by S. Todd Stolp MD
©July 2008
In 1927, Dr. William Mayo, of Mayo Clinic fame, wrote an article in the Annals of Surgery about the relative importance of the various “five special senses” to a surgeon. He expressed the opinion that sight was the most important, followed by touch, then hearing, smell and finally, taste. He points out that even the sense of taste could be recruited as a tool for health care, having been incorporated in the first detection of “sugar in the urine of the patient with diabetes.” While vision remains the sense that most sighted people prefer to exercise as they pursue their daily responsibilities, an understanding of our visual apparatus and the diseases that affect it deserves special attention.
The eye is constructed very much like a camera, with a lens which allows us to focus an image onto the back of the eye (the retina) so that one group of photo sensors are able to pick up color and other photo receptors are able to perceive light intensity. With apologies to endless generations of romantic prose depicting the eye as a “window on the soul,” light energy is translated by the retina into electrical signals which are in turn interpreted by the back of the brain to reconstruct an image based upon our scrapbook of prior images. This process occurs instantaneously, and can be used to illustrate two interesting phenomena about perception.
First, we can be misled. Optical illusions are fun because they expose our propensity to make assumptions based upon our prior experience. When the magician causes the maiden to emerge unscathed from the flaming telephone booth, this is beyond the understanding of our visual memory banks, at least from our available angle of observation.
Second, recent research has found that certain individuals with blindness utilize the part of the brain which normally processes visual information to process sound and other sensory signals in order to create a perception of the physical world. Our brain’s ability to seek sensory data from the ears or from touch when it is not provided by the eyes is a tribute to our machinery’s appetite for information about the environment.
The most common causes of blindness develop over years. Macular degeneration is the most common cause of blindness over age 65. It is caused by a breakdown of a part of the retina but the reasons it occurs are not understood. Diabetes is a common cause of blindness, as are cataracts (loss of clarity of the lens) and glaucoma (increased pressure in the eye). Careful control of diabetes can protect significantly against blindness. While Macular Degeneration typically causes a loss of vision in the center of our visual field, glaucoma causes an early loss in the peripheral, or outside edges, of what we observe.
The most common cause of one-sided blindness is direct eye injury. To prevent this common cause of visual loss, it is important to remember eye protection while operating machinery or participating in other high-risk activities.
An eye exam should be done at the time of routine physical examination. Particularly for people with a family history or personal history that puts them at risk for vision loss, a formal eye examination should be sought. Vision loss can be prevented if conditions such as glaucoma are detected early.
Perhaps most overlooked in operating our visual apparatus is the fact that the owner/operator is ultimately responsible for awareness. While beyond the scope of these shared thoughts, it is a puzzle that the ability to see perfectly well does not always coincide with the willingness to look.