written by Michael Cole, OD, Child and Family Eye Care Center
The human eye works like a tiny, intricate telescope. A series of interfaces bend, refract, and focus light so that the brain can interpret the images captured from the scenes around us. Most of these components are fixed, unchanged since shortly after birth. There is, however, one adjustable part of this system. Inside the eye, directly behind the iris, sits a malleable lens. This lens is surrounded by a tiny muscle that changes the shape of the lens, allowing us to focus up-close and far away.
When we are born, the visual system has not yet learned how to control the change in focus of this lens, which we term “accommodation.” The focal distance of the eyes of newborn is relatively fixed at about eight to 10 inches, about the distance to their parents’ faces. Over time, we learn to quickly and accurately change focal lengths in order to clearly view different distances. During this early developmental time, it is critical that children have meaningful visual experiences: Tummy time, crawling, and outdoor play are essential to understanding the visual world we live in and for normal vision development.
Sometimes, children don’t fully gain control of this system and it does not function as it should. Often, children will complain of blurry vision that comes and goes, or that they cannot see the board at school. Even when distance vision is the complaint, it is frequently their accommodation, or ability to change focus, that is the problem. These issues are treatable with compensatory lenses for near vision, multifocal lenses, or by training the visual system with vision therapy.
As we age, the lens becomes less and less pliable. While the muscle that surrounds the lens maintains its strength, as the lens hardens, we slowly lose the ability to change focus, unbeknownst to us. Usually, around age 40, the amount of “focusing power” has been reduced to the point where we start to notice problems seeing up close. This condition is known as presbyopia. Often, the first symptoms that are recognized are eyes getting tired after a day’s work, or the eyes taking a long time to readjust focus when changing viewing distances from near to far, and vice versa. This is usually when the rite of passage of wearing multifocal lenses or reading glasses begins. At first, the additional correction for up close helps mostly with fatigue and comfort. With additional age, extra help will be more and more necessary to function normally while reading, using the computer, and reading anything else up close. Technology has improved greatly since Benjamin Franklin’s bifocal lens centuries ago, and the digital progressive lenses of today are much more functional and appealing. However, while we have many options to compensate for a non-functioning lens, there is no “cure” for presbyopia.
Another normal change in the natural lens that happens later in life is cataracts. Because the original cells in the lens are never replaced, over time the optical quality of this lens begins to degrade. The lens becomes more and more cloudy very slowly, and later in life the opacity in the lens starts to affect our vision. When the lens becomes cloudy enough to not allow light to pass normally through it, it is called a cataract. Cataracts are normal changes that happen to everyone, and if one lives long enough, cataracts will eventually set in. The good news is that cataracts are easily treated with a cataract surgery. In modern cataract surgery, the old cloudy lens is broken into smaller pieces and removed completely. It is replaced with an artificial lens that is perfectly clear, and, because the natural lens is removed, the cataract cannot recur.
Understanding the anatomy of the eye can help us make sense of the visual changes that occur as we age. While the lens may not function normally later in life, children should not struggle with treatable vision issues such as accommodation problems of the natural lens. Comprehensive eye exams for children should include the testing necessary to identify these types of concerns.