Almost one in 13 soft contact lens wearers in a recent study had abrasions on their corneas severe enough to lead to infections or other problems.

More than half of the 500 people taking part in the study had signs of mild abrasions - called corneal staining - on at least one eye.

Corneal staining happens to nearly everyone from time to time, whether or not they wear contact lenses, said Kelly Nichols, a study co-author and a clinical assistant professor of optometry at Ohio State University. However, people who wear contact lenses may be particularly susceptible to developing such abrasions if they don't clean and replace their lenses regularly.

Nichols and her colleagues wanted to look at the prevalence and potential causes of such abrasions in people who wear soft contact lenses.

"Contact lenses are still a very safe and effective form of vision correction," Nichols said. "The chance of developing serious visual complications related to wearing contact lenses is very low."

The surface of the cornea regenerates much like skin does -- old cells slough away to make room for new cells. But sometimes too many cells are shed, providing a gateway for bacteria, viruses and other microorganisms to enter the eye.

"The abrasions usually heal in less than 24 hours and don't cause problems," Nichols said. "We were looking for extreme cases where the subject had a lot of staining."

The research appears in a recent issue of the journal Optometry and Vision Science.

The researchers asked optometrists at 20 clinics and private practices around the United States to evaluate some of their contact lens-wearing patients for corneal staining. All 500 participants were required to be full-time soft contact lens wearers - that is, a minimum of eight hours a day, five days a week - and to have worn the lenses for at least a month.

Corneal staining gets its name from the orange-yellow fluorescein dye used to temporarily "stain" the surface of the cornea. After staining the eyes, the optometrist looks at each eye under a special light that causes the dye to fluoresce. The abrasions show up as bright spots on an otherwise smooth, dull surface, thus the name "corneal staining."

"Optometrists monitor staining to make sure that the cornea is healthy and that it's interacting normally with the contact lens," Nichols said.

The surface of the cornea - the epithelium - protects the four underlying layers of the cornea. Optometrists graded staining on a scale of zero to four, with zero indicating no staining and four being the most severe case.

"While any breakdown of the corneal epithelium could provide a possible route for microorganisms to enter the eye, more severe cases of staining could lead to ulceration or scarring," Nichols said.

More than half (55.7 percent) of the patients had mild staining in at least one eye. Eight percent of the subjects had moderate to severe staining in at least one eye.

The severity of staining seemed to depend on how often the subjects replaced their lenses with a new pair; if they wore conventional rather than disposable lenses; and how meticulous they were about cleaning their lenses.

Of the nine subjects who reported using daily disposable lenses, only one had evidence of any staining, and that was mild. Nearly two-thirds of the 120 patients who used conventional contact lenses - lenses replaced yearly - had evidence of mild staining, and 14 percent had signs of moderate to severe staining.

More than half (58 percent) of the subjects wearing the most popular lenses in the study - those replaced every one to two weeks and worn by 225 participants - had some corneal staining, while only 16 subjects in this group had moderate to severe staining.

Not surprisingly, less meticulous hygiene habits made subjects more prone to staining. Patients not cleaning their lenses properly were nearly three times as likely to have moderate to severe staining.

Two-thirds of the subjects who used rewetting drops (164 total subjects) had evidence of mild staining. Half of the subjects not using rewetting drops (333 total subjects) had mild staining.

Lens power also played a role in moderate to severe staining: Farsighted subjects and those with moderate to high nearsightedness were more likely to have severe cases of staining.

Corneal staining is inevitable in most people. Anything that causes friction on the surface of the eye - such as rubbing the eyes, blinking and wearing contact lenses - can cause cells to slough off. Problems like dry eye can also cause staining, Nichols said. Although corneal staining is innocuous most of the time, she suggests that optometrists monitor their contact lens-wearing patients for such abrasions.

"A person with even mild corneal staining could get an infection, though it's less likely if there are fewer openings on the eye," Nichols said.

"If staining progresses, the underlying layers of the cornea may be affected," she said. "Like a cut that doesn't heal properly, the eye could eventually scar. But that's rare - the disease has to be pretty advanced. It's nothing you'd see from the normal wearing of contact lenses."

In most instances, the stains heal on their own. More severe cases usually require intervention. People with moderate to severe staining may simply have to stop wearing lenses for a while or replace their old lenses and adopt better cleaning habits. In rare cases a patient might have to wear a bandage contact lens to keep the eyelid from removing new cells.

"We recommend that soft contact lens wearers be monitored for corneal staining and also consider replacing their lenses at least every six months," Nichols said.

She conducted the study with G. Lynn Mitchell of Ohio State; Karen Stonebraker, an optometrist in Tempe, Ariz.; Dawn Chivers, an optometrist in Albany, N.Y.; and Timothy Edrington, of the Southern College of Optometry in Fullerton, Calif.