted lens is clear and allows the image to pass through the eye nondegraded. As for presbyopia, the solution is even simpler--reading glasses or bifocals.
Finally, the retina and the optic nerve work in tandem and deterioration in either can lead to serious, even blinding, conditions. The retina is an extremely complex tissue that converts the light image entering the eye into a neural signal. This signal is then transmitted to the brain by extensions of the ganglion cells. It is the extensions of these cells that form the optic nerve so, in a sense, the optic nerve is an extension of the retina. Numerous studies have shown that, in the absence of disease, there is no significant age-related deterioration in visual capacity of the retina or optic nerve. Unfortunately, age-related diseases of the retina and optic nerve are not entirely rare. Macular degeneration (loss of central retinal function) and glaucoma (damage to the optic nerve due to increased intraocular pressure) lead the list. Early detection of these diseases can often prevent or minimize the extent of vision loss, particularly as new and improved therapies become available.
In summary, as our population ages, society is not fated to having millions of people with "dim" eyes. Rather, with proper proactive ophthalmic care and an emphasis on the development of new therapies for blinding diseases we can continue to make "great vision" a reality for those past the first blush of youth.
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