If you are lucky enough to have good eyesight, you may be surprised when your diabetes care team recommends that you make an appointment with an eye doctor. If your vision is stable, and your eyes don’t bother you, why should you have your eyes checked?
The answer is that many potentially devastating eye problems develop without causing discomfort or distorting vision. Glaucoma and cataract are examples of eye problems that occur commonly in older adults and more frequently in people with diabetes. Generally, these conditions are treatable, but if not caught early enough, they can lead to vision loss or even blindness.
In addition, there’s diabetic retinopathy, a serious complication that is more likely to occur in people with Type 1 diabetes but may develop in anyone with diabetes. Tight blood glucose control can significantly reduce the incidence and severity of diabetic retinopathy, but the only way to identify this and other eye problems in their earliest and most treatable stages is to have regular, comprehensive eye examinations.
There’s no reason to avoid an eye exam; it involves a series of painless tests that check your visual acuity and general eye health and screen for signs of disease. Before we discuss what to expect at the exam, let’s take a look at the eye and how it works.
The eye is a hollow organ about the size of a Ping-Pong ball, with an opening at the front that lets in light, and a gelatinous substance called vitreous filling most of the inside. It functions in a manner similar to a camera.
The aperture through which light enters the eye is the pupil, the black-seeming hole in the middle of the eye. The iris, the colored ring of muscle tissue surrounding the pupil, controls the amount of light coming in by narrowing or dilating the pupil. The “white” of the eye, or sclera, is a hard shield of tissue that encircles and protects the opening of the eye. A thin layer of tissue called the conjunctiva protects the sclera and connects the eye to the eyelid.
The eye’s main focusing element is the cornea, a clear, hard tissue covering the iris and the pupil. The curve of the cornea bends, or refracts, light rays, focusing them on the retina at the back of the eye. A pool of fluid called aqueous humor fills a cavity between the cornea and the iris. Directly behind the iris is the lens, an elastic disc about the size and shape of an M&M candy, which flexes to fine-tune focus.
Lining the back of the eyeball is the retina, a complex, photosensitive membrane of many layers. This is the “film” of the eye and its most important part. When light is focused onto the retina, photosensitive cells translate the light into electrical impulses, which are then sent via the optic nerve to the brain, where an image is formed. (See “Anatomy of the Eye” for illustrations.)
The eye exam
Like most doctor visits, the eye exam begins with a little paperwork. You will be asked to answer questions or fill out a form, providing information about your general health, any medicines you take, allergies or eye problems you have, and your family medical history. While some of this information may seem irrelevant, asking these routine questions is the only way to establish background information that really does matter. Having high blood glucose or even taking a common, over-the-counter medicine can cause fluctuations in your vision that might make a difference in your exam.
Background complete, the next step in most eye exams involves assessing your visual acuity, or how well you can see. Vision is measured by the size of the letters you can easily read on the eye chart, which is usually about 20 feet away. If you cannot read all of the letters on the chart, it’s because the shape of your eyeball, lens, or cornea causes light to focus either in front of or behind rather than right on the retina. Using a process called refraction, the eye doctor can find an eyeglass or contact lens prescription that bends the light correctly and enables you to see clearly.
Refraction can be done in several ways. The doctor or a technician may hold up vario