Dangerous infections, blinding ulcers in the eyes: These are just some of the troubles that can come from wearing your contacts for too long.

Contact lenses are generally considered safe, but wearing them while asleep significantly raises the risk of developing serious complications that can cause permanent visual loss, the U.S. Centers for Disease Control and Prevention warns.

In one instance, a 34-year-old man who both swam while wearing his contact lenses and wore them overnight three to four days a week developed a rare but potentially blinding form of keratitis caused by a microscopic organism.

Keratitis -- an inflammation of the cornea -- is responsible for an estimated one million outpatient and emergency department visits each year, according to the CDC.

Another case involved a teenage girl who developed a corneal ulcer and scarring after wearing unprescribed lenses to bed.

Yet another involved a 57-year-old man who nearly lost vision in one eye after wearing his soft lenses nonstop for two weeks without daily cleaning.

In that case, the patient ultimately required a corneal transplant in his right eye after suffering a bacterial infection and a perforated cornea.

"The excess risks of developing corneal infection with overnight wear of contact lenses has been recognized for many years," explained Dr. Oliver Schein. He is a professor of ophthalmology, and vice-chair for quality and safety with the Wilmer Eye Institute at Johns Hopkins University in Baltimore. Schein was not involved in the current case study report.

With an eye toward the 45 million Americans who regularly wear contacts, the CDC is highlighting the concern in collaboration with the Eye and Contact Lens Association and the disease surveillance network EMERGEncy ID NET.

Together, the organizations published a new report in the January issue of the Annals of Emergency Medicine that outlines the experiences of six patients who recently developed corneal infections after routinely wearing their contact lenses to bed. The report was led by Dr. Jon Femling of the University of New Mexico's School of Medicine.

Despite the introduction of silicone hydrogels in the 1990s, overall corneal infection risk has held steady over decades, Schein said, affecting about one in every 2,500 lens wearers.

But that risk is "at least 10-fold greater for those choosing to sleep with lenses in. So I discourage that practice whenever I can," he added.

As to why the risk is greater, Schein cited a number of factors that "favor microbial [germ] growth" whenever the eyes are closed, including "microtrauma to the surface of the cornea," a drop in tear production, and a rise in temperature and humidity.

Amy Watts, director of optometry and contact lens service, and director of the vision rehabilitation service at Massachusetts Eye and Ear in Boston, said corneal function also plays a role.

"Our cornea is the only part of the body that receives its oxygen from the atmosphere and not our blood supply," she noted.

So, "when we close our eyes at night, we reduce the amount of oxygen to our cornea by cutting the corneas off from the oxygen-rich atmosphere," Watts explained.

"Wearing a contact lens further reduces the oxygen while asleep, and may put us just past the tipping point for keeping our corneas in optimal condition to fight off microbes," she said.

Another issue is that people who sleep with their lenses also tend to shower with them in place. And "tap water -- just like water in lakes, pools, ponds, oceans -- can have microbes that may cause a serious eye infection," Watts said. "In extreme cases, these infections can cause a severe loss in vision."

Both Watts and Schein said preventing corneal infection is best achieved by wearing single-day disposable lenses.

Schein said, "This approach also removes the need for solution use and, most importantly, obviates the need for a contact lens case, an important advantage, since contamination of the case is certainly a risk factor for infection. However, if the user tries to save money by not discarding the lens daily, then the safety advantage is entirely lost."

Meanwhile, he advises any wearer who develops a painful red eye to remove their lenses and see an eye care specialist within 24 hours.

"Most emergency rooms are not equipped with eye specialists or the equipment -- [such as a] slit lamp biomicroscope -- necessary to distinguish a simple abrasion from an infection," Schein cautioned. "Therefore, those who go to emergency departments initially should be seen in follow up as soon as possible by an eye care professional."

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