Last month, Frank King asked his girlfriend to see if there was a stray eyelash in his left eye, as they worked the night shift together at the Wal-Mart in DuBois. She couldn't detect anything.

But then, before he left for home, Mr. King noticed that the vision in the lower left corner of the eye was getting cloudy.

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Diabetic retinopathy / Condition and treatment

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As a 20-year diabetes patient, the 33-year-old resident of Reynoldsville in Jefferson County knew that could spell trouble.

He was right. The cloudy spot meant that vessels growing from his retina had leaked blood into his eye, one of the problems that can occur in a disorder known as diabetic retinopathy.

Five days later, he was sitting in front of ophthalmologist Thomas Friberg of the UPMC Eye & Ear Institute in Oakland, watching as the doctor burned 779 tiny spots on his retina with a green laser beam.

Retinopathy is not the most common complication of diabetes -- it ranks third behind kidney problems and nerve damage -- but it still affects nearly 20 percent of people with diabetes, and it may be the scariest side effect of them all.

"Your kidneys, you can get transplanted or get dialysis," Mr. King said. "But if your eyes are done, your sight is gone, period."

It is also one of the sneakiest complications. Diabetes patients "often can maintain excellent vision in spite of having fairly significant disease," said Dr. Andrew Eller, one of Dr. Friberg's colleagues at Eye & Ear.

"Someone can come in with 20/20 vision and I might examine him and say, 'Boy, you're in trouble. We should do a laser treatment now.' "

The retina is the brain's gateway to everything we see in the world. Covering the rear two-thirds of the inner wall of the eye, the blood-rich retina contains 120 million light-detecting rods and about 6 million color-sensitive cones, which transmit visual signals through the optic nerve to the brain.

Diabetes does its damage by attacking the blood vessels that feed the retina.

The first stage of retinopathy, affecting about two-thirds of diabetic eye patients, is called macular edema, Dr. Friberg said. It occurs when vessels in the center of the retina leak colorless fluid into the eye, causing swelling and sometimes distorting vision.

Macular edema shouldn't be confused with macular degeneration, a more serious condition in which damage to the center of the retina -- the macula -- can lead to loss of vision from the center outwards.

The next stage of diabetic retinopathy kicks in when blockages in some of the retina's vessels deprive it of the oxygen it needs.

To counteract that, he said, the retina starts growing new blood vessels, which creates two problems. First, the new vessels are inherently fragile and prone to hemorrhaging; second, they often grow into the white of the eye instead of hugging the retina.

Both macular edema and new blood vessel growth, which is known as proliferative retinopathy, call for laser treatment, which has been around for about 40 years.

It might seem logical that the laser is used to coagulate the leaky vessels themselves, but that's not how it works, Dr. Friberg said.

With edema, he said, the laser is used to lay down a grid of spots around the leaky vessels. Doing that "stimulates the eye wall tissue to pump more fluid out of the eye."

With new blood vessel growth, the laser is used to put tiny scars on the retina, often in the corners of the eye, to reduce its demand for oxygen, which in turn often reverses the vascular proliferation.

When retinal vessels hemorrhage, the effect is like squirting ink into an aquarium, which explains why Mr. King's vision was partly clouded.

In some cases, doctors have to remove that blood surgically so that the laser beams can get through to the back of the eye. In other cases, like Mr. King's, the laser can be aimed on either side of the hemorrhage, in hopes that the eye will then reabsorb the blood and the new vessels will atrophy.

In a few cases, though, the new vessels can age and become fibrous, which can lead to the final stage of retinopathy -- retinal detachment.

The force exerted by the fibrous vessels can pull parts of the retina loose from the back wall of the eye.

"If you think of the retina as wallpaper on the back of the eye, then a traction detachment is like some little kid walking by with sticky fingers and pulling some of the paper loose," Dr. Friberg said.

Even worse is when a hole develops in the retina and fluid from the eye infiltrates between the retina and the wall of the eye.

"That's like if we took a knife and made a little hole in the wallpaper; eventually the water would diffuse under that hole and all the wallpaper would come off," he said.

Eye surgery is necessary to correct retinal detachment.

With traction detachment, he said, surgeons put three tiny tubes into the white of the eye. Two of them take out the fibrous areas, while the third pumps in salt water to replace any vitreous jelly that's being removed and maintain the eye's shape. If it's done right, the parts of the retina that have pulled loose will snap back into place.

When there's a hole in the retina, surgeons use a tube to suck out any fluid that's flowed behind the retina. They then seal the hole with a laser, and inject a gas bubble into the eye to act as a natural clamp.

As the eye heals, the eye will replace the gas with liquid.

Researchers have discovered that the culprit in proliferative retinopathy is a protein called VEGF -- vascular endothelial growth factor -- that the retina secretes when it isn't getting enough oxygen.

There are some experimental medications being tested that block VEGF, but Dr. Eller said he isn't confident they will be commercially available anytime soon.

The problem, he said, is how to deliver the drugs safely and effectively.

He cited the example of one experimental drug for macular degeneration that has to be injected directly into the eye once a month -- not a very appealing method for routine prevention in diabetes.

If people with diabetes are able to control their blood sugar and get regular eye checkups, they often can maintain their vision throughout their lives, the two doctors said.

If not, there is a huge cost not only to the patient, but to society and his family, Dr. Eller said.

A diabetic who goes blind often gets state vision services and goes on Social Security disability payments, he said, so "not only are you not providing society with your skills, but you're taking from society because you're drawing down benefits."

Family members often have to take time off from work to ferry blind relatives to doctors' appointments and make other kinds of trips, he added.

Often, the ones most likely to lose their sight are the patients who are in denial, Dr. Friberg said. "There are quite a few patients who know diabetes can cause you to go blind but they don't want to know about it," he said.

Don't put Frank King in that group.

"My kids are 8 years old and 20 months. I want to see them grow up. It makes you think."