People with diabetes are at higher risk for eye diseases that can lead to vision loss or blindness, but they might not have symptoms at first. Regular eye exams can provide early detection, when treatment is most effective.

Diabetic retinopathy, one of three major forms of diabetic eye disease, is the leading cause of blindness in American adults aged 20-74, according to the National Eye Institute.

Two other major eye diseases associated with diabetes are cataracts and glaucoma. People with diabetes are more likely to get cataracts — clouding of the eye lens — at an earlier age and they are twice as likely to get glaucoma — fluid pressure in the eye that leads to optic nerve damage.

The good news is that 90 percent of diabetes-related blindness is preventable through early detection and treatment.

The most common diabetic eye disease, diabetic retinopathy, usually affects both eyes and is caused by changes in the blood vessels in the retina, the light-sensitive layer at the back of the eye. Fluctuating blood sugar levels in diabetics can cause the blood vessels to swell and leak fluids, leading to vision loss.

The leaking vessels also can cause macular edema, in which fluid accumulates and causes swelling in the area of the retina that controls central vision, the macula. It can occur at any stage.

Diabetic retinopathy is a progressive disease. In its later stages, blood vessels grow abnormally and leak blood, which causes severe vision loss or blindness.

The longer patients have diabetes, the more likely they are to develop retinopathy.

"There are a lot of diabetics in the country. Fifty percent of people with diabetes will show signs [of retinopathy] after 14 years," says Dr. Craig Sklar of the Eye Care Group, with offices in Waterbury, New Haven, Southbury and Branford.

"It's essential for people to keep their blood sugar under control" to prevent rapid changes in blood vessels, he says. "It's not a disease [diabetics] can take a vacation from. They really need to be disciplined throughout their lives. It's a major, major cause of blindness."

In 2012, the NEI reported that 7.7 million Americans were affected by diabetic retinopathy, an increase of 3.7 million in the past decade. That number is expected to grow to 11 million by 2030.

"Even though it's a leading cause of blindness, there's still a lot we can do," Sklar says.

The NEI recommends dilated eye exams yearly for diabetics. Patients should not wait for vision changes or pain because symptoms develop gradually.

The NEI also recommends that diabetics control their blood sugar, blood pressure and cholesterol levels through healthy eating, exercise and medication.

According to the federal Centers for Disease Control and Prevention, in 2012 about half of those affected by diabetic retinopathy were not getting their eyes examined in time for treatment to be effective.

An advantage of early detection is the opportunity to make lifestyle changes, including making more frequent visits to the eye doctor, who will check to see if the disease is progressing, and inform the patient of what to watch for, Sklar says.

It is recommended that people with diabetic retinopathy get eye exams more often than once a year. With treatment, even people with proliferative retinopathy — a severe stage of the disease — can reduce their risk of blindness by 95 percent, according to the NEI.

Laser surgery and drugs are used to treat proliferative retinopathy and macular edema. If someone has proliferative retinopathy, "the risk of being legally blind within five years is about 35 percent, but with laser treatment they have a better than 90 percent chance of preserving their vision," Sklar says.

Laser treatment for proliferative retinopathy might result in loss of side vision and some night and color vision. But a patient's remaining sight can be saved. It works best before new blood vessels have started to bleed. If there is severe bleeding, a surgical treatment, vitrectomy, is required, experts say.

To find an eye doctor, use referrals from sources around you. Talk to family, friends, family doctors, hospitals, professional associations and your insurance company. And when you meet with your doctor, ask questions until you clearly understand the information provided. Bring a family member or friend to help with notes and questions, ask for written instructions and where to go for further information.

If you are diabetic, maintaining stable blood sugar levels and scheduling regular eye exams can save your sight. Sklar says he has seen improvement in the way that patients take care of their eyes: "I see them better managed than several years ago. I think people have more of an awareness."