The 42-year-old UK woman at the heart of a disturbing new case study had no idea what was causing her upper eyelid to swell and droop. When her doctors ordered an MRI, they saw there was clearly a mass in there, but it wasn’t until they operated and removed it that they realized. It was a rigid, gas-permeable contact lens, dislodged by a badminton shuttlecock 28 years earlier, that had gotten stuck behind her eyelid and formed the base of a growing cyst.

Warning: There’s a moderately gross picture of a corneal infection near the bottom of this article.

“The patient assumed that the RGP lens fell out and was lost,” wrote her doctors in a case report, published August 10 in the British Medical Journal. “However, it can be inferred that the lens migrated into the eyelid and resided there asymptomatically for 28 years.” It may be a freak accident, but according to the Morbidity and Mortality Weekly Report released by the US Centers for Disease Control and Prevention on Friday, contact lens-related eye problems are more common than you might think.

This woman's drooping left eyelid turned out to be the result of a migrated contact lens from 28 years ago. BMJ Case Reports 2018

Contact lenses, the CDC warns, are not without their risks. Though the UK woman’s injury was due to accidental trauma, her case serves as a reminder of what can happen when a contact lens migrates to a place it’s not supposed to be. In the new report, the CDC warns that one of the major reasons this happens is because people break one of the main rules of contacts: Take them off when you sleep.

For the estimated 45 million contact lens wearers in the United States, that’s just Contact Lens 101. But according to the CDC, around one-third of contact lens wearers leave them in while they sleep. In the new report, the agency published six case reports in which patients suffered from corneal infections that were all associated with sleeping in their contacts.

“To illustrate their serious health implications, six cases of contact lens–related corneal infection, in which sleeping in lenses was reported as the main risk factor, are presented,” CDC doctors write. “Consequences of infection reported among the identified cases included the need for frequent administration of antibiotic eye drops, multiple follow-up medical appointments, and permanent eye damage.”

In one case, a man who’d worn contacts for 17 years and who slept in them three to four nights a week developed an infection that took eight months and multiple different treatments to resolve. He ended up with visual acuity decreased even more than it had been before he started wearing contacts. Two of the six patients examined in the case reports required surgery, and all of them had some kind of long-term eye damage or vision impairment.

Here’s a picture of what a typical corneal infection looks like:

This is an example of what a typical contact lens-related cornel infection looks like. CDC/ Deborah S. Jacobs, Jia Yin

Rest assured that these corneal infections, together with the UK woman’s horrific injury, represent worst-case scenarios that don’t represent typical experiences. After all, we all know somebody who sleeps in their contacts regularly. Still, they warrant consideration.

Interestingly, three of the cases mentioned in the CDC report involved cosmetic, non-prescription lenses that were not purchased through a doctor, despite the fact that the FDA considers contact lenses to be medical devices. As buying glasses and contacts online becomes the norm, it’s likely that the number of people buying contacts without ever coming into contact with a doctor will only increase in coming years.

So no matter why you wear lenses, keep in mind that there are risks associated with putting objects into your eyes. Make sure you regularly wash your lenses and don’t sleep in them, as the CDC reports that sleeping in contacts increases your risk of infection by six to eight times.