An eye exam, at its most basic, determines how well these miraculous little spheres look out on the world. It can also spot medical issues going on inside the body.
"People need to realize that looking into your eyes is more than checking your vision," says genetics counselor Linda Robinson of University of Texas Southwestern Simmons Cancer Center. "Your eyes can tell us so much more than if you need glasses."
Most notably, they can show signs of diabetes, hypertension and cardiovascular problems. An eye exam can also reveal lupus and multiple sclerosis as well as various kinds of cancer.
That's a lot of secrets in a relatively small space. One reason is that the eye is so exposed, explains Andreane Fagala, an associate and therapeutic optometrist with Arnold M. Stokol O.D. and Associates of Richardson, Texas.
"It's the only part of the whole human body where we can visualize the arteries and veins in a noninvasive way," she says. "When you look at any other blood vessel, you have to cut into the tissue or have a scope done."
Thus, seeing the narrowing of those vessels could indicate serious cardiovascular issues, she says. Hemorrhaging or swelling of the macula -- an oval-shaped spot near the center of the retina -- might mean diabetes. Spots and colors can mean the incidence of, or proclivity for, cancer.
"Other than dermatologists, who see the skin, with everything else you don't see the tissue," says Dr. Karl Csaky, head of the molecular ophthalmology laboratory at Texas Retina Associates. "You don't see the organ. The retina, we do. Everything that's there, we see. Even if we can't see it, we have great tests to further visualize it."
People sometimes ignore symptoms in other parts of their bodies, but tend to be diligent about eyesight because they need it for every aspect of their lives, Fagala says.
"A lot of people don't make the connection: How can we see this systemic thing going on, and that the whole body is reflected in the eyes?" she says.
Here are some health issues -- some common, some not -- that the professionals might find when they examine your eyes and thus, why a yearly eye exam not only can save your vision, it can save your life.
For diabetics, an eye exam is routine because diabetes can affect vision, Csaky says. But people may have the disease long before they have trouble seeing, he says.
"It can be relatively advanced and still not be affecting the vision, which is one of the scarier things about it," he says. "The vast majority of people, if they have vision problems, what do they think? That they need glasses. Those are the group for whom this diagnosis is more than surprising. It scares them, so we need to spend a lot of time talking to them."
Diabetes affects vision starting from the periphery of the retina and working inward toward the center of vision. Macular degeneration, on the other hand, begins and ends with vision being blocked from the middle of the retina, which is far more apparent to the patient. In one form of diabetes affecting the retina, Csaky says, "abnormal vessels grow, but they're away from the center of the retina, so the change will not affect vision, and patients don't notice any change."
Left untreated, though, "abnormal blood vessels on the surface of the retina grow and can lead to a detachment of the retina," he says. "The retina dies, and in the worst cases, the patient can go completely blind."
When there are hypertensive changes, Fagala says, "we can see the narrowing of the arteries. They look thinned and make us ask, 'What's going on?'"
Additionally, because the arteries and veins are so visible, an eye exam can show intravascular plaque, she says. The diameter of vessels in the retina is narrow, she says, especially compared with a larger artery like the aorta. Thus, plaque is more easily lodged and can cause damage.
It can look like a yellow fleck in the artery, she says, but patients can't feel it.
"Sometimes patients don't know their cholesterol is high. We see the plaque in their arteries and say, 'Hey, this patient is at risk.' " They'll notify that person's primary-care physician, who may order additional tests or prescribe medicine to get the cholesterol under control.
Extremely high blood pressure can affect vision, Csaky says. "The blood vessels in the retina become constricted, and the retina will look unhealthy. We check their blood pressure and it's dangerously high. Many times we send them from our office to the emergency room."
"Inflammation of the eye can be associated with systemic manifestations," Csaky says.
One of those might be lupus, he says, which most commonly affects younger women. Their eyes might get irritated, turn red, and become sensitive to light, which prompts them to go to the eye doctor, he says.
"We'll ask, 'Have you been having joint pain?'" he says.
"Yeah, something aches," they'll tell him.
"Have you had fever?" he'll ask.
"A little," they'll say.
He tells them, "I need to send you to your internist right away. I think you have inflammation in another part of your body."
Recently, he says, "We've seen leukemia. Again, people notice some bruising, but it's amazing how some symptoms people blow off. If you notice bruises, oh, well. But if it's affecting vision, you go to the eye doctor. Rather than a few minor changes in the retina, we see lots of changes."
One such change might be multiple blood specks, he says, which usually are caused by high blood pressure and diabetes.
"But if we see lots of little bleeding and different configurations, we ask, 'Have you been sick?'
"If they say they've had some weight loss and bruising, we send them right away to the internist."
Robinson, the University of Texas Southwestern genetics counselor, sees patients who are referred because of a strong family history of cancer.
"There have been times we've seen stuff in the eye that led us to do genetic testing that found the patients had a high risk for cancer," she says.
Finding a "freckle" on the eye during an exam isn't all that unusual and could be there for a variety of reasons, she says. One of the more serious is called CHRPE, for congenital hypertrophy of the retinal pigment epithelium.
"It's related to a condition called familial adenomatous polyposis, a disease where you get thousands of colon polyps," she says. "The chance of getting cancer when you have this is almost 100 percent."
In addition, "There's a little vascular tumor you see on the back of the eye," she says. "We see it as early as 5 or 6 years of age."
It can cause blindness and kidney cancer, she says. "You can laser them out and save the vision and the retina." They also might be indicative of tumors in other parts of the body.
The first sign of another childhood cancer, retinoblastoma, is often found in photographs. Instead of red eye, these children have "white eye," she says. If caught early, the disease has a very good survival rate, she says.
"People have to realize your eyes are part of your body just like your skin, and can tell you a lot about your health," she says.